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MG Midget and Sprite Technical - Phenolic spacers on custom intake manifold

I have a 1967 1275, stock with a 12G938 head, and am fabricating a custom aluminum intake manifold for my twin HS2s (together with a nifty cold air intake--but that's a different post sometime). Can't see spending the cash for a Maniflow steel job; 6061 T6 drawn aluminum tube is cheap; and I'd like improved flow. I know I can do the Vizard thing and radius the stock manifold at the crossover, etc., but I'd like to try rolling my own.

I note that the longer Weber manifolds do not use phenolic spacers, and I gather that on the stock setup they seem to perform two functions: moving the carbs away from the heat shield the right distance so the throttle stop cam functions; and insulating the carbs from the conduction of heat through the short stock manifold.

So, here's my question for the experienced tuners and racers. If I make runners whose total length is roughly equal to or greater than that of the existing manifold runners plus the spacers (and modify the heat shield to retain it), will the loss of the spacers cause the new manifold to conduct enough heat to cause a problem?

Joel.
JM Young

I can't answer your question, but 6061 aluminium moves heat very quickly and efficiently, so I'd guess that you'll need some sort of insulator.
Trevor Jessie

Thanks for responding, Trevor.

What you say makes sense, now that I think about it, since that was one of the selling points of the Speedwell aluminum radiator I installed, and I've read that (ironically), welding aluminum can take more heat because it transfers heat so well.

As I said, my main aim is increased flow, so with my contemplated, straight-shot design, the only impediments would be the joints between the carbs, spacer, heat shield, and manifold runners. I'll have to pull the carbs and examine how close the bore of the spacers is to that of the carbs at the manifold end. I seem to recall that the original SU spacers are bored at fairly close tolerances, and I don't think the heat shield is that far off, either (but we'll see). Of course, if stock gaskets aren't just right, it's easy to cut more accurate ones out of a sheet of the right material.

So, for now I'm leaning toward retaining the spacers--unless someone else has a different experience to report. It will certainly make retaining the heat shield easier! Maybe I'll rivet some of that neat ceramic waffle stuff on the back side of the shield (and repaint the thing, too).

The only other design consideration in my case is clearance for the MG Metro dual-outlet, cast iron exhaust manifold I've been wanting to mount later on, when I get around to putting in a Kent fast road cam. Unlike what people have said elsewhere, this manifold does not foul the footwell or direct the downpipes into the steering column. I'll just have to aim the downpipes forward a bit, and perhaps extend the cutaway in the sheet metal aft as far as possible.

Anyway, the crossover for the stock intake manifold just contacted the #2-3 center branch of the metro exhaust manifold, If I remember correctly when I had things apart to R&R the drivetrain for the Rivergate conversion. So, I'm going to configure my crossover pipe/vacuum source for the crankcase breather differently--which will also greatly reduce interference/turbulence, I think.

Joel.
JM Young

Ive been there and done that...

With out good quality tools and shop experiance...this is a tough job, and not one id take lightly... You have to be very presice....i tried 5 times to replicate the maniflow design that vizard has...in the end, i could never get it to perfection....its so easy to not get it to seat flat which is extrmely important...if its sitting even a few thousands off center you can creat a vacume leak and toast your engine, even makimg one bend sharper then the other side can screw up the speed of the fuel charge

Something elase to consider if your talking flow is if you make the inside bigger... How bad will you slow down the flow of the fuel charge

The purpose of the spacer is to help the fuel and air atomise...the longer the spacer the more the air and fuel mix giving a more explosive charge... But to long a spacer is as bad as 2 short a spacer...so you got some math looking at you

Anywqy... Yeah a new high performance manifold is expensive... But i think they are worth it for a high end name..it just takes all the if and buts out of the soup
Prop and the Blackhole Midget

Hey JM

ARE YOU sure you have a ... 12G938 cly head, if so, id upgrade to a 12g1319 ??? Head other head without the smog ports...im not sure but i think the head you have goes to a 1098 block, but they can be made to fit on the 1275

Hopefully someone can verify what im saying as im going from pure memory... No memorex here....so i hope im wrong

