Hi Andy, Joachim, Pete,
I’ve just rebuilt a GFR engine and being concerned about the GFR’s reputation for seizures (deserved or not) I did a little research before I decided on what to do for a piston.
Lets start with the fact that the GFR type piston is now obsolete and is not obtainable from Asso (or anyone else), this has been confirmed by Uncle Bob, even he can’t get them.
On to the differences, the first thing to take note of is that a Mr Giulio Biaghien of Asso stated on 14th May 1998 that the ‘GFR piston is different from Crono/CX/Apache pistons in ovality’. From this I’m guessing that because of the extra heat created by the higher revs in a GFR engine the piston is finished to give more clearance around the exhaust port side of the piston.
Mr Biaghien also stated that a ‘CX type piston should not be fitted in a GFR engine as it could seize’. So as far as Asso are concerned don’t do it but as the GFR type piston is obsolete we have no choice but to use a CX type piston do we?
Another very important thing to note is that the GFR runs a higher piston/cylinder clearance than the Crono/CX engine. Therefore, a size ‘A’ CX piston is bigger than a size ‘A’ GFR piston! This might not be a problem with worn parts but if a new CX type piston is being fitted in a re-plated GFR cylinder the piston/cylinder clearance should be finished to at least the minimum specified for the GFR which is 0.050 mm.
This is what I’ve chosen to do with my GFR engine and after two track days (well they were only half days due to other failures and events) so far so good, maybe 0.060 would have been safer, time will tell!
By the way Uncle Bob can still get CX type pistons from Asso but there’s a minimum order required so sometimes he might wait until he has a few ordered before buying a batch. I’m fairly sure the PJME guy gets the Asso pistons he sells via Uncle Bob!
I wouldn’t risk those cheaper pistons personally, the quality of the piston is so important in these highly stressed two strokes that paying the extra for an Asso might be cheaper in the long run, if the intention is keeping and using your bike that is.
The changes to the GFR from the Crono /CX engines beside the piston that I can remember are a lighter crank, timing advanced, auxiliary exhaust ports raised by 1mm, larger cooling ways around exhaust port, larger big end crank pin and bearing, different emulsifier tube, slide and main jet in the carb and revised exhaust pipe.
That reminds me, anyone got a good GFR exhaust pipe for sell? Or a complete bike in bits for that matter?
Leigh