Here’s the deal: I spent a whole whack of cash last summer getting my 89” stroker Evo rebuilt. Crank trued and balanced. Bored over .020”, new S&S pistons, new Hi-4N, new oil pump and a few other odds and ends. Done by a reputable v-twin shop in my area that a lot of people recommend. Ran great for the break-in period, which I babied it for. Changed the oil after 800km and then carried on. About a month ago it started running hot and the idle suddenly dropped and became lumpy. I checked all the simple stuff like timing and carb tweaks but I couldn’t get it to run right. Noticeable loss of power.
So I take it back to the shop and they are telling me that there are vertical lines on the cylinder walls and my pistons look damaged. They’re claiming this is due to getting on the bike cold and ramming the throttle without it being warmed up properly. I know in my heart of hearts that I have never just got on the damn thing and just started blasting away. Sometimes it’s only been a few minutes before I start riding, but I certainly don’t hit anything over half throttle before it’s at operating temp.

I guess my question is; assuming that I’m telling the truth, what is the likely cause of scored cylinder walls on a fresh rebuild? Also, how the would the piston be damaged? They haven’t blown apart the motor, they’re just looking through the plug hole so they can only see the top of it. I’m going to call them back tomorrow, I just want as much info going into the conversation as I can.

Cheers folks!

 

Have them do a compression test and a leak down test before pulling anything apart...

 

Possible your bike had a partial seizure at the time it started running hot. That would account for the loss of power and the damage to bore and pistons. Cause? Don't know without looking, but seizing is usually down to lubrication failure. As said a compression test will confirm ring / bore damage and go from there.

 

Perhaps they mis-diagnosed the reason, particularly if the engine has yet to be pulled apart?
Only a thought.
Scott

 

A bad compression test could also be the result of valves not seating in the head, right? How can you tell the difference between piston/cylinder issues and head issues? The bike doesn’t seem to blow smoke at all (blue or black) and it doesn’t use oil from what I can tell.

 

Do a Leak-Down Test, that will reveal the problem.

 

Can you elaborate on that? I have a compression tester.

 

My quickest explanation would be to goggle search "engine leak-down test'...........as long as AOC is not involved...
Scott

 

Yes, it could be but, the chances of valves suddenly not seating (unless you over revved and bent a valve stem or two are slim I would say. For me personally I would be looking to do a visual inspection (strip) of the top end to ascertain exactly what has gone **** up.

 

As has been said, the OP should arm himself with accurate compression and leak down test results before returning to the builder. A failure like this might be the result of poor piston to cylinder fitment resulting from a poor bore/hone job or cylinder bores that are not true. The OP should have bores checked with bore gauge and piston to cylinder fitment checked with bore gauge and cylinders in torque plates, probably by someone other than the builder.