But id recheck the head before you get very far
Prop and the Blackhole Midget

Whow..... Im wrong, ... This is a rare performance head...it goes to the same sprite engine with the EN40B crankshaft

Dude... If you really do have one of these rare engines...i wouldnt mess around... Buy a nice high performance intake manifold...id hate to see something this rare get destroyed to save $75

Prop
Prop and the Blackhole Midget

Here is a little insight

http://www.spritespot.com/phpBB3/viewtopic.php?f=10&t=5526
Prop and the Blackhole Midget

Prop,

Yes, the configuration of the motor and the ‘1275’ tab suggest to me that it really is a 1275. However, the mysterious head casting number has always vexed me, and you are the first among the persons, books, and organizations I have queried who has been able to explain it (thank you). I am thrilled to learn that my little orphan is a sleeper of sorts.

Turns out there is a story behind this car, and its engine, that may explain how I lucked out.

I bought the car from a retired investment banker and amateur pilot behind the nearby Sandia mountains who bought the car from the previous owner who suffered a minor rear-end collision (inspecting inside the boot and the undistorted unibody confirm this). Banker man's friend did a nice job of repairing and repainting the rust-free body and replacing all kinds of stuff from seat covers to hood, but couldn't make her run well and the economy tanked, so no takers.

Here's the engine part. Banker man has three bugeyes, one is concourse correct, one is a daily driver, another is a racer that looks professionally prepped. His airplane hanger, where my car and the racer were kept, had several long blocks lying around among boxes of odds and ends. As I haggled and my wife started to look increasingly uneasy, Banker man started to talk.

Banker man said that he and his friend took the engine OUT of this car to assemble a special vintage racer from various barn-find stuff they had collected. He said something about rebuilding ‘the engine’ but changed his mind because it ran so well. Thing is, the engine in the car ran terribly. Wrong distributor, leak between no. 2 and 3 cylinders, carbs adjusted poorly, etc. So, what I’m thinking may have happened is either they got mixed up and put the engine they had intended for their project racer into my midget, or she had this special engine all along, and they didn’t know what they had, couldn’t make her run right, and wanted out of the money pit.

Anyhow, Prop, I appreciate your concerns about the manifold and my newly discovered treasure. I have a very precise compound miter saw and excellent, 100-tooth aluminum cutting blade, so no problem making precise, clean cuts in the 6061 drawn tube for nice, squarely mounted flanges at each end. I think I need a 20 degree angle to keep the float bowls level, right?

I use a good machinist who has a vacuum table to test for leaks when it’s tig welded up by another guy who used to work at an aerospace plant and is a masterful welder. Essentially, the runners will be just slightly longer than stock to keep the spacers and leave room for air filters--but they should flow much better.

Wow, with the Kent fast road cam and some light porting, it sounds like she’ll be a hummer. What’s the benefit of the EN40B crank you mention? How can I tell if I have one of those?

Joel.
JM Young

Joel

The EN40 crank is stronger than the EN16 crank. It was fitted to some early stock 1275 engines and also used for racing.

It has cross-drilled big-end journals and *should* have the number AEG556 on it, as well as EN40B.

Does your head have the raised bosses with brass plugs in them, as in the photo?


Dave O'Neill2

Hi, Dave.

Cool, next time I have the oil pan down, I'll check it out. Thanks for the information, which I've written down.

The bosses you mention look slightly different, but more importantly, where yours says 12G940, mine says 12G938. The stamping outside by the thermostat housing is very small, but clearly reads the same.

Like I said to Prop, this odd casting number has always vexed me--until now.

JM Young

Well congrats JM

Its a welcome surprise to learn you got something special instead of the avg norm

Sort of like the guy that went to a yard sell and purchased an old bottle of johnny walker blue label only to find some years later after drinking the whole bottle it was actually a bottle of johnney walker blue label 1809


YREEEAAAYYYY... im the man ! Hipp Hipp HoRay I own a bottle of 1809.....Oh NOoooo, now i remember.

