Hepolite Pistons (£83 + VAT type) pin fit

Now't wrong with sliding fit gudgeon pins....usually a sign of quality.

What you have to realise is that when any piston heats up the fit becomes very loose at least a thou or so. The clearance difference between a cold sliding fit and warm sliding fit is minute so this tiny amount is lost or overshadowed when things are heated to hundreds of degrees Celsius when the engine is running and the fit becomes extremely slack. The advantage is that you have the luxury of being able to fit the pistons so much more easily too.

So what you have is common practice nowadays made possible by precision engineering so relax.

Les

The gudgeon pin should be a snug fit in the piston and not turn when running. All movement should be done in the small end bush.

If not, then the piston would wear out. The hole is not a bearing.

Check that the piston is not a shitty chineese job.

It only takes a hot air gun or boiling water on a rag to get the gudgeon pin moving on the original pistons so they are going to start spinning anyway regardless of these new type pistons being a sliding fit cold. Can't remember ever coming across any worn pin holes in a piston and I have changed a few over the years and all loosened off the pin below 100C.

Current Hepolites are Taiwanese made by JCC and are excellent pistons, their rings aren't too good hence in these Heoplites they use Hastings rings.

Apart from what I and John Holmes has said above;think about it this way: Do you really think piston makers would take the trouble and expense to ream the piston to get a perfect cold sliding fit when all they have do is make the hole, needing less precision, and simply make it too small so you have to heat it up and bash in the gudgeon pin?...It would be so easy for them to do so if this was better.

Further, I have spoken to expert precision Piston makers where they actually ream the piston by hand to achieve this type of fit.

Les

PS: As an experiment to demonstrate the minute amount of difference...pop one of the pistons in the freezer for 5 minutes probably the temp is minus 10 or so...then try to slide in the pin by hand...bet it is very tight or impossible. This is just a 30 deg change...thus demonstrating how the fit changes with so little temperature change...Now imagine how slack the fit will become when you heat the piston from 20 deg up to 250 deg!

Taking this one step further, pop your old tight fitting pin piston in the oven set at 250C...take it out (with glove of course) and see how slack the pin is....OK to be fair the pin has to be left in although in practice in the engine the pin will actually be running slightly cooler than the piston. Turn the piston sideways and I bet the pin falls straight out on its own showing it is so loose and indeed free to turn when operating.

Les

PS...If you don't mind, when you have phoned Les at Norvil tomorrow and heard what he says, could you post what he has said please...I'm sure many members would like to know...Thanks....Les

Please correct me if I am wrong but, I seem to remember from an old physics class that the ID of a circlular item (hole in piston) will reduce when heated. Conversely, the OD of a rod like device (piston pin) will increase when heated.

So.... If when heated, the hole in the piston closes up and the pin grows, will not that make the pin tight in the bosses?

Mike

The steel pin does expand but the alloy piston and the pin hole increases its size faster due to its higher coefficient of expansion. Your physics teacher was telling porkies.

That being said when removing steel bearings from an alloy crankcase if you only heat the alloy crankcase just around the bearing then the cold alloy surrounding it does not expand and will force the hotter section to expand inwards gripping the bearing more hence why its best to heat the whole lot in the oven.

All of the Pistons (wiseco) in my race bikes have pins which slide in without heat, don't over think problems that aren't there!

As an experiment, I removed a very old Atlas piston from storage, heated it slightly, removed the pin and allowed the piston to cool. The pin and hole were both measured and found to be identical at .6875. With zero clearance the pin and hole are an exact fit which requires a slight heating to allow the pin to slide in and out. Using an adjustable ream a very slight amount of material was removed from the hole in the piston until the pin could just slide in without heating. The measurement of the hole was now a tight .689. I don't think a difference of .0015 would make a difference as some pistons run this way as manufactured.