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Author Topic: Help! Bore clearances and ring gaps for oversize pistons  (Read 352 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).

Remember "Its always in the last place you look" COURSE IT IS YOU STOP LOOKIN THEN!

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 »
CB750 K6-76 1005cc JMR Billet block.
CB750 K2-75 stock looking project started 2017

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.

Offline Joao_f3rr3ira

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Re: Help! Bore clearances and ring gaps for oversize pistons
« Reply #6 on: October 19, 2018, 02:18:35 am »
In regards to the oil rings, my rings are made from 3 pieces (top ring, wave shape ring and bottom ring).
Should I grind all of them to the desired clearance or should I just grind the top and bottom rings and leave the wave shaped one alone?

Thanks

Offline dave500

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Re: Help! Bore clearances and ring gaps for oversize pistons
« Reply #7 on: October 19, 2018, 02:20:41 am »
leave the waved shaped one alone,dont modify it!

Offline Joao_f3rr3ira

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Re: Help! Bore clearances and ring gaps for oversize pistons
« Reply #8 on: October 19, 2018, 02:25:17 am »
leave the waved shaped one alone,dont modify it!

Thanks Dave!

Offline PeWe

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Re: Help! Bore clearances and ring gaps for oversize pistons
« Reply #9 on: October 19, 2018, 10:23:49 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.
One more, thing. Do not mix-up piston-bore clearance with ring gaps. Ring gap is a minimum distance to let ring expand when hot without letting the ends to meet and expand more, not good.
CB750 K6-76 1005cc JMR Billet block.
CB750 K2-75 stock looking project started 2017

Offline bryanj

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Re: Help! Bore clearances and ring gaps for oversize pistons
« Reply #10 on: October 19, 2018, 10:40:30 pm »
In 40+ years i have never had to adjust the gap on HONDA rings
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).

Remember "Its always in the last place you look" COURSE IT IS YOU STOP LOOKIN THEN!

Offline dave500

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Re: Help! Bore clearances and ring gaps for oversize pistons
« Reply #11 on: October 20, 2018, 02:20:10 am »
with a rebore bryanj depending who does it etc youll have to check the gap!its not a Honda bore no more etc,dont fly blind check em!

Offline bryanj

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Re: Help! Bore clearances and ring gaps for oversize pistons
« Reply #12 on: October 21, 2018, 12:13:58 pm »
If genuine Honda rings are used and the gap is small in a rebore the bore is too tight
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).

Remember "Its always in the last place you look" COURSE IT IS YOU STOP LOOKIN THEN!

Offline Joao_f3rr3ira

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Re: Help! Bore clearances and ring gaps for oversize pistons
« Reply #13 on: October 21, 2018, 01:42:18 pm »
with a rebore bryanj depending who does it etc youll have to check the gap!its not a Honda bore no more etc,dont fly blind check em!

I think it's really important to blueprint every part even if it is a brand new one, thats why I'm really concerned to check and leave the appropriate gaps on the rings. I will ask my local machine shop to bore the cylinders and the pistons will be non OEM +0.25mm from CMSNL that come already with rings. I've seen some guys boring their cylinders and place the rings into them as they come out from the box without checking the gaps. I just don't want to end up with a seized piston on a fully rebuilt engine. That why I wanted to know on average what gaps can I leave on the rings.

Thanks

 

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