Clearance on a coated ProX piston?

[1976RD400C]

I'm going to try a set of these on my bike after using Wisecos. Should I allow anything in the clearance for that coating to wear off? If and when the coating comes off how much smaller will the piston be? I have a real good hone and bore gauge and can get the bore just about within .0001"-.0002". I'm thinking set the clearance at .0018" measuring the piston with the coating on it???  Thanks

[DesmoDrew]

Lightly scuff through the coating at the front and the back of the piston skirt. That is where you can measure your piston for clearance. The coating is sacrificial and should not factor into your final clearance measurement.

[SoCal250]

Yup, as Drew said. Here's an explanation from Chuck on the topic https://www.2strokeworld.net/forum/index.php?topic=1681.0
You'll need to trim the tab too of course for an AC engine

[1976RD400C]

Thanks guys. Now I remember Chuck talked about this. I took some steel wool and cleaned the coating off a little spot on each side and I'm coming up with about .0005" smaller diameter, I'll check that again because Chuck came up with a bit more. My Lisle hone seems capable of getting the cylinder spot on. It just seems a bit sketchy having .001"ish clearance when assembled with a coated piston. The Wisecos I took out had collapsed skirts and the cylinder had a little wear, the clearance measured .007". They were hanging in there, but the ringing sound was kind of ear piercing. I hope the cast piston with tighter clearance will help the motor inhale better. The crank was spreading apart at the big end pin on the rotor side even with the light HPI rotor. This isn't the first time I've had this happen, with other cranks. It moved about 1mm. When I pressed it back, it seems like a nice tight fit, so I don't know. It wasn't out of true when I check that.  The bearings are all real good. There isn't that many miles on it but it is a hot rod. My old TDR single petal reeds seem tired.  There's a very visible gap when they should be closed, ordered new ones.

[m in sc]

hate to say it but a tiny tig weld on the pin will keep it from spreading. one that easy to remove later.  yeah. .007 is a lot of room to dance.   

[Frank B]

Yup, as Drew said. Here's an explanation from Chuck on the topic https://www.2strokeworld.net/forum/index.php?topic=1681.0
You'll need to trim the tab too of course for an AC engine

Like this

[1976RD400C]

Oh my goodness gracious! Sure am glad I torn this down for inspection. I saw how the lower edge of the intake side skirt was wearing where you cut the tang off on my Wisecos, that I am replacing, and was making a plan to radius that edge on the new ProX I'm installing.


Then I flipped it over and see this horrendous crack going right up the middle of the shirt between the two holes. The other piston had a crack forming also.



Having it wear to the point of .007" clearance can be troublesome.   

[sav0r]

'

You got some work out of those.

[Plasticman]

Is it a crack or a casting mark?

[irk_miller]

Is it a crack or a casting mark?

That what I see.  Flashing from the seam of the mold. 

[1976RD400C]

I didn't think it was a crack at first but then...   I think I should have smoothed that area where the tang was removed better. Looks like another one is forming up to the left. Big intake ports and .007" clearance made the skirt bang away on the bottom of the port is what I'm going with as the cause. The ProXs should be good to go in a couple days.

[SoCal250]

It almost looks like there's a small impact mark on the outer edge of the skirt right where the crack begins.
After trimming the tab and smoothing out the area, the skirt should be radiused slightly (chamfered) on the outer edge similar to the OE machined finish

[Striker1423]

Hmmm.... *Looks over at my bike with interest*.

I think that could be some of the noise I'm hearing...

[1976RD400C]

Should be good to go tomorrow 

[1976RD400C]

Kinda had a freak out. Once it got hot, I heard a noise on the right side. Pull the pipes off and took a look in there. All looked good. That coating on the piston is coming off and making a paste like Never Seize grease. Looks like it is in the hone marks too. There is a puddle of oil/piston coating in the exhaust port. Put it back together and started again. As it got hot here comes the noise. I could feel it touching the head. Then I thought about it, is the piston hitting the head gasket with 1.25mm oversize pistons? I remember something about this could happen with pistons over +1.00mm. Took the head off and that sure looks like what is going on. The hole in the gasket is smaller than the bore. Between new pistons, and having the bottom gasket surface of my cylinders resurfaced flat at one time (.004"), there is no clearance. You think taking a die grinder and carefully making the head gasket hole bigger is the way to fix them?