Tr4 Lee Posted February 23, 2019 Report Share Posted February 23, 2019 Hi guys can anyone tell me what the numbers on my pistons mean 065 205498 u g thanks Quote Link to post Share on other sites
Chilliman Posted February 23, 2019 Report Share Posted February 23, 2019 The 205498 relates to the original number given to a standard 86mm piston, can't help you with the others... Quote Link to post Share on other sites
Tr4 Lee Posted February 23, 2019 Author Report Share Posted February 23, 2019 Thanks for the info I guess they are standard then Quote Link to post Share on other sites
RogerH Posted February 23, 2019 Report Share Posted February 23, 2019 I'm sure the 'g' is the fit tolerance. Roger Quote Link to post Share on other sites
Charlie D Posted February 25, 2019 Report Share Posted February 25, 2019 Hello, TRs and, I think, Heralds and Spits and GT6’s all had pistons that came in three grades “F”, “G”, and “H”. I think that as the letters went up the diameter increased by a small amount, (one thou?) to take into account discrepancies in bore diameter. Why they did not use A,B and C is a mystery. And why were all the bores not exactly the same? I would think Mick Richards would know more about this sort of thing. Maybe the 065 was a date code. Charlie. Quote Link to post Share on other sites
Motorsport Mickey Posted February 26, 2019 Report Share Posted February 26, 2019 (edited) As to why the fit variation being itemised as F. G or H I have no idea. However the idea that close running components can be machined to be exactly the correct size ( especially 50 years ago) ignores the manner of mass production. As machine tooling produces components engine blocks, liners, con rods, pistons etc It wears, only by small amounts but enough to make a difference to the engine build. Rather than scrapping the different offerings which would increase machining costs by 2/3 thirds “ batch” engineering is used where many stillages ( containers) of machined components would be “ sized” and sorted, allowing a slightly oversize liner to be used with the same oversize piston etc etc. If you.check the workshop manual it gives the optimum piston to bore fit. Don’t get anal about it, the fit will change as the piston and liner ‘beds” into one another and for maximum performance we would deliberately machine the difference to be as much as possible just within limit of it starting to lose compression. For me that means any fit within 5 thou is fine, just don’t expect another lifetime of life 50k ? to be achieved without increased breathing etc. Mick Richards Edited February 26, 2019 by Motorsport Mickey Quote Link to post Share on other sites
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