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MG Midget and Sprite Technical - Engine Build - Advice needed!

So im building a new engine for my car. Ive decided on the following spec and would like to know if anyone has anything similar or could advise i do something different. At the moment the block is with the engine builders and I intend to ask them to do the following:



Use the linered 1275 block. Rebore the existing liners +020 to take to 1293.

Use 21253 pistons

Deck the block so its square to the bores but also so the pistons are level with the top. Desired CR is 10.5:1

Line bore and fit new cam bearings

Line bore the crank tunnel to ensure striaght.

Regrind the crank and journals +020 (it needs it) re tuffride and balance.

Machine the centre cap to take a strap ( do I want to use the 4bolt type (PeterMayEngineering or am I okay to use the 2 bolt steel strap type - cars intended rev limit will be 7k)

Drill block to take extra studs (future proof it if you like whilst builders have it)

New core plugs.



Rebuild using the metro turbo oil pump

New dizzy drive

544 (style) camshaft (tempted to go 649 but that maybe too much?)

New standard followers

New duplex gears and chain

Timing given by offset key rather than vernier

ARP conrod bolts.



Top end, for the moment at least, a 12G940 MG metro head with standard 29.4 exhaust and 35.6 inlet valves, standard rockers, twin HS2 11/4 SU's







Is this engine sound like a good useable build? Would you do anything different? Is there anything I have missed or anything thats not needed? I appriciate the head is nothing special and I could replace in the future with something a bit more special. Are there any tolerances I should ask the engine builders to build too? For instance I am best building it to slack tolerances or tight ones? (As I need to ring them early next week thats perhaps the most important question!)

Also, what do I do about the rear oil seal? My current engine is pouring oil out and I dont want that with this new one. Am I best having a PeterMay lip seal conversion or is there something else I can ask the machinists to do during the overhaul to make sure the original "system" is going to operate effectivly?



I hate to ask this last question, because I dont really care about figures, but the inevitable, how much power do you think I would be looking at with this build?
C Mason

You don't say what the intended use is - road or competition?

Steel strap with two bolts for the centre main should be fine.
Dave O'Neill 2

Car will be used for autotests, spririted road driving and hillclimbs possibly.
C Mason

The changes id make

Instead of .20 for the pistons ...id do .60+ and make it a 1330cc

The real power is in the head... id have it lightly ported, with the valve metal blobs removed, hardened seats, and bigger/over sized stainless steel wasted racing valves ,(1.41 / 1.15)

Id also have the crank indexed (Cha Ching) but nice, id also have the rods lightened and balanced and the crankshaft lightened, then have the clutch, rods, crank, pistons, crankshaft pully (harmonic balancer also lightened) all balanced

Plus id source a aluminum or stainless rear engine plate


Id also go with a roller tipped 1.5:1 high lift rocker

Scrap the fan, and use an electrical fan

And one last tip... install gapless piston rings (agian CHA ching), but that would probably cure your rear oil seal leak

And just a throw in... a comitic steel head gasket

Prop

Prop and the Blackhole Midget

I wouldn't bore a linered block past +20. If the liner is factory fitted (they did this to blocks that didn't pass quality inspection) it is very thin. Gold seal and Silver seal engines may also be fitted with liners. Conversely, the liners were a lot harder than the block material so potentially last longer.

Rob
Rob aka MG Moneypit

Just to add my tuppence-worth: I didn't strap the centre main on my 1360. I'm using an RE130T cam (about 276 degree duration) and won't be revving much beyond 6500 rpm. As Prop says, the head is a big factor in power production, so I'd put the money saved from the machining towards head mods.
I'm running a conservative compression of 10.2:1 on a modified head using 1.4" inlets, but I'm only using the standard stud arrangement. I bought the Mini Spares upgraded ones along with a good quality gasket and followed the torque instructions.
I balanced the reciprocationg parts myself using an accurate balance. I did the rods end-for-end, and mixed and matched the pistons with gudgeon pins to allow for the least amount of metal removal. I was lucky with the crank; it came with a spare motor and was already balanced. I used ARP big end bolts. The flywheel was a lightened one as a consequence of fitting a five speed and 7.5" clutch.
Whilst working on the block I had a rear main seal from Keith Calver fitted. I believe its slightly different in design to others on offer. The rear main cap is machined flush though, so there isn't an easy way back. It was straightforward to install and has proven good so far. You need to make sure that the area of the flywheel that the lip of the seal bears against is smooth.

