Piston Clearance

[Ozmadman]

Just measured as best I can, firstly with the piston inverted (skirt up, keep it clean!!)into the top of the bore with the piston facing the correct way(ie, front to front). About and inch down the bore I can squeeze a .004" feeler gauge between the piston skirt and bore (fore and aft) and at the side an .008", tried it from other end of the bore,now with the piston the correct way up ( ie, being pushed up from the bottom as if it was in the engine) and the measurements were the same. I worked out from the manual that the difference between the piston skirt and barrel diameters should be just over .006" so it looks like I am probably ok?

Also just to confirm the differences in clearance between the fore and aft and side to side measurements I put some callipers(don't have any verniers) across both fore and aft and side to side parts of the piston skirt and the side to side measurement is definitely smaller. I understand, well I do now, that the piston is oval so are they smaller side to side as in my findings? Also can I de-glaze the bore manually without buying a proper tool?

Thanks paul

[Rangy65]

Hi Alan, i got a new a new piston and rings from F W Thornton, Orleton Lane, Wellington, Telford, TF1 2BG, Shropshire, ENGLAND

Phone: 01952 252892 e mail - enquiries@fwthornton.co.uk

They were very helpful and the piston came the next day. It was £135 including delivery and was complete with rings, gudgeon pin and clips. Compared with genuine honda stuff i thought it was a good price.

There web site says they stock or can get just rings. I suggest you give them a call but expect to pay around £50 a set. Alternatively you could also try Les at Russell Motors.

Cheers
Steve

[iansoady]

I understand, well I do now, that the piston is oval so are they smaller side to side as in my findings?

Thanks paul

Yes.

The only trouble with using feeler gauges is that they don't bend to the radius very easily so can give a false impression. My old copy of The Motorcyclist's Workshop suggests using bits of wire calibrated with a micrometer.

WRT measuring devices, I almost always use a digital caliper (< £10 from Maplins) these days - quicker, easier and just as accurate as the old Moore & Wright mic I've had for decades.

[Ozmadman]

Whilst playing around with the above barrel and piston I thought I would remove the gudgeon pin from the piston, thinking it would be as the manual says "a sliding fit" was it hell!!! once the circlips were out it still wasn't budging. I had to heat the piston up and knock it out!! I have cleaned the piston bosses out with some Solvol Autosol but the gudgeon pin is still very tight, not what I would call a sliding fit, any ideas? Also if the pin is a sliding fit is there rotation with the con rod and piston bosses when the engine is running?

Paul

[SPRIDDLER]

Whilst playing around with the above barrel and piston I thought I would remove the gudgeon pin from the piston, thinking it would be as the manual says "a sliding fit" was it hell!!! once the circlips were out it still wasn't budging. I had to heat the piston up and knock it out!! I have cleaned the piston bosses out with some Solvol Autosol but the gudgeon pin is still very tight, not what I would call a sliding fit, any ideas? Also if the pin is a sliding fit is there rotation with the con rod and piston bosses when the engine is running?

Paul

Paul, 'sliding fit' tends to refer to the pin in the little end; it should just slide through under its own weight.
I've always had to heat pistons with a hot air gun, supporting the piston in a leather gardening glove. I believe that's how it should be. The steel pin doesn't (mustn't) turn in the alloy piston when th'engine's running. That's why the phosphor/bronze little end bush is used.

[Ozmadman]

Thanks Neville, that's exactly what common sense tells me BUT the manual does say "sliding fit in piston and con rod"??? think I will stick to common sense on this one..

Paul

[iansoady]

I've always had to heat the piston on all the engines I've stripped. As Spriddler says it's important that the pin is free in the bush but not in the piston.

[Ozmadman]

I've always had to heat the piston on all the engines I've stripped. As Spriddler says it's important that the pin is free in the bush but not in the piston.

Thank you
Paul