I recently rebuilt my 3120 that I purchased used. Piston had evidence of a four-corner partial seizure, with piston transfer on the cylinder, mostly on the exhaust side. After carefully removing the transfer with acid (yes, this works fine if you take it slow), I smoothed all the ports (some felt a bit rough even though the cylinder was OEM but it was an AB-sized cylinder with an AB-sized piston) and installed a new OEM piston with OEM rings (lazer-etched 'KO B8', I'm guessing Komarov rings). I was hesitant on using these rings as their gaps were quite large at 0.44mm and 0.45mm (for a 60mm bore). For reference, the gap should have been closer to 0.24mm if using the rule of thumb 0.1mm gap per 25mm bore (0.004"gap/1.0"bore).
Before installing the piston, I did a quick cylinder 'honing' with a pad of 3M scotch-brite on a cloth buffing wheel (slightly oversized to the bore) attached to a drill on low rpm, moving it up and down the cylinder about five times. The result was barely noticeable cross-hatching in the nikisil. After that, I thoroughly cleaned both cylinder and piston in hot water and soap. The cylinder was lightly-coated in 2T oil during assembly, as were all the bearings. Just documenting these steps so the typical naysayers here don't have much to discredit this top rebuild.
After only approximately 1.5 hours of varying work, I took off the exhaust to have a peek. I saw some very light scratches on the intake side of the cylinder. After taking off the cylinder, I found some very light scratches on the exhaust side as well. Piston skirts and rings appeared to be fine.
I took off the rings from the piston and refit them into the cylinder to recheck the end-gaps. After only 1.5 hrs of use, both rings gaps had grown to 0.635mm. This measurement is beyond the suggested replacement-time gap of 0.6mm = 0.25mm gap per 25mm bore. Close examination of both rings show that there is still the circumferential ridges on the outside of the rings, except nearest to the ends where it has more wear, but the ridges are still slightly evident there. There appears to be very slight rounding on the circumferential edges. The edges on the NIB OEM rings on my standby piston look far sharper. However, those standby rings have the same exact gap as the now slightly-used rings had when initially installed.
The cylinder looks great except for the very slight scratches above the inlet and exhaust ports.
The engine seemed to run very strong, but so much ring-wear is concerning.
For more reference, I'm using 100LL fuel at 25:1 ratio using Maxima K2 oil (full synthetic for high-heat engines).
A friend sold me an extra OEM cylinder and piston set a while ago, but they have more wear on them than what I am using now. But the rings look to be non OEM compared to what came in both of my new OEM pistons. They're stamped with 'SP' and they're finish appears like those of Caber F-cast rings -- shiny rather than black chromate like the OEMs. The rings had been sitting in the piston ring grooves for who-knows-how-long, and the top ring appears to have some transfer on it's bottom surface, so not too thrilled about using these, even though the outside face appears almost new, circumferential ridges hardly worn at all and very even in appearance. They're obviously made of a much harder cast material.
The big question is, where can I find real, official Caber rings, and should I use their softer cast rings, or go with the F-cast, as that is what seems most easily found. However, only the skeptical dealers who sell the chinesium saws have any 'Caber' stock. Anyone know of reputable dealers having Cabers in 60mm x 1.5mm(t) x 2.5mm(w)?
Thanks for your helpful and constructive comments.
-doug
Before installing the piston, I did a quick cylinder 'honing' with a pad of 3M scotch-brite on a cloth buffing wheel (slightly oversized to the bore) attached to a drill on low rpm, moving it up and down the cylinder about five times. The result was barely noticeable cross-hatching in the nikisil. After that, I thoroughly cleaned both cylinder and piston in hot water and soap. The cylinder was lightly-coated in 2T oil during assembly, as were all the bearings. Just documenting these steps so the typical naysayers here don't have much to discredit this top rebuild.
After only approximately 1.5 hours of varying work, I took off the exhaust to have a peek. I saw some very light scratches on the intake side of the cylinder. After taking off the cylinder, I found some very light scratches on the exhaust side as well. Piston skirts and rings appeared to be fine.
I took off the rings from the piston and refit them into the cylinder to recheck the end-gaps. After only 1.5 hrs of use, both rings gaps had grown to 0.635mm. This measurement is beyond the suggested replacement-time gap of 0.6mm = 0.25mm gap per 25mm bore. Close examination of both rings show that there is still the circumferential ridges on the outside of the rings, except nearest to the ends where it has more wear, but the ridges are still slightly evident there. There appears to be very slight rounding on the circumferential edges. The edges on the NIB OEM rings on my standby piston look far sharper. However, those standby rings have the same exact gap as the now slightly-used rings had when initially installed.
The cylinder looks great except for the very slight scratches above the inlet and exhaust ports.
The engine seemed to run very strong, but so much ring-wear is concerning.
For more reference, I'm using 100LL fuel at 25:1 ratio using Maxima K2 oil (full synthetic for high-heat engines).
A friend sold me an extra OEM cylinder and piston set a while ago, but they have more wear on them than what I am using now. But the rings look to be non OEM compared to what came in both of my new OEM pistons. They're stamped with 'SP' and they're finish appears like those of Caber F-cast rings -- shiny rather than black chromate like the OEMs. The rings had been sitting in the piston ring grooves for who-knows-how-long, and the top ring appears to have some transfer on it's bottom surface, so not too thrilled about using these, even though the outside face appears almost new, circumferential ridges hardly worn at all and very even in appearance. They're obviously made of a much harder cast material.
The big question is, where can I find real, official Caber rings, and should I use their softer cast rings, or go with the F-cast, as that is what seems most easily found. However, only the skeptical dealers who sell the chinesium saws have any 'Caber' stock. Anyone know of reputable dealers having Cabers in 60mm x 1.5mm(t) x 2.5mm(w)?
Thanks for your helpful and constructive comments.
-doug