Rebore & Piston Clearance

[Granty]

Interesting one this -- had a rebore done & piston moves freely through the barrel from top down but is just sticking slightly right at the bottom of the barrel -- obviously a high spot somewhere around the inlet port. quick question -- on a standard stroke how far down the barrel does the piston travel and would it be wise to emery cloth around the neck or hone? Comments welcome dudes -- G

[Fast n Furious]

Are the piston rings fitted?

[Scooterdude]

At BDC the piston deck would normally be in line with the lower edge of the two transfer ports. Regarding cleaning up with emery, I wouldent, you've paid someone for a service that should be right. Take it back to them to sort out.

[Adam_Winstone]

I once helped a mate with a rebuild and we checked the ring gap in the bore and also piston skirt to bore, part way up the bore, and they were both 100% right. However, he repeatedly nipped up and I thought it was down to his inability to control his wrist whilst running in, as timing, jetting, compression, fuel flow, etc. were all fine. In the end I stripped it again to check and found that the piston was still fine in the bore, however, I then went to push the piston out of the top of the barrel and it locked up on the skirt! Turns out that the rebore was off line and conical!!!! Done by an a many decade established UK dealer.

As we didn't have time to send it back, a 3-leg hone was used to increase clearance at the top of bore, not much we could do about slight alignment issue. This barrel then ran faultlessly 2-up to Milan and back in 97.

[Granty]

I once helped a mate with a rebuild and we checked the ring gap in the bore and also piston skirt to bore, part way up the bore, and they were both 100% right. However, he repeatedly nipped up and I thought it was down to his inability to control his wrist whilst running in, as timing, jetting, compression, fuel flow, etc. were all fine. In the end I stripped it again to check and found that the piston was still fine in the bore, however, I then went to push the piston out of the top of the barrel and it locked up on the skirt! Turns out that the rebore was off line and conical!!!! Done by an a many decade established UK dealer.

As we didn't have time to send it back, a 3-leg hone was used to increase clearance at the top of bore, not much we could do about slight alignment issue. This barrel then ran faultlessly 2-up to Milan and back in 97.

This is pretty accurate to what I am finding -- at the top of the barrel (head end) no issues at all, piston slides free & can turn full circle in the barrel. Push the piston into the barrel 1/2 way the same -- move the piston into the neck of the Barrel (crank end) it's not 100% free turning -- it's like the rebore is slightly conical. I wonder if this very slight resistance has bee enough to screw my standard Sil Crank at the Big end? Ah well at least I'm not just making it up, further hone required - G

[Granty]

Are the piston rings fitted?

No Buddy -- this is with no rings -- G

[Fast n Furious]

Bad re-bores do happen, especially from gear that is old an not meticulously maintained.
Good luck on your fettling.