Engine build compression issues
Turbo Mini Forums

Following on from this thread:

http://www.turbominis.co.uk/forums/index.php?p=vt&tid=582018

I decided to go for 26cc head chambers (by Turbo Phil) as any larger would lose the shape and cause an inefficient burn. Then, also get the Piston (21253+60) dish increased from 8.4 to around 11cc. If the pistons sat 10 thou below the block deck, then this would be about right for 9:1 compression I believe?

However, I test fitted a piston the other day and the 21253's sit 55-60 thou ABOVE the block deck. Looks like the Morspeed engine I bought years back has had the block skimmed.

Not sure what to do now, as using these pistons would mean skimming them 65 thou, which will reduce the dish volume (not sure how much by though), and then re-dishing the pistons even further to get 11cc.


Any suggestions? I assume skimming 65 thou, and then increasing the dish to 11cc is a bit much for these pistons, and will leave the crown a bit too thin in places?

1994 Mini Mayfair, 1330, Mspd Ph2 cam, Benross turbo head, 9:1, GT1752, TT Intercooler, MegaJolt, BMW M3 leather seats.

You could source another block.

Was common practice for Morspeed to sell a "performance engine" with 21251 pistons in then decking the block the get the CR up.

If something is worth doing, it's worth doing half of.

Correct, morspeed used to deck the blocks and use 21251 Cadburys Chocolate Pistons.

Your going to need a different block

1/4 Mile 14.3secs 96Mph Terminal 10psi of boost.


Fibreglass Parts? - http://www.tdkracing.co.uk/
Split Rims? - http://www.force-racing.co.uk/

short comp height pistons for a stroker, then skim the block more?

What pistons would they be?

Would Minispares mega pistons be any better? I was told they have thicker crowns and would be better for skimming/dishing?

1994 Mini Mayfair, 1330, Mspd Ph2 cam, Benross turbo head, 9:1, GT1752, TT Intercooler, MegaJolt, BMW M3 leather seats.

I'm still in the find a new block camp.

However! Forged omega's are 40thou shorter than 21253's I believe which would leave you 15-20 though proud. You could prod take 20 though off the top of an 18cc forged omega easily enough as the dish is pretty big on them.

If something is worth doing, it's worth doing half of.

cant find the opnes i was thinking of, only expensive forged ones... in which case cheaper to get another block...

http://med-engineering.eazyti.gr/catalogue...s-special-short

http://med-engineering.eazyti.gr/catalogue...istons-020-18cc

But still IMHO cheaper to get a new block and bore it to suit your pistons.

If something is worth doing, it's worth doing half of.

Ok, well I do have another block - my pre-Morspeed engine. Was a smokey old low-oil pressure engine.

So, looks like that may be the best way to go?

It's 1275 at the moment. So, should I go back to +60 again, or would I be better going for +20/+40? Is there much difference in torque/spool time?

Also, should I go for:

21253 machined to 11/12cc (maybe less if I went for smaller bore)
Mega +40 10cc dish pistons?
Or, should I bite the bullet and buy Omega Cast 10cc - still not cheap at £320?

Actually, I've seen the "Short 11cc dish Omegas" (also £320) but they are 1380 so would still need machining. When going 1380 turbo, do you suffer any more headgasket reliability issues?

Edited by ian2000t on 3rd Jul, 2015.

1994 Mini Mayfair, 1330, Mspd Ph2 cam, Benross turbo head, 9:1, GT1752, TT Intercooler, MegaJolt, BMW M3 leather seats.