Im assured thats a true story, but who knows, it would be funny as heck if it were true.... Can you imagine...god how sucidial that guy must have been... Buy the bottle for probably $5, then drink the whole bottle thats probably worth around $250k. Then learn of its true value, and rarity, several years later...hahaha

Prop
Prop and the Blackhole Midget

Joel
My 20 cents worth

The Weber manifold doesn't need a heat insulator as such as the carb is mounted on softish/flexable insulators which stop any heat transfer and the retaining nuts are also spring loaded which helps against heat transfer through them.
A possible suggestion if you really want to fabricate a longer manifold without the SU type phenolic spacers, is I reckon you could adapt a pair of the Weber "O" ring type spacers and spring loaded mounting hardware.
This would give you the long manifold/non big spacer look that you want and would also do your carbs a favor by reducing vibration through the flexy Weber mounts
Your floats and needle&seats would be happy I'm sure and would say thankyou and go very fast
Right Prop- what a good idea, you should do that instead of messing about with that timing light

Cheers Willy
William Revit

Also
If fitting any angry camshaft the balance tube size can be reduced to smooth out the idle a bit
Willy
William Revit

Hey willy

Can you expand on your above comment about reducing the balance tube on the camshaft...interesting thought, ive never heard of that before

Im guessing balance tube is the core of the camshaft that holds the lobes

Prop
Prop and the Blackhole Midget

Hmmmm....sorry the blond hair got in the way..lol

You mean reducing the size of the balance tube ...in the intake manifold. ???


I was over thinking balance tube as language oddity of OZ vs hill-billy
Prop
Prop and the Blackhole Midget

Some people block off the balance tube altogether and smooth the tract and report an improvement in measurable HP - and having them only slows the speed (and produces turbulance) to the incoming charge. The argument cited is that 1) Webers work without them and produce good power and 2) SU's if properly set up shouldn't need a balance tube.
F Pollock

Willy - You have me intrigued about the Weber-style 'O' ring style spacer-insulators and spring-loaded hardware. I'm ignorant of that setup and am going to look into it. Of course, I'm making my own flanges and can configure them however I want, so this sounds like a neat idea.

Willy and F. Pollock - I had wondered about whether the crossover was necessary, given that many racers use Webers to great effect, and Vizard does not indicate that one might want to modify the Weber manifolds to add a crossover.

Really, the only reasons I was going to include a crossover were: (1) a vacuum source for my stock breather valve, as I like negative crankcase pressure and also like the look of the stock setup; (2) a structural member to join the runners--though now that I think about it, the heat shield probably adds sufficient bracing once the carbs are bolted on; and (3) a way to let air into the runners, should I elect to switch over to EFI and wish to include an idle air control valve. Of course, I could add quite small nipples and connect them with hosing for the latter purpose without causing much turbulence.

That said, can any of you think of any downside to eliminating any crossover, and having the crankcase fumes sucked into either no. 1 & 2 or 3 & 4 only? Right now, the only vacuum source I need is for my distributor advance, and that's obtained from the port on the forward HS2.

Joel.
JM Young

Joel
For a road car on SU's I'd run a balance tube
It evens out the pulses in the manifold and effects low speed mixture and also as you mentioned it provides a smoother vac supply for breathers and brake boosters etc
With wilder camshaft grinds------
A normal size balance tube gives the normal brp, brp ,brp lumpy style idle but
With no balance tube at all and a "big" cam the idle is like with a side draught weber where no's 2&3 become very weak and the engine basically idles on two cylinders

I had a pair of 2" SU's on my hillclimber without a balance tube and you could pull the plug leeds off 2 or3 at idle and there wasn't any effect at all It was very tractable down low even with it's monster camshaft but for the road I don't think it would be much use

I'd be running a balance tube if it were mine for the road
A friend spent a lot of time setting up tripple SU's on his Healy. it had a 30-70 cam He tried several different sized balance tubes and ended up with 1/2"i.d tubing which compares to about 1" on the standard manifold Idles as smooth as

Willy

The weber mounts are called soft mounts
If you have trouble finding pics yell out and i'll find a pic.
William Revit

The other option to the "soft mounts" William mentioned is the bonded rubber mounts as used for side draughts on Alfa Romeos. I used them when I made a manifold to fit a 40 Dellorto to my 1420cc A series and they worked well.
David Billington

Well, Gents, things are starting to settle in my mind.