The motor was rather rough at the bottom end initially but pulled well further up the range. In fact I questioned myself with regard to cam choice. The 1.5:1 roller rockers are a bit noisy as well.
I had the motor set up by Pete Burgess and he saved the day. Its a lot better now and ok in traffic or in motorway queues. 2000-6500 is really strong. The tick over is still quite lumpy though. Mapped ignition is next on the list, followed by efi. The fiddling about is part of the fun for me.
I probably wouldn't want to commute with it every day, being spoilt with 'flappy paddles' and a good stereo in my company car, but the traffic light grand prix is fun, and early morning blasts around the Lake District are great. Not having driven a 'real' car for a long time, I just need to learn how to drive it properly.

Regards, Mark
M Crossley

All interesting stuff. Turns out Keith is 7 miles from my parents house. Never heard of him before! So I shall be calling in there and see what he reckons.

Can I ask how did you get the compression ratio? Do you just do it on shaving the head alone, or is it a factor of pistons used and/or having the block skimmed?
C Mason

When I built my motor, I chose the pistons early on to give the capacity I wanted. Standard stroke and 72mm bore gives 1360, but with a slight offset still allows for another rebore at a later date if needed.
A larger capacity motor is generally more tolerant of a lumpy cam, and a lumpy cam tends to need more static, calculated compression, because the volumetric efficiency isn't as good. It is sometimes difficult to compare like with like so beware of figures being bandied about without knowing the bigger picture.

I did a dummy build of the bottom end, and had the block decked to leave the pistons .010" short of the face. Apparently this helps with gasket reliability on larger overbores. I measured the volume in the the block, which took into account the dish in the pistons, and gave the figure to Keith Calver who modified my head to suit the bottom end and the cam I had chosen.
So the short answer is that the pistons, the block and the head are all factors.
On a different point; as Rob says, if your block has been sleeved, I'd have a word with your machine shop, or the piston manufacturers before you go too far. I've had a sleeved block refused as unsuitable for an overbore in the past.

Please bear in mind that I'm not an expert. I've read a couple of books and tried to apply some of that knowledge to this engine. There are people here who have written some of my reference works, and who really know what they are talking about. Similarly, there are those here who have built and raced for years.

Mark
M Crossley

Rather than line boring the mains tunnel, it can be measured by your machine shop of choice and if ok all the money you save from the expensive line boring process could be better spent on a crank with full size journals

willy
William Revit

I would be extremely wary competing with a linerered block because the liner wall is so thin.

Before you 'spec' the engine decide what class you want to compete in for hillclimbs. If you are doing much autotest you need to pull from low revs. Bear in mind hillclimb folk who 'mean it' will be running max class capacity engines.

Peter
Peter Burgess Tuning

Good comments, been making mean question a lot my original plans, which is what I was after.

Interesting points about linered blocks. Because thats real conflicting advise from other sources ive had. I actually have two blocks with the engine builders. One is a linered midget engine and the other is unlinered marina engine. Both need a Rebore. Ive been advised to just use the midget on a 020 as the liners give a better seal than the block itself. Last longer. Absolutely wont break up or crack at just a 020 Rebore (infact theyd go 030) and that alot of racers on the mini scene actually use liners on engine builds as part of the course now...

I had just taken that all on board and decided not to use the marina block...
C Mason

So you won't use the stiffer marina block in a compitition engine....
Onno K

Pete Ottewell, has a Marina engine that he'd like to exchange for a Spridget engine. Both need a rebuild. Swap? Then you'd have two the same to play with, and Pete would get back the originality he wants.
Lawrence Slater

Why do you think the marina block is stiffer? Thats the ital a+ block..... although i guess you could say the arch in the webbing is missing and it uses a larger flange, but id think that was neglible?

I have a few midget engines 1275 available. Quite happy to swop/ sell.
C Mason

We had a race Midget engine disintegrate at +20 on liners. Have not gone near linered A series blocks for Comp eversince! Don't forget the bit about maximising the engine size for comp.
I also feel the Midget rods and cranks are much better quality than the A+ ones. The bypass style heads are certainly better cast than A plus and have more meat to skim off to get small ccs on race engines.

Peter
Peter Burgess Tuning

This thread was discussed between 25/06/2015 and 28/06/2015

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