Piston clearance

[Verner Hansen]

Does anyone know the piston bore clearance for the club vh pistons

[david.anderson]

For an iron cylinder VH I use .0065” to .007” for Australian conditions. They will nip up at .006” here but may be ok in a colder climate.
David

[Simon.Gardiner]

For the Club pistons, they ought to be machined to work in the nominal bore size (ie you shouldn't have to look at th ebore size and calculate clearances, the manufacturer should have made the piston so that it works in that bore size)
So for an 81.8mm bore size, +0.040" (as commonly called 40 thou oversize) those are the dimensions to bore to.
(Apologies for mixing metric and imperial but for some reason that's our usual way of doing it...)

SG

[dave.owen]

Hepolite recommend 4-6 thou I believe

[nevhunter]

These days you don't just bore to the Next size as you can't ever put that Metal Back and sleeves weaken the barrel. a lot of barrells were sleeved and reused the original pistons . There's a lot wrong with that idea . Keep the bore as close to standard as you can. A good Hone will true it up or even create some CHOKE if you want it. Smaller at the top as some aero engines are. Then you get a piston to fit your bore size with the clearance you decide on. Be a bit loose IF in doubt, rather than risk a seizure which could require another rebore. JP will make a piston to YOUR requirements and others may too as it's not that hard to do at the point of Manufacture. Nev

[david.anderson]

This piston here was bored at .065” clearance (measured bottom of skirt). It did almost 10,000 miles without a problem. I then had a carburettor come lose on the mounting flanges, the bike ran lean for a short time as it was sucking additional air from behind the flange, obviously got hotter and this is the result. Only a minor nip but not acceptable. However it does show that I was clearly running a minimum clearance. While the piston is a JP, contrary to popular belief they do not expand more than original Hepolites.
In another VH I am running a 7.5 (Omega) club supplied piston from the previous batch of club pistons. It was bored at .007” clearance and is completely successful, with no piston slap. When I researched the Omega pistons I found they were forged and a fairly high expansion piston. Forged pistons use a different alloy to cast pistons. I am uncertain what brand the current club pistons are.
David

[tony.barnett]

For the Club pistons, they ought to be machined to work in the nominal bore size (ie you shouldn't have to look at th ebore size and calculate clearances, the manufacturer should have made the piston so that it works in that bore size)
So for an 81.8mm bore size, +0.040" (as commonly called 40 thou oversize) those are the dimensions to bore to.
(Apologies for mixing metric and imperial but for some reason that's our usual way of doing it...)

SG

Exactly. My understanding is that all pistons are made like this.

[Roger Gwynn]

That is how it should work and does virtually all the time, but best practise is to measure the piston and add the clearance. When we rebore a multi cylinder, the pistons are numbered and matched to individual bores.

[Verner Hansen]

Thanks for all your answers.

[david.anderson]

I have just purchased a replacement for the piston shown above (which has problems apart from the minor nip up). It was my intention to lightly run a hone through the bore which is in first class condition, and to if necessary have the new piston skirt coated with an anti-friction coating, if it was too lose. The opposite is the case. Only .005” clearance. I have had to take over a thou out of the bore with a hone to give me the .0065” clearance that I was after. Both pistons from the same manufacturer purchased about 9 years apart.
The photo below shows the cylinder (which was a new old stock cylinder that I managed to purchase 9 years ago that had a broken top fin), the old std JP piston is shown, as is the new std bore JP which happens to be over a thou bigger at the bottom of the skirt. The new honing in the cylinder to fit the new piston can be seen in the photo.
{A further off topic note is that the cylinder head on this bike was the only alloy head that I owned (apart from a new Budgen head) in which I had not had an exhaust guide come lose while in my ownership. It was lose when removed and now will have an oversize guide fitted, (and on removal I found that it already had an oversize guide in there). The exhaust port will be ceramic coated to hopefully help this problem in the future.}
David