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650 pistons

Submitted by peter_bugler on Sat, 19/12/2015
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Heavy Twins

I have just fitted the pistons to the conrods of my 650. The small ends needed heating for the gudgeon pin to fit throughhowever the pistons are loose ie slide in easily and rotate. All items are new and pistons standard bore size. Is this a problem?

I say 'new' however they have run for a few minutes until I proved there was no oil return. This was found to bedue to the oil pump gasket being the wrong way round (I know). Crankcase oil contained finemetal dust. I assumed I hadn't cleaned something properly so stripped and cleaned everything again.

Thanks in advance.

Peter

Submitted by robert_tuck on Sat, 19/12/2015

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Small ends are not supposed to be tight to the pin.This may make removal a problem without straining something. If however the pistons are an easy fit to the pins and they are not too tight and there is enough clearance between the piston bosses and small end to allow the piston to move to centralise freely it will probably come to no harm .Some car engines firmly clamped the small end in the rod.Tolerances have changed and this is not an unknown occurance. I would prefer to have the small ends reamed as they may have worn a little oval in use. I recently PROPERLY cleaned out a 1960 timing cover and was amazed to find a considerable amount of machining swarf from 1960 Bracebridge ST. You could look upon the issue with small ends as good fortune that will allow you to re-size the bushes and correct any ovality. Serendipity??.

Submitted by peter_bugler on Mon, 21/12/2015

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Previously robert_tuck wrote:

Small ends are not supposed to be tight to the pin.This may make removal a problem without straining something. If however the pistons are an easy fit to the pins and they are not too tight and there is enough clearance between the piston bosses and small end to allow the piston to move to centralise freely it will probably come to no harm .Some car engines firmly clamped the small end in the rod.Tolerances have changed and this is not an unknown occurance. I would prefer to have the small ends reamed as they may have worn a little oval in use. I recently PROPERLY cleaned out a 1960 timing cover and was amazed to find a considerable amount of machining swarf from 1960 Bracebridge ST. You could look upon the issue with small ends as good fortune that will allow you to re-size the bushes and correct any ovality. Serendipity??.

Hi Robert

Thanks very much for this advice. As the crankcase is in the frame and I don't have a big enough reamer, I'll slide the barrels on. The small ends free up when hot so may well be OK.

Merry Christmas.

Cheers

Peter

Submitted by les_howard on Mon, 21/12/2015

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I agree with Robert, although it is quite common to have "floating"gudgeon (wrist)pins, having to heat the small end to fit the pin through it is far tootight and I doubt if they will loosen up as the pin in the piston fit is going to become very friction free when the piston heats up...far more than the small end will loosen up, if at all....If you didn't have a reamer to hand I would have used the old pin and coated with very fine metal polish, something like Brasso or even Auto Solvolto lap in the small end and if still very tight on the new pin usecarborundum paste.

Now swarf in the oil is worrying...First check if it is ferrous or aluminium by seeing if it is attracted to a magnet. If it's ferrous it's likely to be wear from newlycross hatched bore....you did glaze bust it didn't you?....if not those new rings are going to have a difficult time trying to bed in.

I'm not sure from your question...but did both holes in the pump get blocked with the gasket wrongly fitted?....If you have run the engine with no oil pressure...it's probably best to start the enginerebuild again as that swarf is likely to be the big-end metal....sorry if you have....Les

 


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