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[Steve T]

Oops missed a pic

[Bullet Whisperer]

Hi Steve. You say you have done 955 miles on it already, since buying it. If something nasty was going to happen it would have probably done it by now. I rebuilt an engine after a piston broke up, I flushed the crankcases out with a petrol / diesel mix, then tipped the whole bike upside down [top end off, also battery] and blew it all out with an air line. It is tuned, gets a hard life from its' current owner and has racked up many thousands of miles since and even marshalled in an enduro. If those bits of 'piston' were in the actual crank area of the crankcase, I would expect immediate scavenging and wet sumping troubles, as they would get sucked towards the small opening to the scavenge oilway below the crank and block it. Put the top end back together, change the oil and you could probably get plenty more useful life out of it. Or, rebuild the lot, at much more cost, never knowing whether it was necessary or not. Winter is approaching - there will be plenty of time for a full rebuild then, IF it needs one. Cheers, Paul.

[Tim NZ]

Your piston is fornicated. It is an early made JP item that has 'ring seized' (which they were prone to do) and in turn the blow-by wiped out the trust face. It predates the revised spec piston that they have been supplying for over 7 years, so I am wondering how you or the previous owner came by it?


In Australia the newer 'improved design spec' pistons are colloquially called 'Busby Spec'.
The latter version of the JP has a 0.004" smaller top-land diameter, an additional 0.0005" taper on the skirt, a slight chamfer on the outer edge of the skirt to force extra oil between the bore and skirt, and much enlarged Oil feed drillings to the gudgeon, and slightly looser (sliding) fit pin.


Typical time frame for death of main bearings (and crankshaft) from the disintergration of the OEM piston is approx 5000 miles.


IF the motor repair is left until the timing side bearing is noticeable audible, then the resultant crank-shaft 'flex' will cause the crankpin eyes in the flywheels to fret oval, the pin so have suffered visible localised stress loadings (Blued) on the shoulder and the shaft will be uneconomical to repair.


Reuse of a crankpin which has badly fretted will lead to it snapping across the oil hole drilling.

[Tim NZ]

The Aussie JP piston is a 'traditional' T slot, Split-skirt item designed for touring and with low mechanical sound levels; its is intended for modest street use. Run it at sustained high speed (Thrash it, Race it) and over time the front of the skirt has been known to break off. As is a common failing with many other similar T slot designed pistons.


If you ride your Bullet like it was stolen, there are stronger and lighter (forged) pistons available from our Hosts.


One big Plus with the JP piston for the average owner is that they will finish a One - Off to suit any nominal bore diameter, and as such there is never any need to rebore the barrel. Simply have the bore parallel honed and order a piston to suit...