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Author Topic: Help! Bore clearances and ring gaps for oversize pistons  (Read 144 times)

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Offline Joao_f3rr3ira

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Help! Bore clearances and ring gaps for oversize pistons
« on: October 06, 2018, 02:56:54 am »
Hello!

I'm starting to assemble my 77 F2 engine, this is the first CB engine I build... The cylinders will have to be bored in order to receive 0.25 oversize pistons. I went through the Honda manual to check what is the best clearance I can get between the piston/cylinders and piston rings and what I see is a range from 0.01-0.04mm and a serviceable limit of 0.7mm for the piston rings. I know that some piston brands give their own clearances but I can't find them for OEM pistons. Can someone tell me what is the optimal clearance that I can use for piston/cylinder and ring gaps? Also, I can still use a 61mm head gasket for the 0.25mm oversize pistons right?

Thanks guys!

Offline bryanj

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Re: Help! Bore clearances and ring gaps for oversize pistons
« Reply #1 on: October 07, 2018, 12:16:25 pm »
Use the same piston clearance and ring gaps as a standard piston/rings and the standard head gasket is OK up to a 1mm overbore
Semi Geriatric ex-Honda mechanic and MOT tester (UK version of annual inspection). Garage full of "projects" mostly 500/4 from pre 73 (no road tax in UK).

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Offline RAF122S

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Re: Help! Bore clearances and ring gaps for oversize pistons
« Reply #2 on: October 07, 2018, 12:58:16 pm »
This kind of information really needs put in a sticky thread along with forged vs cast piston, OEM, and clearances for bore size as well as ring gap if not using OEM std overbore pistons. It would be most useful I think in the projects thread area.
David stuck in the curvy back roads of "Upper East" TN

Offline Joao_f3rr3ira

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Re: Help! Bore clearances and ring gaps for oversize pistons
« Reply #3 on: October 07, 2018, 01:17:07 pm »
Use the same piston clearance and ring gaps as a standard piston/rings and the standard head gasket is OK up to a 1mm overbore

So in your opinion, do you think I should go for a piston to cylinder clearance of 0.02mm and a ring gap of 0.35mm?

Thanks

Offline PeWe

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Re: Help! Bore clearances and ring gaps for oversize pistons
« Reply #4 on: October 08, 2018, 12:22:42 pm »
Not more end gap than needed. When bore will wear, end gap will increase.
Honda manual say 0.2-0.4 mm for top and 2:nd rings
0.1-03 mm for oil ring.
I guess the oil ring the manual describe is the first version with 1 piece ring. Later was 3 piece ring.
I should go for 0.2 mm if 3 piece.

Honda say 0.025-0.055 mm on Piston-bore clearance. this is total.
Hondas bore spec 61.01-61.02
piston spec  60.965 - 60.985 mm
Tightest bore with biggest piston will give you min clearance. Biggest bore with smallest piston is max clearance.

EDIT: I'll aim to min 0.05 mm on my 849cc setup. (0.0015"/0.038mm is often written to be used on this forum for those pistons)
 I ran 836cc with clearance of 0.2 mm without smoking. Rather high compression too.
« Last Edit: October 11, 2018, 03:07:19 am by PeWe »
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Offline Joao_f3rr3ira

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Re: Help! Bore clearances and ring gaps for oversize pistons
« Reply #5 on: October 09, 2018, 01:30:43 pm »
I didn't want to risk leaving a big clearance because as you said when the bore wears the gap will become bigger. I was thinking more of values between 0.02mm to 0.035mm. I believe my oil ring is a 3 piece one.

 

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