This was started in another thread and we were directed to continue it here as it was getting lost.
The discussion is how to modify a stock XS400 to accept a slightly larger piston and ring set that is commonly available. Not for a performance gain so much (though that could be worked into the equation), but just so we have repair parts (since pistions/rings are about impossible to find for the XS400).
As I explained in the other thread, I’m a car guy and just now getting into these bikes. I have technical data sheets for all kinds of cars that lists rod length, piston diameter, piston height, etc, etc, etc. I do not have this information for bikes and am having trouble just Googling such info. That said, here is my thought.
If I am correct, the bore of the XS400 sleeve is 69mm. I do not know how thick the sleeve is, nor do I know if the jug itself has any room to be bored. So without knowing all that information, I’m thinking there are two options.
First, if the sleeve could be slightly bored, bump up the bore to 70mm (depends on how much it can be bored, if any, and what pistons you could possibly use) and find pistons that fit that bore. There are lots of 70mm pistons. Now you’d have to go through all of the pistons that are 70mm and find one that has the same height from the wrist pin center to the top of the piston. Otherwise compression would be nil (if too short) or it’d smack the head (if too tall). The overall length of the piston isn’t a problem. If the skirt were too long and were to hit the crank (very unlikely) it could be easily ground down to fit.
The other route would be to pop out the stock sleeve, bore out the jug (if that is at all possible) and press in another slightly bigger stock sleeve and again use those pistons/rings.
Or… along that same route, but if another parts combo had a sleeve with the same outside diameter as the XS400 but had a larger inside diameter (thinner sleeve… ) then it’d be same result as if you were able to bore out the stock sleeve.
So find the parts that fit the bore and piston wrist pin to piston top height, press the Frankenstein piston onto the stock XS400 crank, and continue as normal. If the piston had a slightly different size wrist pin, it could be bored to accept the new pin/piston combo. No modifications of the head or crank would be needed.
With a search of inside bores, here are some possible candidates:
A 1984-1985 Kawasaki KXT-250 sleeve/piston combo looks like it'd fit 70.00 Bore
70.00mm Bore, for use on Honda®, ATV, Each
Part Number: LAS-FL106
Cylinder Sleeve, Chromoly, 70.00mm Bore, Arctic Cat, Snowmobile, Each
Part Number: LAS-FL1094
Cylinder Sleeve, Chromoly, 70.00mm Bore, Arctic Cat, Snowmobile, Each
Part Number: LAS-FL1095
Cylinder Sleeve, Chromoly, 69.50mm Bore, Ski-Doo, Snowmobile, Each
Part Number: LAS-FL1149
Cylinder Sleeve, Chromoly, 70.00mm Bore, Arctic Cat, Snowmobile, Each
Part Number: LAS-FL1159
Cylinder Sleeve, Chromoly, 69.72mm Bore, Polaris, Snowmobile, Each
Part Number: LAS-FL1189
Cylinder Sleeve, Chromoly, 70.50mm Bore, Ski-Doo, Snowmobile, Each
Part Number: LAS-FL1222
Cylinder Sleeve, Chromoly, 69.50mm Bore, Ski-Doo, Snowmobile, Each
Part Number: LAS-FL1224
Cylinder Sleeve, Chromoly, 69.75mm Bore, Ski-Doo, Snowmobile, Each
Part Number: LAS-FL1225
Cylinder Sleeve, Chromoly, 70.50mm Bore, Yamaha, Snowmobile, Each
Part Number: LAS-FL1226
Cylinder Sleeve, Chromoly, 71.00mm Bore, Arctic Cat, Snowmobile, Each
Part Number: LAS-FL1234
Cylinder Sleeve, Chromoly, 70.50mm Bore, Polaris, Snowmobile, Each
Part Number: LAS-FL1254
Cylinder Sleeve, Chromoly, 70.50mm Bore, Polaris, Snowmobile, Each
Part Number: LAS-FL1261
Cylinder Sleeve, Chromoly, 71.00mm Bore, Arctic Cat, Snowmobile, Each
Part Number: LAS-FL1272
Cylinder Sleeve, Chromoly, 71.00mm Bore, Yamaha, Motorcycle, Each
Part Number: LAS-FL510
Cylinder Sleeve, Powersports, Chromoly, 70.250mm Bore, Each
Part Number: LAS-FL5239
Cylinder Sleeve, Powersports, Chromoly, 70.000mm Bore, Each
Part Number: LAS-FL812
Cylinder Sleeve, Powersports, Chromoly, 70.000mm Bore, Each
Part Number: LAS-FL845B
Cylinder Sleeve, Powersports, Chromoly, 70.000mm Bore, Each
Part Number: LAS-FL88
Cylinder Sleeve, Powersports, Chromoly, 70.000mm Bore, Each
Part Number: LAS-FL942
Cylinder Sleeve, Powersports, Chromoly, 70.000mm Bore, Each
Part Number: LAS-FL949
Cylinder Sleeve, Powersports, Chromoly, 70.000mm Bore, Each
Part Number: LAS-FL988