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Advice On Engine Build

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#1 Coolingthunder


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Posted 22 June 2020 - 11:11 AM

Allright, so back with a new question.

I am in the process of stripping and rebuilding a 998 block.

I bought a 12G295 head, which has had all the work done (i.e. stage 2), including a skim, from a good friend of mine.

As I am approaching the stage where I will be sending the block off to the machine shop for boring and such, I am wondering the following: does the block need decking too?

The block is in very good shape, so I don't expect any issues. I am planning on swapping to flat top pistons in combination with the Cooper head, as that should give me a bit higher CR. Decking the block should help in that area as well, but I have been told that might lead to issues with the tops of the pistons being too high up...

What should I do here?

#2 johnv


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Posted 22 June 2020 - 11:15 AM

It really has to go to the machine shop twice! Get the clean, bores, cam bearings, crankshaft etc done, then you need to 'dry' build it and measure the piston to block clearance to calculate the CR you need then get the block back to the shop for decking if need be.

#3 Turbo Phil

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Posted 22 June 2020 - 01:04 PM

If you’re set on using the head you have, the first thing you should do is measure the current chamber volume. You can then buy some pistons that will give you a suitable compression ratio. If the head’s been massively skimmed you might find flat top pistons aren’t suitable and you might need dished. Or possibly the opposite and that more needs removing from the head to give a better ratio.
Until you have some numbers it’s all just guesswork.
I’ve covered calculating compression ratio, measuring chamber volume and 295 cylinder heads in my YouTube videos, you might find them helpful.



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