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MG Midget and Sprite Technical - 1275cc engine - budget build ideas?

Hi

Any suggestions please for a specification for a road 1275cc engine (good torque that could also do the odd hillclimb/sprint which would be under 1400cc so 1310 could be OK)? Guess that is at best 90bhp. To keep costs down and to learn something I would want to use not too exotic components, assemble it myself having farmed out any necessary machining jobs. I realise a challenge will be finding a suitable donor engine that has not been linered or rebored too much already and also has retained its engine number (should have snapped up the sweet sounding runner David Smith had for sale recently).

What is the large bore budget go to set of pistons, i.e. the equivalent of using 998cc Mini pistons in an overbored 948cc engine? What conrods?

I would be tempted for rebore that is not offset, so not 1380cc, so more like 1293cc.

I realise I would need an LCB and decent exhaust, but what cam and where to get a modified head or what mods to do to the head? I note Alan Anstead’s advice on AC Dodd cams and also see some useful info on Keith Calver’s website (including a superb acronym for faithful disciples of the Vizard book): https://www.calverst.com/technical-info/engine-1275-95bhp-sports-tourer-test-result/ I also see some very old advice in CCC: http://austinhealeyspritemk2.blogspot.com/2015/ plus the BL Special Tuning booklet. Old days would be selling something to buy a Longman head or get the grinding bits out and having a go.

Machining - rebore and crank machining if needs oversized bearing shells, work for to fit a centre main strap? If getting more serious/got a not quite flat block then deck the block??

Probably put twin HS2 carbs as have them to hand (perhaps a single 1 3/4” if came by a carb and manifold at the right price).

Make me think a 1293cc built short engine from Bill Richards Racing is probably a strong option, similar in spec to the one they do for Minispares: http://www.minispares.com/product/Classic/ENG004.aspx

Thoughts? Your favoured recipe and ingredients? Tips and pitfalls to avoid?

Mike
M Wood

Mike

As I'm sure you're aware, I'm no expert but here are some thoughts following the rebuild I did myself on my 1275 in 2020.

This was a straight-forward re-bore and re-build that I had diagnosed as just needing new piston rings until the machine shop pointed out that a re-bore was required. I didn't have the funds to throw anything exotic at it, so most replacements were standard apart from being over/under-sized following machining. As far as I'm aware, it is a standard cam.

The machine shop bill came to just over £1000. This included +0.040" re-bore to 1310cc (it was +0.030" when I got it), head skim, block skim, valve seats re-cut and head reassembled. It also included the Nural pistons and a full gasket set. I wasn't expecting them to re-build the head but I'm glad they did because I think I might have missed that the valves needed re-cutting.

The only modifications I made were to use ARP bolts on the bottom end, fit a spin-on filter that I already had and vernier timing gear, from Mini-Sports, which allowed me to advance the cam timing. I had been advised to go for this anyway by a local performance engine builder who said that the head and block skimming would affect the standard timing a bit.

I've been pricing replacement pistons following a recent mishap and the going price for the standard compression Nurals is around £50 each.

I considered getting the centre main bearing strapped but was told this would only be needed if I was circuit racing using sustained high revs rather than squirting it up short hills.

The Sprite already had a Maniflow three-branch manifold and straight-through silencer. It has twin 1 1/4" HS2 SUs with K&N filters. The distributor is a 23D4 with condenser and points and could probably do with rebuilding as it's never been touched.

I've absolutely no idea what hp it puts out, just that it can clock 71.99 at Loton and 79.51 Harewood. It was booked in for a rolling road session with Peter Burgess but I had to cancel that when it swallowed a spring washer last month. I'm hoping to visit Peter next year when it has new pistons and, just possibly, a Weber carburettor.

Colin
C Mee

You can take counsel from A.C.Dodd via Fb.
Mr Burgess may give you advice.
Bill Richards, who built the Minispares engines, has retired

Unfortunately taking counsel from ‘all & sundry’ can involve some just repeating old ‘Mantra’ that they have read or heard but of which they have no hands on experience nor engineering background.

You dont necessarily need variable cam gears just offset keys.





Alan Anstead

Colin,

I can see why a block and/or head skim would alter the valve timing on an overhead cam engine but not on an A series with the cam in the block.
David Billington

It depends on what you class as a budget build!

It’s amazing how expensive it is to build a decent (or even not so decent) engine and its also hard to find someone to do it.

Brief spec of my last Hillclimb engine was:

1310 cc - so plus 040”
Standard but HC pistons from Minispares - AE = Nural I think now.
10.7:1 cr
Kent 286
Decent head with 29x37 valves
1.5 Roller rockers
HIF44 carb
Standard bottom end apart from ARP rod bolts and Turbo mains.

