1968 T500 Cobra Pistons

[Bloop2]

Hello.......I'm in the process of putting my Cobra engine back together, have had the cylinders bored out to first .5 oversize. I just realized I have two different pistons. The left piston is the original part number 12120-15700. The right piston had 12110-15700 P/N on the box with a hand written new part number of 12110-31002 which is a superseded part number. The superseded piston is not identical in dimensions, the port window is a different size and the ring location is just a hair lower. I'm assuming I'm going to have to find a newer LH piston to match the RH, or an earlier RH to match the LH.

According to the parts listing at Alpha, the left piston part numbers went from

12120-15700 to
12120-31712 to
12120-31002 to
12103-31870

the RH piston went from
12110-15700 to
12110-31002 to
12103-31830

What is the difference in the newer pistons? Do they still have the closed window on the port? Are there later pistons without the closed window that can be used in a 68 Cobra?

Thanks........Fred

[oldjapanesebikes]

What is the difference in the newer pistons?

Fred - the newest version pistons are the same as are used on the GT750 - so no port windows as seen on the old original style. The part numbers sync'd up with the GT500. And I don't know how critical it is, but the guy that does my cylinder boring likes to have the replacement pistons in hand when doing the job just to be certain of fit/clearances.

[Bloop2]

Thanks Ian, my boring guy does the same thing, wants a set of pistons before he rebores. So, can the later windowless pistons be used in the Cobra, as long as I have a matched part number set? I guess my concern is some of the later pistons had different porting.

Fred

[tz375]

If the barrels were bored to match those pistons, I would modify the window piston to match the other and go with them like that.

As long as the skirts are the same length differences at the bottom of the transfer port should not make much of a difference at normal street riding speeds.

[Bloop2]

Thanks TZ, I measured the length of the opening to the bottom of the skirt and it is the same, it is the width that was off a bit. I'm going to clean them up and use them.

[argo1974]

Stock pistons come with very accurate dimensions and were not divided into tolerance groups like on many other bikes.
In fact, you can have a set of Suzuki cylinders rebored blindly to factory oversize. Personally I never give pistons out of my hands since bore shops tend to add few hundredths mm of clearance for easier running in period. At the cost of engine's quietness as piston clatter starts much earlier. So a "matched" set of barrels/pistons often means 15.000 to 20.000 miles shorter life of top end.

You can't cut out port divider on earlier pistons to match them to later style since they are cast differently. In front and rear area, piston skirt thickness is reduced compared to other areas. With dividers removed, they tend to break (in fact, they broke often even with that divider back then). You should measure cylinder bore, which should be 70.50mm on both barrels, and also the pistons. Newer type pistons (from GT750) run much less cylinder clearance so it's highly probable, they have higher silica content. Depending on the measuring results, you can decide, what way to go - earlier style pistons with more clearance or later style with less clearance.