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Re: GFR piston fitting HELP ! 2 years 5 months ago #6874

  • 72degrees
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Dunno. I can find part numbers (on the motospares site) for CX and Crono pistons as 943757001. The GFR parts list I have states:
953590 1 Pistone comp. Piston comp. Piston group Kolben komp. Piston comp.
1/2/3/4

So there must be a difference of some kind. Given the higher revs it might be a subtle difference in final finishing on ovality and 'taper'. We had a talk by a custom piston manufacture at the local VMCC last month - fascinating.

My seized one had to have the bore re-plated (by Langcourt) so I just got a piston from Uncle Bob and handed the entire engine to Nick at Revs in Halesowen with some pages form the manual.

The chap at PJME used to be on here so they should know what's what.

Worth rebuilding a GFR? Not a lot of them left, but I didn't get that good a price for my #2. Depends on how much work required and what state the cosmetics are in. As is usual with 25 year old bikes I fear a really nice original looking one is always going to be worth more than a restored one with pattern fairing and non standard paint job etc.

To ride though - definitely. I must have been lucky with both mine (apart from the seizure which was down to a cooling system air lock by incorrect refilling by a previous owner). Both have been thrashed round Cadwell track days and there's only one way to ride a GFR - change up at the red line ;)
"Do not adjust your mind, there is a fault in the reality"
1957 175 Sport

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Re: GFR piston fitting HELP ! 2 years 5 months ago #6875

  • Joachim
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as far as i know (what might be not the truth) Luigi has changed several things at the GFR engine.
The bolt in the crankshaft is 1mm bigger.
The conrod is longer.
The bore in the piston for the pin is at a different place
The boss of PJME owns a GFR. If i remember right he has made something japanese fit and is selling that stuff.
Maybe for my spare engine.... sometimes

ciao Joachim

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Re: GFR piston fitting HELP ! 2 years 5 months ago #6876

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Thanks Joachim,

So it looks as if it's got to be a GFR piston or find something else with the same dimensions or can be modified.

The pistons PJME sell are Asso:

"Piston Is Made By The Original Supplier To Gilera Asso Werk

!!Limited Supply!!"

Hence £155. Still cheaper than forged Morini 350 pistons in 62.5 or 63mm and you need two of those ;)

The PJME GFR conrod kits are Japanese.
"Do not adjust your mind, there is a fault in the reality"
1957 175 Sport

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Re: GFR piston fitting HELP ! 2 years 5 months ago #6879

  • Valdeziscoming
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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

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Re: GFR piston fitting HELP ! 2 years 5 months ago #6880

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Very useful info - thanks Leigh. Good to hear an Asso CX one can be made to work well.

Any chance a piston specialist like Omega could re-machine a CX bare piston to meet GFR spec or do they use a completely different casting or forging?

The chap who gave the VMCC talk (he made a set of pistons for the Beast of Turin) might be able to. He will make a piston for anything from drawings or 'pattern' - starting at £400 or so for a one off (which he has done for some very rare and valuable vintage bikes).

I have spare GFR system in shed #2 (David Champion got me a new one years back when he was at Piaggio), but it's not that good (dented where a previous owner or mechanic had used a trolley jack under it to lift the bike for a wheel change!) and I might need it one day ;)
"Do not adjust your mind, there is a fault in the reality"
1957 175 Sport

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Re: GFR piston fitting HELP ! 2 years 5 months ago #6885

  • palban100
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Many thanks for all your help guys.
I'm thinking Leigh's plan with using a Crono piston is a goer, but in a sensible manner - decent piston/bore tolerence and proper running in/warm up. Once I know the motor is good and the rest of the bike is coming along, then I'll splash the cash on one of Bob's GFR OEM jobbies. I'm not sure of the cylinder quality, as the piston currently fitted shows signs of an unwanted piston to bore interface. There's a lot to do on the motor alone, as I was stupid (21 year old) when I tore in down in 2000/2001 and left ports, etc. open to the elements, allowing the crank ends and conrod to get rusty.... you live and learn.

I am also keeping an eye out for a Honda CRM 250, kawasaki KR1S or KDX 220 or TZR 250 2MA for a potential motor swap. The steel frame is easier to manipulate than the Mito/RS's alloy frame.

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