Honda CB500RS

[steveD]

Blimey that's tight!

To my knowledge the skirt is 'Hemispherically' ground and the only part of the piston is the part controlling the rings. That's where I would measure.

[tevie54]

According to the Wiseco instruction sheet the crown of the piston and the ring landing area are smaller than the skirt so it is the skirt where the measurement is taken.
I can only assume the seizure was down to a combination of things, possibly too tight a piston to liner clearance, the piston rocking in the bore due to play in the small end and me giving it too many revs before it was fully run in, it had done 290 miles since the rebore although i would have thought that last point would have caused a ring seizure not a skirt seizure.
Many thanks to all who have contributed to this post.
Steve

[iansoady]

According to the Wiseco instruction sheet the crown of the piston and the ring landing area are smaller than the skirt so it is the skirt where the measurement is taken.

That's been the case on most if not all the pistons I've ever worked on.

[guest564]

I dont think 10.5 to one is excessive compression and with the mild porting job and the XR cam it should be just quick enough but reliable as well.   

That would be very similar to my first race engine which pulled like a train with plenty of top end power. Did you need to have the little end bushed? when my engine was still 500 the only Wiseco piston was from the earlier RFVC engine and had a smaller gudgeon pin.

[tevie54]

No Wiseco listed one for the 1980 XL500 with a .5mm overbore making it 503cc.
To be honest I didn't really notice much difference in outright power after fitting the high comp piston but it did rev up much quicker,
I've fitted an XR400 36mm carb now so hopefully the power will be a bit more progressive after the Mikuni 40mm flatslide which was a bit abrupt.

[tevie54]

Hi all, I've refitted the piston together with the rebuilt crank and new cam chain and new Elextrex generator, I took it out for the MOT and it goes fine, except for the initial rattle on start up, which was kind of the original reason for the rebuild in the first place.
Ive read conflicting reports on the internet about forged pistons and their tendency to to rattle when cold due to the greater clearances but I'm not convinced. Could it be the higher comp piston? My XL250 has a 10.5 compression piston fitted as standard and that's quiet as a mouse on start up. It all goes quiet after a few seconds so am I worrying unnecessary.
I'm considering going back to the standard comp cast piston as if I'm honest I didn't really notice any real difference in power when I fitted it.
Any ideas would be greatly appreciated