It put out around 100bhp, revved to 7500 and lasted for many years of hillclimbs, sprints and finally racing - until the crank snapped!

At a guess and going off 2 recent engine builds (one mild tune and one full race) I’d say budget for about £2500 minimum if you do it yourself or £3500 if you get someone to do it. That is a complete guess and depends on what parts you use, the state of the engine core you have and what head you find - some places want £1000 just for a fairly mild tuned head.

Recently I found it very difficult to get someone to do the work, most of the major A Series tuners now only do full builds at high costs.









John Payne

My recent rebuild cost about US$2,300, including machine work (boring to .030, crank grinding, etc.) parts, (including higher compression ratio pistons, bearings, etc). I also had the machine shop assemble the engine since they do that every day and I do it only occasionally.

With the new clutch and other bits, the cost was probably more like $2,600. I am the original owner of the car and recently came across the window sticker from the dealership. The car was US$ 2,690, new in 1971 and if I recall correctly, the dealer knocked off fifty bucks off as part of the deal.

So now I have a new running midget again for about the original cost.
Glenn Mallory

Engine specs will always bring out varying opinions and options.

More performance for what purpose.
How often do you use over 4000 rpm. If the answer is not a lot then you probably don't / wont need a cam that starts and finishes high.

After an audience arranged, some years ago, for my group with A.C.Dodd I was called upon to de-tune two cars.
We fitted A.C.Dodd Road Torque cams and set the CR at 9.5:1

No fancy adjustable cam gears just offset keys. Both owners were now happy with a bit more torque and excellent road manners. Both engines had the CR reduced (9.5:1) using Cometic Head gaskets.

Uprated bolts between stock and ARP are available.

Roller rockers can sometimes irritate as they tap away even audible with an alloy rocker cover.

Lightening and balancing the flywheel and crank makes for a smooth running engine.

I now fit billet x-drilled cams to aid lube on the cam lobes.

The engine I am building in the picture above received an A.C.Dodd RT cam but the owner would not allow me to deck the block thus it runs stock CR, lower than AC’s spec (9.5:1) but he was pleased with the build.
Alan Anstead

Thanks all, some very useful advice and realism.

Cheers
Mike
M Wood

Mike
For a good gutsy engine you can't beat cubic inches
I'd be going the biggest bore you're happy with and get yourself a stroker crank done-
willy
William Revit

MED engineering had an ad for stroker cranks not so long ago. I saw them on facebook, circa 250 quid *OFF* special offer.

Link still good.

https://m.facebook.com/MedRaceTechnologies/photos/a.857685667576490/3239623172716049/?type=3

What else is involved, other than changing the crank Willy?
anamnesis

Well, you need shorter pistons for a start ;o)
Dave O'Neill 2

Yep--pistons more or less covers it
If you're not going overboard with the extra stroke a bit can be skimmed off the top of std. pistons to accommodate the extra stroke-
It used to be quite popular here with the mini guys
They'd use a large journal crank and get it reground offset to small journal size and run small journal rods
Normally std pistons were a bit down the bore at tdc anyway so that soaked up a bit of the extra travel then a little slice off the pistons to get the deck height good and away they went
I did the biggest B series engine on the planet for my midget hillclimber years ago, had this crank done with 4" stroke and ran into all sorts of bother with the rods hitting the camshaft which took some sorting out and getting windage tray clearance was an issue. Also getting pistons to suit the height without the top rings overlapping the valve cutouts in the bore.--but well worth the effort-----Had it in a BGT on the road for a while and up the back of home here on the highway there's a steep hill and in a std B in overdrive, if you hit the bottom of the hill at 110kph(local speed limit) you'd have to flick out of od and floor it and go over the top at about 90 in 4th---With this engine you stayed in od and floored it and you'd be doing 150+ over the top--loved it
The big bore mini guys here years ago used to use modified Triumph 2000 pistons if the old memory is working
willy
William Revit

It sounds good. So why hasn't it caught on here? Too expensive, or other downsides?
anamnesis

You can't just leave it at that Willy!!

What capacity was the engine? What was it like fitting a B engine into a Midget? What gearbox did you use? Tell us more :-)

Malc.

