DAICROCKet Posted December 4, 2020 Report Share Posted December 4, 2020 Hi guys, After much deliberation, I managed to purchase a new cylinder kit from molinari in Italy.. on looking to fit, I have found that the piston is a type B of 72.451. I've measured the cylinder and get 72.48/9. On working the tolerance , I get near 0.030mm ... which is around 1 thou.. Will this seize ? Quote Link to comment Share on other sites More sharing options...
jonboy883 Posted December 4, 2020 Report Share Posted December 4, 2020 Possibly the piston, being the smaller metal object will expand more due to heat that the cylinder, which also benefits from its cooling. Id say the tolerances were too close, and you'd need a bit more. Have you contacted John Lampkins for proper advice, they'd know for sure! Quote Link to comment Share on other sites More sharing options...
collyolly Posted December 4, 2020 Report Share Posted December 4, 2020 Not wanting to split hairs but 0.030mm is in fact 0.75 thou inch so I would say much too tight. Quote Link to comment Share on other sites More sharing options...
DAICROCKet Posted December 4, 2020 Author Report Share Posted December 4, 2020 Does anyone know offhand, what a new cylinder measures? I'm getting 72.48 with telescopic guages and calipers... Quote Link to comment Share on other sites More sharing options...
DAICROCKet Posted December 4, 2020 Author Report Share Posted December 4, 2020 1 thou is 0.0254mm so 0.030 is larger ? Quote Link to comment Share on other sites More sharing options...
huski Posted December 4, 2020 Report Share Posted December 4, 2020 Think it's too tight,0.05mm would be better.Is the cylinder or old piston marked A,B,C ? Quote Link to comment Share on other sites More sharing options...
DAICROCKet Posted December 4, 2020 Author Report Share Posted December 4, 2020 Piston is new and marked 72.457. Cylinder is also new which is why I'm slightly worried about the clearance! The seller in Italy states it's paired what ever that means Quote Link to comment Share on other sites More sharing options...
DAICROCKet Posted December 4, 2020 Author Report Share Posted December 4, 2020 The skirt diameter for an A piston is 72.451, so that's why I can't understand how I've got a B piston bud Quote Link to comment Share on other sites More sharing options...
collyolly Posted December 6, 2020 Report Share Posted December 6, 2020 My apologies, you are correct, I got my conversion turned around, I still think it’s too tight though. I have a feeling that 0.001” per inch of cylinder bore has been a guide. Quote Link to comment Share on other sites More sharing options...
suzuki250 Posted December 6, 2020 Report Share Posted December 6, 2020 (edited) On 12/4/2020 at 9:45 AM, DAICROCKet said: Does anyone know offhand, what a new cylinder measures? I'm getting 72.48 with telescopic guages and calipers... Will will never measure it correctly with a Caliper You need a bore gauge and micrometer to get the correct measurement (a mic that measures 0.001mm) Good quality feeler gauges would probably give a better clearance measurement, if you cannot use the above 250cc bores normally need between 0.04 to 0.05mm clearance Edited December 6, 2020 by suzuki250 1 Quote Link to comment Share on other sites More sharing options...
DAICROCKet Posted December 6, 2020 Author Report Share Posted December 6, 2020 Thanks for the info guys... I've read many threads stating 1 thou per inch = 0.0028 for a 72.48mm bore. So If the piston is 72.457mm leaving a tolerance of 0.023mm equating to .0009 of a thou!! So if I put a standard 72.451mm equating to 0.029mm or 0.00114 I'm confused as that would also be tight!! Does anyone know where I can access beta tolerance data? Am I measuring the bore wrong as it is stated as 72.5?? Quote Link to comment Share on other sites More sharing options...
DAICROCKet Posted December 6, 2020 Author Report Share Posted December 6, 2020 I've tried feelers with piston top flush and guess what.. can't get a 1.5 thou down there!! Quote Link to comment Share on other sites More sharing options...
suzuki250 Posted December 6, 2020 Report Share Posted December 6, 2020 (edited) 5 hours ago, DAICROCKet said: Thanks for the info guys... I've read many threads stating 1 thou per inch = 0.0028 for a 72.48mm bore. So If the piston is 72.457mm leaving a tolerance of 0.023mm equating to .0009 of a thou!! So if I put a standard 72.451mm equating to 0.029mm or 0.00114 I'm confused as that would also be tight!! Does anyone know where I can access beta tolerance data? Am I measuring the bore wrong as it is stated as 72.5?? The bore is 72.5mm + or - whatever the tolerance is for the plating, (that's why they have size graded pistons) the clearance depends on the piston material, if its hypereutectic then 0.023 would be fine Measuring it with a caliper you cannot get the accuracy required, therefore if the seller says they are paired, then its probably correct. Build it and ride it Edited December 6, 2020 by suzuki250 Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.