Fast Eddie
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- Oct 4, 2013
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My understanding was also that the early 920 kits from RGM and Norvil used Ford pistons of some kind, which were heavier than std Norton pistons. And that the larger gudgeon pin was due to this piston choice, rather than being a deliberate design feature of the kit.
Therefore, my understanding is that if you have the larger pin, you have the heavier pistons.
The best thing would be to weigh them though, and weigh any new ones you buy, and ensure they are a) close to the weight of you old ones and b) the same as each other.
However, I just read the very informative technical notes on 920 conversions on RGMs web site. Sadly, these notes are not very favourable to these pistons! Quote:
“The old (and no longer made) 920 piston was adapted from a ford cross-flow engine, it had the disadvantages of a larger gudgeon pin, excessive weight, and too much length both above and below the pin. This piston required extensive alteration before it could be used, and in its non Powermax version had a taste for self destruction. The nominal bore size for this piston at STD size was 3.188”. Thus our piston should be suitable as a replacement for a worn 920. Once arrangements have been made to accommodate the standard size pin and the bore has been honed to 3.190“/3.191”
Here’s a link to the RGM page with a lot more 920 information:
https://www.rgmnorton.co.uk/csi/1245184/f/pdf/rgm_920_kit_instructions.pdf
Therefore, my understanding is that if you have the larger pin, you have the heavier pistons.
The best thing would be to weigh them though, and weigh any new ones you buy, and ensure they are a) close to the weight of you old ones and b) the same as each other.
However, I just read the very informative technical notes on 920 conversions on RGMs web site. Sadly, these notes are not very favourable to these pistons! Quote:
“The old (and no longer made) 920 piston was adapted from a ford cross-flow engine, it had the disadvantages of a larger gudgeon pin, excessive weight, and too much length both above and below the pin. This piston required extensive alteration before it could be used, and in its non Powermax version had a taste for self destruction. The nominal bore size for this piston at STD size was 3.188”. Thus our piston should be suitable as a replacement for a worn 920. Once arrangements have been made to accommodate the standard size pin and the bore has been honed to 3.190“/3.191”
Here’s a link to the RGM page with a lot more 920 information:
https://www.rgmnorton.co.uk/csi/1245184/f/pdf/rgm_920_kit_instructions.pdf