Clearance above piston crown.

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Posts: 184
Joined: December 2nd, 2012, 3:45 pm
Location: Lodz,Poland

Clearance above piston crown.

Post by Piotrek125p » October 30th, 2016, 11:52 am

Hi GC & members,

Last days I studied a lot of GC site and 2010 GC book. One of the most interesting things for my is a GC pistons design. A lot of info I got from this site and book, amazing source.

Unfortunately, I have not found a infos about choosing right clearance abouve piston crown.

I got this GC tables:
CR_JC 2 liter StIII.GIF
CR_JC 2 liter StIII.GIF (24.7 KiB) Viewed 8015 times
NIX piston data.GIF
NIX piston data.GIF (23.03 KiB) Viewed 8015 times
I know that you makes a really detailed pistons project so I think that right clearance above piston crown is also important thing when you doing your projects.

In my engine I want to get a 0,40-0,50 deep clearance above piston crown. I want have it because:

- I will have some metal when I will rebuilding this engine after season or after headgasket failure
- I will have more material in pistons when I will make a dry building op
- I heard that running pistons lower makes a headgasket live easier - it is true ?

GC, could you tell me how looks your way to get a correct clearance above piston crown and what factors are important when you make this decisions.
I saw that you using different clearance from engine to engine.

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Guy Croft
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Location: Bedford, UK

Re: Clearance above piston crown.

Post by Guy Croft » November 3rd, 2016, 5:17 pm

there is no 'correct' for this and I have no back-back dyno data on the effects. There is no data anywhere, only usual internet guesswork here and there that more squish is better.

Some older engines have no squish at all!

It's 100% not practical for me to change my piston designs every single time I do an engine. I could but it would drive me and JE nuts. When I need very importat changes I do it but I don't class squish as such. There are other features that help burn rate without worrying about crown-head distance!

You must have at least 0.5mm piston to head, that I can assert. Less than that and at 7500 rev/min the 2 liter pistons will hit the head...

Beyond that if the pistons are 0.5mm low or whatever I don't care. Nor did Fiat.

Guy Croft, owner

Posts: 184
Joined: December 2nd, 2012, 3:45 pm
Location: Lodz,Poland

Re: Clearance above piston crown.

Post by Piotrek125p » November 3rd, 2016, 6:58 pm

I will go with my Beta 1585 cast piston 0,40-0,50mm below the deck, I want to have some material for dry building in future.

Thanks for your reply dear GC, for me this thread is 100% done.
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