To bore or not to bore?

I have a question for all you inside mill guys. I have been helping a friend with his 1982 Suzuki GS650 (I know how could I) because he is clueless and thinks I'm some kind of genius since he witnessed part of my carb rebuild, and he is envious of my XJ550 and how it runs-BTW thanks everyone on the forum who gave me great advice!!

Anyway it died on him the other day and although we got it to start, it immediately started sputtering and wouldn't rev up, then it would die. My first thought was to do a compression test. It was pitiful, all four were low, 45, 85, 105, 109. Wet test 60, 147, 144, 149. So right away I am seeing worn rings and possible valve problem with #1. We pulled the head. It seems fine it's holding gas in the combustion chambers, but 1, 2, & 3 did eventually leak after about 2 hours, 4 held indefinitely. The bores all look good except for #1 which has about 12 longitudinal marks (deepest opposite thrust side) that can 'just' be felt with a fingernail. I measured them in about 8 locations and they are very close, within .02mm to original spec and not out of round.

So long story short that's broken rings on #1 right? My question is can the cylinders just be honed and replace all the rings or is he due for a rebore?? We will replace the valves or at the least lap them with new seals-Thoughts and opinions welcome.

 

If youve got the honing gear and the inclination to help him then hone it, do the valves and try it. Outlay would be nominal for that exercise...

 

As far as the head goes you need to look at the valve guides. If the guides are loose you'll kill the new valves in a short time. Air cooled engines have a tendency to wear guides from all the heat. Valves are expensive consider having them machined.
The cylinders need to be pulled and the bores inspected and checked with a bore gauge to check for taper and out of round before and after honeing. Also you are going to have to inspect and measure the pistons. If the pistons are out of spec then you will need to see what's available for replacements.

 

It will all depend on how deep the gouges in the cylinder walls are. If removing enough metal to get them out puts the cylinder out of spec, then you would need to re-bore.

 

Ok that sounds right. I pulled the valves to the extent of the their travel and checked their deflection it seems pretty minimal, I guess I need to get the specs and a gauge to measure the tolerance. He wasn't burning oil before this happened does that indicate the guides are probably ok?

I did check the top 40mm or so of the bores (above the BDC pistons for taper and out of round and they are very close all around within .01mm. The gouges are hard to measure but they seem very slight-if you're running your finger you can barely notice them but a fingernail will find them and they are visible. I guess do the hone and measure again to see if it's still in specs. Thanks

One other question what's the difference between normal, shallow, and deep oil ring grooves? Would the deep compensate for getting near or just out spec on the bore?