11 replies to this topic

[nobbycameron]

hello, as in the title realy, is it possible to machine a bigger dish into a 1275 A+ pistons to lower comp ratio? i have the higher compression A+ which i think is 9.?:1

if not then are there any pistons availible which i can do this to?
thanks

[smorg]

hello, as in the title realy, is it possible to machine a bigger dish into a 1275 A+ pistons to lower comp ratio? i have the higher compression A+ which i think is 9.?:1

if not then are there any pistons availible which i can do this to?
thanks

well wouldent it make the metal of the piston alot thinner making it more suseptable to a hole melting through

[Jordie]

You can, but id suggest you would be better having new pistons with a dish allready in. Omegas or something?

Either that or have the head machined with bigger chambers to lower the CR

[nobbycameron]

yeah that is what i am thinking, just wondering what the options are for lowering comp ratio...

[Pooky]

Decompression plate?

[nobbycameron]

trying to avoid a decompression plate because they seem like a bit of a bodge, i may just get the chambers enlarged.

[Jordie]

trying to avoid a decompression plate because they seem like a bit of a bodge, i may just get the chambers enlarged.

Very sensible

[Cooper-202S]

If you engine numer starts 12H then it should be 10.1-1 CR, 12A then its 9.75-1 if its been rebored then it will be slightly off these numbers.
J Cooper conversions after 1992 run 10.5-1 but will be marked 12A you will need to check how close the piston comes to the top of the block to tell.
For low boost upto 7 psi the you could use a standard piston with an enlarged head, but the piston would have a limited life span.
The standard pistons such as the 21251 and 21253 arent suitable for re machining, I cant speak for others.
If you are running a serious boost or long life forced induction build then you will need a set of Omegas with a bigger dish
to drop the Compression ratio.
It would also be worth you talking to Hypertec in Australia or the FW Thornton in Shrewsbury to seeif JP make something suitable, as the Omegas are a tad on the expensive side, Avonbar would also be able to advice you.

Good luck on your journey.

Edited by Cooper-202S, 05 June 2009 - 09:47 PM.

[Jordie]

If you do get omegas, dont order from avonbar. They tell you 2 days delivery, but they themselves order the pistons from MED which state 2 days delivery to avonbar.

Go direct to MED if you want Omega pistons. Cheaper and quicker service.

[mini13]

Its perfectly feasable to machine the pistons out, but i would be a bit wary of doing t to std a+ pistons. I know the turbo pistons can be taken out to 15ish cc pretty safely, and i am currently running some 73.5mm powermaxes that i have taken out to 17cc (from 10).

the trick i use for machining them is to hold th paiston against the chuck on a lathe using a bar through the gudgen pin hole with a peice of threading goingthrough to the back of the lathe and held there on a bush. then just take the dish out squarer with a tool with a generous radius on it.

[wolfie]

you can dish N/A and turbo pistons, i run dished AE pistons in my turbo theres a few on turbo minis that have done the same

[1987_ParkLane]

Get the chambers lowered in your head and get some nice Omega's from MED. Like you said, don't go for the decomp plate as its a bodge.