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16 Dec 2020
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Re: GFR piston fitting HELP ! 5 years 6 months ago #4889

  • Valdeziscoming
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Hi Joachim,
Uncle Bob has drawings of a GFR con rod and he confirmed today that it is 106mm between centers, the same as the Crono/CX engines.

This makes me wonder how Gilera increased the compression ratio to 13 to 1 on the GFR, is it with the piston? We know that both engines use a piston from the same cast but that they are ground differently on the sides.

Maybe the GFR head is different, back to Uncle Bob again!!
Leigh

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Re: GFR piston fitting HELP ! 5 years 6 months ago #4895

  • Joachim
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if you check the Crono and the GFR workshop manual you will see that there is a difference.
There is an part describing the selection of a cylinder foot gasket for getting the right distance above the piston.

The big end is the same at both conrods?
I remeber that there is also a differnce.
If i remeber right the GFR one has a bigger diameter (1mm plus?)
But i think that is the difference compared to the rods of the SPs, not to thath from the Crono or CX.

ciao Joachim

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Re: GFR piston fitting HELP ! 5 years 6 months ago #4896

  • Valdeziscoming
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Hi Joachim,
Using various thickness cylinder base gaskets to achieve the correct squish clearance is the same on GFR, Crono/ Apache and SPO1/2, the squish clearance is the same on all those engines. The manual doesn't state what it is but it says get 7.45 mm above the piston.

The Crono series engines had a problem with the big end bearing picking up on the crank pin so the GFR is fitted with a bigger crank pin, as you say 1mm bigger I believe.

The SP01/2 also run 13.1 compression ratio, they use a longer con rod, different barrel height and different piston, the piston pin is in a different place in the piston. These engines use what appears to be the same head as the Crono. I was wondering if the head on the GFR is different? I don't have a GFR head but uncle Bob's trying to find out.

Leigh

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Re: GFR piston fitting HELP ! 5 years 6 months ago #4897

  • Joachim
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Valdeziscoming wrote:

... the squish clearance is the same on all those engines.


???
I will have a look.
I had problems with that at my GFR engine. So in the end just used the only gasket i had ;)
Late ri got hand s on a GFR manual and (as far as i remember) there are other
values printed.

I was wondering if the head on the GFR is different? I don't have a GFR head...


I have no Crono head, but i have a spare GFR head ;)
Should i take some pics and measurements?

ciao Joachim

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Re: GFR piston fitting HELP ! 5 years 6 months ago #4898

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Thanks Joachim,
I just opened my e-mails and I have one from Bob saying that he has found a GFR head in his stock for me to look at. I'll have a look at it next week and let you know if it's any different from the other engines.
Leigh

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Re: GFR piston fitting HELP ! 2 years 5 months ago #6873

  • palban100
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Hi All,

Sorry to revive an old thread, but did you ever get to the bottom of the differences between the GFR and Crono piston, and why the GFR runs a higher comp ratio using the same piston/cylinder dims?


I'm only asking, as I need a replacement piston for the GFR and they are ridiculous costs, whereas the Crono piston kit is half the price, and I'm wondering if it's worth a punt to get the old gal going and run in again. Once I've got all the work done and kinks ironed out, from rebuilding a tempermental italian bike before being stored in bits for 18 years, then I can splash the cash on the real deal..... is it worth it do you think?

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