Willy - I think that I will include a balance tube after all, since in my emergent design it can serve three functions: even out the intake pulses; provide a smooth vacuum source (and fair distribution of nasty crankcase fumes, so no pair of cylinders feels cheated out of NM's thin, mountain air); and add some structural rigidity.

Size does matter, evidently, and it seems to me from reading Vizard's tome that a combination of the size and nature of the orifice inside the runners is key.

So, what I'm thinking is that I'll drill a 9/16" or 1/2" hole in each runner and put a generously feathered radius on the downstream side, and perhaps just break the edge every so minutely on the upstream side. Then, use 1/2" drawn 6061 tubing or the bar stock I've already got (rectangular 1x1.5") with a 1/2" hole drilled through it to connect the runners. I like the bar stock idea, as it'll be easier to drill and tap holes for fittings of various sizes, attach a flange for an integral IAC down the road, etc.

David - Any way you could send a link to the bonded style rubber mounts you are talking about? I think I saw something like that on-line, but there seem to be lots of 'soft mount' configurations for Webers, so I'd like to know I'm looking at the right thing.

You know, one favor I'd ask of someone who runs webers and may have a spare gasket lying around. Any chance he/she could measure the stud spread, so I can compare it to that of my HS2s? If I'm going to use a prefab bonded 'soft mount' instead of machining an 'O' ring groove into the carb end flange of the manifold, I'll need to make sure the stud spacing is somewhere close, I would think.

Joel.
JM Young

JM Young,

This is the sort of thing http://www.alfaholics.com/2010/07/carb-rubber-mounts-original-pirelli-40mm . I note that that bunch offer an aftermarket aluminium alternative as well.
David Billington

David,

Very cool--thank you. I found the aluminum version that you referenced here: http://www.alfaholics.com/2010/07/solid-aluminium-carb-mounts/

Zooming in on the image of the carb bolted onto the mounts, I see that the grooves are machined shallow enough so that the chubby cross section of the 'O' rings stands proud a bit, even after being compressed. I take it this is how you get reduced vibration as well as less heat conduction from the manifold.

Zooming in on the mounts themselves, I guess that I could simply machine grooves into the flat stock I'm going to use for the flanges, and get 'O' rings fat enough to create the slight space I see in the above pictures.

Since the drawn 6061 aluminum tube I'm using has fairly thick walls, I could also throw the runners on a lathe and machine some shallow 'fins' to dissipate conducted heat along their length--leaving a flat space to attach the crossover piece. I'm not sure whether that would be preferable to leaving the walls thicker to avoid convection from the nearby exhaust manifold... so many considerations!

Second alternative would be to make separate, relatively thin spacers with 'O' ring grooves on one side and bolt them to the manifold flanges, which could also have 'O' ring grooves. Two rubber rings, two air gaps--which would plummet conduction but perhaps increase movement enough to warrant some sort of support at the air cleaner end. I think I read something about that in a post related to the 'soft mounts.' On the other hand, SU HS2s are quite a bit smaller than Webers, aren't they?

Anyway, I'm getting closer...

Joel.
JM Young

Also search for Misab plates

With DCOE Webers it is important that they are mounted with a degree of flex because bolting them up tight leads to fuel frothing in the fuel float chamber. SU's obviously don't suffer from the same problem.

