[Half-Inch Stud]

Can you imagine having the oil mist system drilled into the rod throws? some expert drilling.

Maybe just drilling the Rod Caps Parting line is practical. Oh hey PMD did that.

[Elarson]

The Boss Bird had scuffed and cracked piston skirts when we were using the recommended piston-to-wall clearance. We tried loosening the clearance (maybe .002" extra?....memory fading) and the problem disappeared.

The Pontiac piston is less massive than a hemi nitro piston so it was growing a lot quicker and binding in the bore.

I'll see if I can find a picture and post.

Edit: don't have pix anymore but the piston is at work in my show-and-tell stash so I can take a picture Monday.
The skirts would get 2 cracks on the side perpendicular to the pin bosses. The pin bosses are heavy and take awhile to heat up. 90 degrees from that, the piston and skirts are thin and heat quicker. So the piston goes oval while it is warming up.

I never could warm up to the idea of piston squirters being necessary since the crank is flinging oil up into the cylinders. But since the OEM's go to the expense on some engines, there must be a benefit. I'm willing to be educated.

Eric

[Elarson]

Originally Posted by Elarson

The Boss Bird had scuffed and cracked piston skirts when we were using the recommended piston-to-wall clearance. We tried loosening the clearance (maybe .002" extra?....memory fading) and the problem disappeared.

The Pontiac piston is less massive than a hemi nitro piston so it was growing a lot quicker and binding in the bore.

I'll see if I can find a picture and post.

Edit: don't have pix anymore but the piston is at work in my show-and-tell stash so I can take a picture Monday.
The skirts would get 2 cracks on the side perpendicular to the pin bosses. The pin bosses are heavy and take awhile to heat up. 90 degrees from that, the piston and skirts are thin and heat quicker. So the piston goes oval while it is warming up.

I never could warm up to the idea of piston squirters being necessary since the crank is flinging oil up into the cylinders. But since the OEM's go to the expense on some engines, there must be a benefit. I'm willing to be educated.

Eric

Here's the pix of the piston that got killed by binding in the bore.

To make a nitro engine survive, the thing has to be super loose.
Piston-to-wall has to be pretty sloppy
Mains need .007+ clearance (when you can grab the crank nose with your hand and make it clunk up and down, you're about right)
Valve guides need to be loose so you don't hang a valve and grenade the whole engine
If the exhaust valve heads are distorted a bit and won't seal after a run, it's ok.....they'll straighten up the next time you pour the coals to it.

Eric

[mgarblik]

Originally Posted by Elarson

Here's the pix of the piston that got killed by binding in the bore.

To make a nitro engine survive, the thing has to be super loose.
Piston-to-wall has to be pretty sloppy
Mains need .007+ clearance (when you can grab the crank nose with your hand and make it clunk up and down, you're about right)
Valve guides need to be loose so you don't hang a valve and grenade the whole engine
If the exhaust valve heads are distorted a bit and won't seal after a run, it's ok.....they'll straighten up the next time you pour the coals to it.

Eric

That skirt damage was at .009"-.0095" piston to wall clearance. Problem went away almost entirely at .0115"-.012" The engine will tell you what it wants to be happy. Just have to look and listen to what it says.