modifying pistons
Turbo Mini Forums

the rebuild of my 998 is going well so far but i have a wee question. im using the standard 998 slipper pistons (AE 22463) but to get the compression down to a reasonable level i need to dish or skim the pistons... now

I have an old hepolite slipper piston 21964 which looks identical to the AE

so out came the hack saw and I chopped the old piston in half, the crown thickness is 5.5mm thick at the thinest part.

how thin in the crown do you think is safe? 4.5mm?

cheers lads

That mini is determined to Kill me!!!!

or would it be a better idea to use one of these and leave the pistons alone?

http://www.minispeed.co.uk/classic/index.c...ct&id=c4146-120

steel shim head gasket 120thou thick im using a 12g940 head with pocketed block :) sorry for all the questions

That mini is determined to Kill me!!!!

i would leave the pistons there not up to the job

get some hypatec pistons from minisport australia cheap as chips and up to the job
mind you the pound is down now big time

the minispeed gasket is a bodge you are better using a 12g295 head or prefer the 940 head with the block pocketed ..... wil-h uses the 940 on his blistering 155bhp 998 mini in modded form togeater with hypatecs

Ive seen some forged pistons with a crown thinner than that...which amazes me it holds together.

No way would I even think about making it thinner.

9.85 @ 145mph
202mph standing mile
speed didn't kill me, but taxation probably will

My first recommendation would be to fit a 12G295 head with larger chambers.

Otherwise fit 20773 dished pistons.

These pistons would be your best bet:

http://www.minisport.com.au/prod23.htm

Edited by Paul S on 16th Jan, 2009.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

my motor was originally running NA with a 12g940 head and pocketed block so a 12g295 isnt an option for me sadly....

but i do have a good set of 20773's

so hypatechs or a set of mega pistons from minispares could be on the cards....

That mini is determined to Kill me!!!!

Minispares Mega pistons are Hypatecs, but buying them from Aus will save about £100.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

Best post I thought will get the following information :

I asked Minisport in Australia if the 998 +120 pistons an withstand 3.75 mm taken of the top.
According to my math, 3.75 should be the difference in deck height between 1098 and 998

Their answer is as follows :
quote :
No I do not think that the crown would have the strength to remove 3.75mm of the top, but it is your call


1098 at -120 will do 1203

Trick question :

How much is to be pocketed when fitting a 12G940 on a small bore engine ?
Will a 060 offset boring clear that ?

Rusty by nature