The only head I've seen marked 12G938 was a very special race head done by Bill Richards many years ago and which ended up on David Shannon's Class A Midget - I believe the original casting came out of the Special Tuning stores when it was closed down in the 70's. Of itself I don't think there is much different about the specification of the head, but I have a feeling that the casting left a bit more material in certain places which meant that subsequent modification could be more radical...
James Bilsland

The Alfa mountings are a lot bulkier than the mounts I had refered to-
A pic. of the type I meant can be seen here-------

http://www.weberperformance.com.au/index.php?cPath=115_101
William Revit

Yeah - Misab plates, that's them.

http://www.dellorto.co.uk/merchandise/products_details.asp?PartNo=Cosx4&CategoryID=8&PartsectionID=46

and

http://www.dellorto.co.uk/merchandise/products_details.asp?PartNo=Misab&CategoryID=8&PartsectionID=46

and the job's a good 'un.
P Dodd

The Lynx unit at $19 is the popular one but the "O" rings are a part of the assy. where some of the more expensive ones have replaceable rings
The thing is they don't really wear out very often so one piece is fine
They mount up using nylock nuts and done up so there is still clearance between the coils of the mounting springs which gives the vibration damping effect
It appears the smallest is 40mm so would need a sleeve inside to get to your 1 1/4" OR maybee you could continue your manifold tubes out through your mounting flanges 1/8" or so and fit the flexy mounts over them , then you wouldn't have to rely on the bolt pattern of the mounts as much to hold them accuratley in place
Willy
William Revit

pic. of a weber mounted on softmounts
Misab plates for you Doddy

I'm off to sleep now it's 10-30pm here - goodnight


William Revit

Thanks Willy - I've got one under my bonnet :) (as the actress said...)
P Dodd

Gents - It's looking like the way to minimize obstructions along each runner's bore and minimize heat conduction is to:

(a) jettison the original way of mounting the heat shield--which is sandwiching it between the manifold and phenolic spacers, which requires two gaskets and creates two joints per runner;

(b) dump the phenolic spacers; and

(c) machine a groove for an 'O' ring into the flange at the carb end, as in this design, http://www.weberperformance.com.au/index.php?cPath=115_101, identified by Willy and Doddy, using the nifty 'hour glass' shaped rubber replacements for the thackery spring washers. (Though I do regret the lost opportunity to use 'thackery' and 'bonnet' in the same sentence). I'd simply weld these mounts onto my runners, but the bore size is way off: 40mm versus 31.75mm for my HS2s. I want a perfectly matched manifold-to-carb bore, and the drawn 6061 tube I found is so close I can't see the transition.

I can use long studs and sleeve spacers to both accommodate the carbs and extend inward from the flanges to position the heat shield in its 'original' location vis-a-vis the carbs, so as to retain its throttle stop function. Just need to make larger or differently shaped holes to slip the manifold flanges through. I could then bend up and paint some 'supplemental' U-shaped pieces of sheet metal to cover the gap created.

Small matter to rubber mount the bespoke air filter box I'm making to the inner fender sheet metal, to keep the carbs from drooping and contacting the manifold at the bottom.

I'll post pictures whenever this project is done.

Thanks so much for your generous help.

Joel.
JM Young

James - it does seem to be the consensus that the 12G938 head is a special casting, whose chief benefit is more material to enable more aggressive porting. I do not recall the combustion chambers being so different that it caught my eye, though of course when I had the head off for no-lead seats and decarboning, I wasn't aware that I should be looking for any distinguishing features. The other superficial aspects are the same--number of head bolts, locations of tapped hole for water temp probe and small brass oil gallery plug, etc.

What Prop and Dave note is that it seems to be paired with a stronger, EN40 crank, resulting in a platform for building a durable, high-performance engine.

Everyone - does anyone know whether the rods, pistons, or cam are any different in these engines? She's running, so I'm loathe to break her down to find out until it's time for a rebuild. My block is stamped 12CC/Da/H10448, and the car's VIN is GAN4L56996. The piston crowns, if I recall, were slightly cupped, which I think is a stock feature--right?

Joel.
JM Young

Hmmm... By saying your dumpimg the phenolic spacers are you going to make the intake manifold the same length as the old design with the spacer ??

If not, id think you will have a fuel flooding issue....you need some length for the fuel to atomize properly before escaping the valves to be exploded
Prop and the Blackhole Midget

Hi Joel

Early Midget 1275 engines had EN40B cranks as did Mk1 Mini Cooper S 1275 engines. EN40B cranks were thought to be a necessary performance upgrade on a sporting car in the mid 60s. Some Midgets were also fitted with the Cooper S AEG153 steel conrods

BMC soon realised the cheaper EN16T material was quite adequate for an engine which was understressed at 65bhp at the flywheel.

