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Measuring Bore To Piston Clearance With Paper?

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#1 TGaffney OFFLINE  

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Posted November 10, 2012 - 08:22 AM

I recently aquired a mid-eighties vintage Kohler K161 that was mostly disassembled by the previous owner (my deceased father) after the connecting rod had broken. The engine came along with a new rod, new standard rings, and a new gasket set. Before I put it all back together I thought I would have a machinist look at it.

I took the block and piston to a machine shop yesterday and was advised to rebore with a .020 piston (has a standard piston in it). The machnist took some measurements and looked at the spec sheet I brought and he came up with a measurement piston to bore measurement difference of I think about .010. We didn't discuss what the clearance tolerance in the spec sheet. He also measured the crankshaft according to the spec sheet I brought it was okay.

When I got home I got to thinking about the cost of a piston/rings/reboring and I decided to do a low tech measurement of the bore to piston clearance and I'd like to get some feedback to see if this makes sense!

The Kohler manual says the clearance on a new piston should be .007 /.010 which I'm gueesing means from .007 to .010.
I used a "feeler guage" of 20# printer paper which is approximately .004 thick, and when wrapped/sleeved around the piston is .008 thick. With the paper sleeve the piston fits snugly in the bore - so it seems to be within tolerance.

Based on this I'm thinking of forgoing the several hundred dollar expense to repiston and rebore - especially since I don't even have a use for this engine at this time.

Any thoughts as to my reasoning would be appreciated!

#2 New.Canadian.DB.Owner OFFLINE  

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Posted November 10, 2012 - 09:01 AM

It may fit snug in places, but that doesn't eliminate the possibility of an "egg shaped" bore (can't remember the technical term right now). It may be wider in the middle that at either end. If so, the middle could be out of tolerance but the top & bottom within tolerance.

#3 DougT ONLINE  


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Posted November 10, 2012 - 09:04 AM

.007-.010 seems a little much for piston to bore clearance. Are you sure that's not ring end gap? I use a new ring and check the end gap as you push it down the bore. Use the bare piston to push it so you keep the ring square. check it at a few spots down the bore and this will tell you how tapered it is too. Usually the bore will be tighter at the bottom.


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Posted November 10, 2012 - 09:53 AM

Rebore and use oversized piston and be done with it!
You put trust in the machinist in the 1st place.......What if he told you everything was OK and you put it back together and it smoked, lost power, low compression, etc?........

#5 TGaffney OFFLINE  

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Posted November 10, 2012 - 10:36 PM

I'm pretty sure the clearances I wrote for the Style A piston are correct. See pages 1.6 and 11.3 in the attachment.

I'll put on the rings and try the test Doug suggested. From what I can observe with the paper test the bore does get tighter as it goes down as the piston snugs right up halfway down.

If the ring test works out (I'll use the feeler guages to measure) I'll probably put it back togehter as is and save the $300 it would cost for something more worthwhile - especially since I don't really have a good application for this engine right now.

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#6 whst400 OFFLINE  

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Posted November 13, 2012 - 04:08 PM

I'll put on the rings and try the test Doug suggested.

What Doug is talking about is placing a new ring, NOT mounted on a piston, inside the bore and measuring the end gap. Taking several measurements at varying depths and comparing those readings will give you a good measurement of whether the bore is tapered, but it is VERY hard to determine an out of round bore this way. It also doesn't take into account whether the ring is in fact the right size. It is not uncommon to have to file the ends of a new ring to get the proper end gap.

I suggest using a dial meter and or telescoping bore measuring devices to get accurate measurements.