72 Spitfire 1300 little end bush to Gudgeon pin clearance

[Pauld]

Hi. 

In the process of rebuilding my spit engine and have pressed in new little end bushes. However, they now need honing to suit the Gudgeon pin. All I can find online and in the Haynes manual is “light push fit at 20degc”. Can anyone provide info on what the measured clearance I should tell the machinist to hone to on the new Gudgeon Pins that came with the pistons. 

Thanks 

Paul

[Bfg]

Paul,   Any decent machine shop will know what is meant by the directive " light push fit at 20˚ " (room temperature).  It's the same as you'd feel when using feeler gauges to set your car's tappets, so 0.010" means just that. ..ie., only just enough for light oil to lubricate.  In practice negative tolerance would be nil, and positive tolerance would be anywhere between two and a few tenths-of-a-thou.  But if in doubt - I'd suggest you just give the machine shop your rods and quote what it says in the book, followed by the commonplace query "what tolerance would you normally work to ?" 

Alternatively,  buy yourself a small three-leg honing kit, those used for smaller master cylinders are usually the right size, and very inexpensive.  Do the honing yourself using a cordless drill.  Add light oil to the honing stones (same as you would when sharpening a chisel on a stone) and keep the drill generally in line with the little end's axis and its speed low ..and you ought to do just fine.  It's not difficult to do as the stones articulate on their legs to keep everything square  ..but of course better to do the honing in small steps until you get the hang of it and make note of how long that particular bearing material takes to cut back.  

Pete   

p.s. the gudgeon pin is usually a just little tighter in the piston than in the little end, but again we're only talking of a couple tenth-of-a-thou difference.