Your engine (if it does contain an EN40B crank which is not a certainty) is likely to be identical in all other respects to later 1275 Midget engines. There is no performance benefit to having an EN40B crank or Cooper S rods unless you use them in a significantly modified engine - they are quite simply tougher than the later crank and rods - however all 1275 Midget cranks and rods are pretty tough and can quite easily handle performance upgrades which double the power output of the engine without significant reliability problems.

Hope that helps

Regards

James B
James B

James... I got "experiance" on my side

I will post some photos of my old 16T crank this weekend, after all, its almost halloween...

Prop
Prop and the Blackhole Midget

Prop

You can break EN40B cranks too

But this one was EN16T

James


James B

To be fair that crank was in a full race 1380 which I used for several seasons - must have done over 50 races...
James B

Hi, James.

Thanks for the additional education on the bottom end parts. I'm perfectly satisfied to have over-engineered bits in my Midget's bowels. I intend to drive her as often as possible and without pampering until something breaks, and a tougher crank can't hurt. I am going to keep the info you and the other gents have provided, and will eagerly examine the innards when the time comes to rebuild her.

Prop - yes, my manifold design seems to have distilled as described above, and will result in runners just as you've described, in terms of their length, that is. My understanding is that longer nets more torque (though that may be a gross understanding), and that outcome is perfect for lurching away from stoplights and climbing on-ramps with overpowered, new-millenium motor cars nipping at my heels.

The other typical scenarios are 75-80+ mph cruising on NM's highways, since our towns tend to separated by long expanses of empty desert filled with careening, cowboy Cadillacs (V8-powered pickup trucks), and baking in rush-hour, stop-and-go traffic in 90+ degree heat (fahrenheit, for you Brits). This is the constellation of challenges for a high-desert Spridget. So far, so good...

Joel.
JM Young

This is what an EN40B crank looks like...


Dave O'Neill2

Thanks james... Now i dont have to post the pics of my crank...as it broke in exactly the same place the same way...just in 2 seprate pieces

All i was doing was driving down the highway at 75 in 5th, it sounded like i lost the exhauste manifold....very little warning, of fluctation oil pressure over the previous 100 miles
Prop and the Blackhole Midget

Dave and James,

Thank you for the images. The two cranks are noticeably different even to my non-machinist's eye. Wish I'd have known what to look for when I resealed the oil pan when doing the Rivergate conversion. I keep running across the Datsun 5-speeds in NM's bone yards.

By the by, could you gents tell me when the best (not necessarily the largest) gathering of MGs is held each year in the UK? My fiancee and I are pining to visit the Kingdom, as tracing my father's roots and the prospect of poking our heads under the bonnets of right-hand-drive Spridgets both seem worth the trip.

Joel.
JM Young

Mine was coming out of the hairpin at Mallory - it suddenly felt a little rough and I hit the kill switch - on the downside I lost a podium finish - on the upside I reckon that crank would have done about 100 more revolutions before it came apart big time and the rod came out the side of the block...
James B

I did the same... As soon as it started banging i shut it down, amd coasted to the shoulder and was able to save the engine for a rebuild

But from what i understand... The 16T that is a common place for it to break... Almost everyone ive talked to, thats where they break...which is hard to belive that a crank with that much meat on it could actually bust...
Prop and the Blackhole Midget

Mine broke just behind the centre main.

It was pulling 7000 in top on the uphill stretch from Eau Rouge to Les Combes at Spa. It didn't half go with a bang!
Dave O'Neill2

Joel

MG Live is the biggest gathering and if you have never been there before also the best.
Racing, spares and a lot of visitor cars to see.
Onno K

Thanks, Onno. Maybe if we start saving our nickels now, we'll be able to afford the tickets in June.

Joel
JM Young

This thread was discussed between 20/09/2012 and 02/10/2012

MG Midget and Sprite Technical index

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