JensH Posted May 10, 2011 Share Posted May 10, 2011 Hi all,I am rebuilding a small crank 1300 spitfire engine. FD/GE prefix; overbore +20.New pistons from County...Note on pistons says clearance should be 0,05mm (0,0015-0,0020'') My BL manual tells me (standard bore size):Bore: 73,64-73,66piston top: 73,03-73,15piston bottom: 73,59 - 73,61.Gives a clearance of min. 0,49mm top and 0,03 mm bottom.Engine shop, that did the overbore, did the 0,05 mm - but after I have fitting pistons, I do believe, its very "tight".What to go for? Link to comment Share on other sites More sharing options...
akuma.no.spitfire Posted May 10, 2011 Share Posted May 10, 2011 Always go by the piston manufacturer's clearance.Different pistons need different clearance and BL doesn't know anything about after market pistons.What brand of pistons are they?Tight is normally good, but did the the machining shop check again after boring and honing?If you're really doubtful measure the clearance now, while the engine is still unassembled. Link to comment Share on other sites More sharing options...
Nick B. Posted May 10, 2011 Share Posted May 10, 2011 JensH wrote:New pistons from County... CheersNick Link to comment Share on other sites More sharing options...
akuma.no.spitfire Posted May 10, 2011 Share Posted May 10, 2011 Oops. ;D Link to comment Share on other sites More sharing options...
tim_trinda Posted May 10, 2011 Share Posted May 10, 2011 I recently rebuilt the engine in my GT6 using County pistons and there was nothing with the pistons specifying the clearance. The guy that did the machining only usually wants one piston to measure before reboring but when he found out they were County pistons he wanted all six ??) When I went to pick up the block he said that after measuring the pistons they were virtually identical and 'County must have improved there quality control'The pistons came with Grants rings and the destructions specified a ring gap of 2-3thou per inch of bore i.e. 6-9thou Link to comment Share on other sites More sharing options...
JensH Posted May 10, 2011 Author Share Posted May 10, 2011 thanks,What is 6-9thou in mm? :oMachine shop did have the pistons with the block, btw very reputable place. County pistons was/is equal, same weight etc. and generally looking like quality items.But was just amazed about the small gap/clearance. Link to comment Share on other sites More sharing options...
thebrookster Posted May 10, 2011 Share Posted May 10, 2011 0.16 - 0.23 mm, if I have managed to read my verniers correctly!! Link to comment Share on other sites More sharing options...
JensH Posted May 10, 2011 Author Share Posted May 10, 2011 that's a factor 10 from my rebore :B Link to comment Share on other sites More sharing options...
akuma.no.spitfire Posted May 10, 2011 Share Posted May 10, 2011 Tim Trinda's talking about ring gap, not piston to wall clearance.Jens, 0.002"/0.051mm to 0.003"/0.076mm is normal for road engines running cast pistons with bore sizes in the 80-110mm range.Small crank 1300 is 73.6mm so could well run a slightly tighter clearance, like the WSM suggests.Forged pistons require more clearance, typically about 0.0012-0.0015" per inch of bore, but I don't think those County pistons are forged. ;D Link to comment Share on other sites More sharing options...
JensH Posted May 10, 2011 Author Share Posted May 10, 2011 Aha, should read more carefully! Thanks - will go on with the rebuild then.County pistons are not forged. Did have a set of mahle pistons, but was +40 - so will use them, when I do meld down one of the county ones 8) Link to comment Share on other sites More sharing options...
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