Piston clearance?

What are your recommendations/opinions for clearance on a new 250cc bore with a forged Wossner piston?
Do you follow the specs on the box of 0.6mm? A cylinder boring company wanted to know what the recommendation was by Husky for the original cast piston before setting it up. I don't know what that spec was or if this is relevant.

The cast pistols require less clearance than the forged pistols.

There is the clearance specs in the tech ref section. On the second page there is piston, honing and boring specs.

The original Husky piston was not cast, the Mahle piston was forged just like the Wossner. Follow the Wossner specs.

Yup, always go with the new piston manufacturer that you are using. Any machinist (engine builder) will know this, if he don't I would personally take it else where. They will bore your cylinder to your new piston with the clearance figured in to the new bore. So take your new piston when you drop off the cylinder . hope this helps.

It depends of if you intend on going ice racing those guys use more clearance. I think the mahle is cast even if it is forged boring the wiseco ones that look very similar to the woosner ones leads to smearing of the piston on the cylinder wall bored to the mahle spec. If you purchace the piston from the boring shop perhaps then the clearance should be on them in my opinion. Is it an air or water cooled cylinder?

air cooled

Mahle pistons are not cast, they are forged just like Wiseco and Wossner. Japanese OEM pistons and Pro-X/Namura/Vertex/Athena are cast.

Ran a Wiseco at .0025 on my 1982 250XC with no problem. Would do the same with a Wossner. Given you live in AZ, don't imagine you will be doing any ice racing.

Intesting, clearly there is more mass in a front to back manner on the sides of the other two. The origional sure has less machining inside. Intresting website, seems motorcycles are small engines to them. Looks like a diesel engine piston is cast.
http://www.mahle.com/mahle/en/produ...ial-vehicles/piston-systems/aluminum-pistons/
The 430 one I had done came back with .004 written on the box I guess the husky shop sent it out to a machinist with that spec it was a mahle. I couldn't find what BigBill was referencing on page 2

In the second page of the tech specs there is a listing on pistons, boring, specs.
The English page won't open. Sorry.

Now the cast piston clearance is tighter than the forged piston clearances.
Are the clearances different again because of the air cooled and water cooled?
The air cooled runs a tad hotter thus more piston expansion.

When measuring a piston diameter I was taught to measure the piston 90 degrees from the wrist pin hole. On the skirt that's the largest size of the piston. It actually tapers towards the ring grooves leaving room for expansion.

Because the molecules are compressed within a forged structure, there are more molecules to expand in thermal expansion as the engine comes up to and maintains operating temperature.

My father had to find that out the hard way when he was using Wiseco pistons in his 1972 Puch 175. The original piston was likely cast as he bored for the Wiseco at the factory value stated for the OEM piston. He seized about 3 or 4 pistons until he found out from our local Husqvarna dealer what he was doing wrong. I do not think that Wiseco was putting the clearance values in the box with the pistons in 1974

Close, in a forging all the molecules are in the same direction like inline so they expand more. Casting has the molecules going in every direction.

I had an auto machine shop bore my 350Chevy. He clearances my forged trw pistols as cast pistons. I had a piston crack when I was in Vermont. The skirt cracked in a vee wedge shape it was captured, rattling but ran I made it home. I couldn't figure it out with 90lbs oil pressure. I took out seven pistons scratching my head. Number 8 came out in two pieces. I worked in a auto machine shop when I was younger. I trust no one now I bore my own engines so I know wherein deep in the woods it's right.

just looking at the wiseco recommendations and they state .004"-.005" clearance for every inch of bore diameter on 2 stroke engines, this relates to ring gap only.

I was just going to ask if there was a rule of thumb for clearance on forged pistols.

The air cooled and water cooled could be different.

.004"-.005" per 1" for ring gap?

I bored a 390cr once to .0045" piston clearance and she was tight. I ran the gear driven adjustable hone and left it at .005+". Open it up too much and we get excessive fin ring/rattle.