Malcolm

2307cc Malc. and wasn't too bad fitting it up but wanted it back a bit and over to the left as far as possible to get the weights right(or close) so ended up on solid mounts(got a buzz out of that)-The body was a bit of a scrapper made up from bits and pieces so chopping it up wasn't an issue-Ended up with exactly the same weight on each wheel with the driver on board --it was a bit of an ongoing experiment and if i was going again it'd get the engine another couple of inches back the get the magic 55% rear weight.--the whole idea was to get it sorted then build a good car but that won't be happening now--There used to be a roadgoing one on the mainland with MGB engine and box that came over here every now and then and that car was on rubber mounts but I've never had a close look at it. i ran a lotus bullet c/r box and a 105e Anglia axle housing with an escort 3.7centre with a homemade lsd---when i sold the car it was on a std crank and 84mm bore(1970?ish cc) and the bloke that bought it was stoked--little did he know what it really went like with the big fella on board
really enjoy talking about this sort of stuff but better stop, we're nicking Woody's thread
willy
William Revit

Love reading about it! But yes, sorry Woody. We will grab our coats and be on our way... :-)

Malc.
Malcolm

Willy and Malc

Very happy as ever for thread drift and hearing about Willy”s MGB engined Midget with a stroker crank. Pics please!

Do love this BBS!

Cheers
Woody
(And it has been too many years since I was called that)
M Wood

Long time ago, I'll see what i can find, didn't take a lot of pics when I had it, too busy having fun,-have some from after but don't like posting pics of other people's cars, see what i can find-
willy
William Revit

couple of fluffy old pics
First start up and start of first hillclimb-hadn't noticed the bloke behind before-he looks a bit worried





William Revit

later on when it was sorted a bit and on some decent wheels--- running a Dellorto instead of the SU's





William Revit

up on his tippy toes here
This is the last time i ran the car---this is about 2 seconds away from a bit of a major excursion down through the trees--the hill comes up and curls left, up over a rise where the car is there then cranks up around to the right---i got in there a bit too quick and got airborn over the rise and landed at rightangles to the road---dissapeared over the edge into the bush, luckily there's a monster wire rope in the bush to catch trucks etc that get out of control going down the hill, so that saved a big trip to the bottom luckily-
IF you get past this corner the next bit is a straight uphill section and the gearing was perfect for that,it'd buzz right out to 7grand in 3rd with the close ratio box and tall axle and was fairly mobile but just back from where you let off the gas it's really rough and she'd start wheel spinning/skipping making it interesting jumping on the skids for the next sharp hairpin bend-bloody good fun
It's a magic hill 33 corners and takes nearly 2 mins to get up--too far to hold your breath-lol
They've banned open top cars and open wheelers on this hill now--"they" reckon it's too dangerous---party poopers



William Revit

That is AWESOME! what a hero!! :-D

Malc.
Malcolm

There's some videos about of guys going up this hill, i'll try and find a decent one that shows how much fun it is----------
William Revit

here you go-
Housey getting some exercise in his old Holden racer

https://www.youtube.com/watch?v=sGbB_4hN-QU
William Revit

That's a proper hill climb! How I wish we had a few hills with a decent length like that here in the UK.

Not many marshals in evidence, or are they all hiding behind trees to be out of the firing line!
C Mee

I LOL-ed when he ran over half a tree in the middle of the track! That wouldn't be allowed to happen here!
Malcolm

branches just drop out of the trees every now and then, fairly rare for one to be on the road like that, someone usually grabs them---not many marshals needed for this hill though as there's no public access on race day, it's the access road to Lake Barrington a hydro/electric storage dam which is also used as an international rowing course--magic spot---
on race days there's a viewing point about half way up the hill with an ambulance there just in case,, and there'd be 5-6 radio contacts up along the hill--it's run quite smoothly and you get 5-6 runs for the day usually about 40 starters-

We better get back on track---Mike's engine--
There must be a decent combo achievable---not too much of one thing just an improvement from std everywhere is the secret and it'll go fine-decent size rebore decent compression to suit your chosen camshaft--don't fall into the trap of overdoing the cam, nice porting job on the head matched to the manifold and twin 1 1/2" carbs or a s/d weber and you'd have a decent improvement over std 1275-
There used to be a cam here called a police special which was a leftover from when the police ran cooper s's -it was a ripper ,hardly lumpy at all at idle but went like a train, they were a bit of a secret weapon for the otherwise unmodified police cars--I'll get onto my little camgrinder mate and see if he's got the specs for it--you might be able to match it up to something there-
cheers
willy
William Revit

Nice.

Interesting we haven't seen many B engines stuffed into Spridgets here. Why is that?

More in the past I mean, before other optioms for engine swaps became more viable.
anamnesis

I'm surprised that there aren't more 1500s fitted with MGB engines. They are able to accommodate longer engines with the modified steering rack cross member. They also have bevelled inner chassis members where the 1500 backplate sits, and the 1500 engine/box can't be much lighter than and MGB engine/box.
Rob
MG Moneypit

Hi Mike
https://www.brayonclassicengineering.com/
They might give you a bit of advice
I found them very helpful
R Deans

This thread was discussed between 18/06/2023 and 08/07/2023

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