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12G295 Head - What To Do?


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#16 Dusky

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Posted 11 July 2020 - 09:22 AM

Sorry to jump on this thread again after a while but can you not deck the head instead of the block to get the correct CR?

no.

#17 Cooperman

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Posted 11 July 2020 - 09:44 AM

Yes you can, but it is important to have flat top pistons. If you don't and the piston tops are too far down from deck level at TDC you may have to machine too much from the head and break into the oil way.
If a 295 is gas flowed properly there is still a lot to be machined off.
The best thing to do is to first deck the block to bring the pistons right to the deck level and themeasure and calculate how much to remove from the head.

#18 blacktulip

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Posted 11 July 2020 - 11:14 AM

With a 998 +.60 with flat top pistons decked flush, the chambers need to be 25cc per chamber. That will give you 9.5cr which is where you want to be with today's fuel from the pump. You can measure the chamber cc with a piece of flat persex (cut CD case) with two small holes drilled about 1" apart. Lay the perspex over the chamber so the holes both lead into the chamber. Keep the spark plugs in the head. Get a kids Calpol serynge and use to fill the chamber with a fluid through one of the holes. The 2nd hole allows the air to come out (I used water but clean and dry after) and keep a count of the ml the goes in until all of the air has gone. You will clearly see the air bubble disappear. One ml = 1cc of chamber volume. Its a good idea to smear some Vaseline around the head face surrounding the chamber so to seal the perspex to the head.

Edited by blacktulip, 11 July 2020 - 11:21 AM.


#19 Ethel

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Posted 11 July 2020 - 11:15 AM

I had one doing 9.5:1 with dished pistons. But I agree it's far better to raise the CR anywhere else than off the head face.



#20 luismx123

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Posted 11 July 2020 - 03:54 PM

Yes you can, but it is important to have flat top pistons. If you don't and the piston tops are too far down from deck level at TDC you may have to machine too much from the head and break into the oil way.
If a 295 is gas flowed properly there is still a lot to be machined off.
The best thing to do is to first deck the block to bring the pistons right to the deck level and themeasure and calculate how much to remove from the head.

exactly what we did. engine is a flyer!



#21 Turbo Phil

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Posted 11 July 2020 - 06:01 PM

There's a load of stuff on my YouTube channel regarding 295 heads and calculating compression ratio.

 

Phil. 

 


Edited by Turbo Phil, 11 July 2020 - 06:01 PM.


#22 Turbo Phil

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Posted 11 July 2020 - 06:02 PM



#23 Turbo Phil

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Posted 11 July 2020 - 06:04 PM



#24 Spider

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Posted 11 July 2020 - 07:08 PM

Sorry to jump on this thread again after a while but can you not deck the head instead of the block to get the correct CR?

 

Maybe. While I haven't looked through Phil's Videos, I'm sure he would have covered the potential issue for the Oil Feed to the Rocker Gear, and a work around.

 

However, I've found that you get considerably better performance from running as flat a top piston as you can (or can find) and getting that as close to the deck as you dare (even protruding) then doing any final adjustments by skimming the head, than running dished pistons and / or leaving anything between the Crown and the Deck of the block and shaving the head to reduce combustion chamber size to achieve the same CR. By building the engine in this combination, you're increase the final Squish volume of the combustion chamber.






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