You got inside diameter measuring stuff?

OHH! How many miles on it?

 
Rings are matched to actual honed cylinder diameters by measuring the ring gap with the ring set square in the bore using the piston top as the setting tool.  "Diameter" is adjusted by the installer by increasing the gap so it gives the right gap when installed in the bore itself.    

First over top and middle rings can be purchased and gapped down to fit a worn stock bore.  Oil scrapers can be generally reused, but if you buy a first over ring kit you get them anyway .....

You have a stock diameter piston that may be worn or warped.

You say you have a buddy with the tools and knowledge to use them.  To judge the fit of a piston to a bore does not require anything fancier than a set of good feeler gauges.

First, you rough up the bore with a fine hone to get the cross hatch pattern back, then you clean very carefully several times before slipping the old piston into the new looking cleaned bore.

You measure the skirt clearance first as that is where your slap or knock noise comes from.  You do this by checking the same spot on the piston with the piston sitting as it will be installed, then rotated 90 degrees from that position to check your bore for roundness.  Then you check all around the piston sitting in the same position to judge the piston roundness.

You do the same again at the ring area of the piston, understanding that the head runs a lot of extra clearance compared to the skirt as the flat top of the piston expands more than the skirt does.

Do both of these at the top, middle and bottom of the bore stroke, checking for taper.

If it exceeds the Clymers clearance numbers for max service on piston skirt or piston ring area then you are either going to bore it out to first over or have the piston skirt moly coated to build it up to get your correct fitment back (assuming your bore has minimal taper and has good roundness and skirt gap isn't over .003" over spec. which is all moly coating can recover).

Then you measure the ring diameter clearance to the bore to see if it is within wear specs.

Then you measure each ring groove for excess ring clearance (slop of the rings in the grooves).

If any of these three items are out of spec, you need a new piston and rings.


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Shops don't do this level of analysis.  Most shops want to sell you a new first over piston and a bore job done by them, so if you take it to a shop understand you are going to get your pocket picked accordingly.

good stuff OF needs to go into the tech section

I have another buddy with the bore measuring instruments and will try and take a lot of photos of the process tomorrow night.  I hope I can use the stock rings coz that's what I ordered...

Thanks for the tips OldFeller, I did not split the cases.  My knock sounded exactly like the deep pitched knock in one of those cam chain tensioner videos and the plunger was out 0.62 inches when I opened it up, so I am hoping that was the noise.  Everything else is so clean I am don't want to think there is any looseness inside.  We shall see...