I am considering buying some +20 thou oversize cam followers as there is a bit of wear in my cam follower tunnels, but I don't think it will be 20 thous worth.

Therefore I would like to know if the 20 thou increase on the +20 followers is achieved by just machining the combined circumference 20 thou larger, or if the increase is achieved by adding 10 thou to each flat face of the follower.

Logic tells me (logic, what me!!) that if the additional material is on the flat faces that I will be able to rub the faces down to achieve the appropriate size to fit the tunnels, but if the extra size is on the circumference that they may not fit without having the tunnels machined to size.

Hope this makes sense

I would say that the followers would be .020" bigger in dia. & would need the tunnels bored out to take them.
The flats should be in the same position relative to the centre of the followers, to keep the followers in the same positions.
It would be well worth searching out a good dealer who can supply them & machine your barrels to suit.

While you're pricing all this work you could compare the costs toward Jim Schmidt's brass adapters to allow the use of lighter BSA lifters:

http://users.gotsky.com/jimschmidt/nortonrods.html

Another option is Steve Maney brand new lightweight barrels.

http://www.stevemaney.com/products.html

Flo wrote;

I would say that the followers would be .020" bigger in dia. & would need the tunnels bored out to take them.

You were spot on Flo, and it is as I suspected (but hoped would not be the case). I spoke to Nick Hopkins at Andover Norton and it is the circumference that is greater in size, and to fit them the tunnels need boring to size. He is going to send me a drawing to show the reference point / angle of bore of the tunnels for my chosen "borer" to follow.

RennieK wrote;

While you're pricing all this work you could compare the costs toward Jim Schmidt's brass adapters to allow the use of lighter BSA lifters:

I did think about this route as it has the advantage of the lighter riciprocating weight, but as I understand it, the followers have to be radiused and I think that this may alter the valve timing on my PW3.

RennieK wrote;

Another option is Steve Maney brand new lightweight barrels.

I haven't absolutely ruled this out, but it will depend on how my rebored barrels and tunnels come out.

Thanks for the info.

Radiused followers slow down the opening & closing rates of cams. Flat followers open the valve timing quicker. They will still give full lift, but less overlap.

Do you need to go to such a large oversize ?

'Standard' size followers always seem to be a an over-tight fit and need lapping into the bore. I'd reckon that you could get away with that several times before it ceased to be a solution.

Well worth having good-fitting followers as a lot of noise comes from that area if they're worn and rocking back and forth.