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Piston Clearance.

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Piston Clearance.

Post  seipgam on Wed Apr 25, 2018 9:40 am

So my Kestrel 13D ran tight today on a long open throttle run. Removed the head and checked the bore (looks OK). It freed up with minimal effort and running again so looks like no harm done.

The piston is a 40 thou oversize (51.016mm) and the bore measures 51.040mm which if my numbers are right, leaves a clearance of 0.024mm.
Is this enough clearance or could this be the reason it ran tight?
The engine was rebuilt as above just prior to my purchase and has done 4000 miles running 25-1 fuel mix the whole time since.

So what clearance should be made on a rebuild?
Would it be advisable to have mine honed out a little more?

Thanks in advance, Geoff.
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Piston clearance

Post  piston 197 on Wed Apr 25, 2018 10:09 am

Rule of thumb for Hepolite/villiers pistons in air cooled engines is .0015" per inch diameter of bore, so should be 50mm ( 1.9685) +040" which equates to 2.0085", so check your piston at bottom of skirt at 90 Deg. to the pin ( effective diameter) and bore should be a minimum of .003" bigger than this ( recommend .004")
the bore size you give 51.040 equates to 2.0095", so is exactly as i would recommend for clearance for a hepolite/Villiers piston
If it has done any dameag or mishapen/picked up on the piston i probably have new old stock ones if you need another ?
JH
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Piston Clearance.

Post  seipgam on Wed Apr 25, 2018 11:18 am

Thanks for the info John.
Some time in the near future I'll lift the barrel and check the measurements and piston as you suggest.

Cheers, Geoff.
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Piston Clearance + Big End problem.

Post  seipgam on Sat Jun 09, 2018 6:56 am

An update to the above issue.

I removed the barrel today and all looks OK with no noticeable piston or bore damage, clearance measures between .003 + .004 thou so as previously suggested that should be fine. So just a bit of a clean up and re assemble.

But looking at the bottom end, when I hold the top of the connecting rod and rotate the flywheel I can feel the big end bearing "rumble" and "grab" a little, that's not good is it? Replacement required maybe?
Only 4000 miles since it was re built by the previous owner so a little disappointing.

The only thing I can think of that may have contributed to the initial problem was a change to a cheap synthetic oil about 200 miles prior (still at 25/1 ratio), any thoughts on this?

Regards, Geoff.
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Big end

Post  piston 197 on Sat Jun 09, 2018 7:20 am

Sounds like the tightening up was the big end/main bearings and not the piston, don't think there is any option, but to replace the big end, but worth checking it is not a main bearing ? these are a few pounds/dollars each and obtainable from any bearing stockist. They should be C3 graded whereas the bigend is going to be considerably more expensive to refurbish.
JH
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Big end

Post  seipgam on Sat Jun 09, 2018 10:40 am

Yes, I fear your summation will be correct John.
There is a guy in our local club that may be able to help me out with a bit of luck.

Thanks again,
Geoff.
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Re: Piston Clearance.

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