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Author Topic:   Piston skirt trouble
Todd83C
Dirt Full Roller

Total posts: 53
posted September 11, 2005 08:32 PM  
I have a 350 40 over with 150 dome pistons in it. Tore the motor down and found the #1 cyinder wall was scored from the piston skirt on the top of block or the side that faces the intake. The rod bearing has a spot about a 1/4 inch long were it wore out the bearing, this is on the push or top bearing all the rest looks good What all would cause this problem?

Xtreme12x
Dirt Forum Champ
Total posts: 874
posted September 11, 2005 08:59 PM  
Probably got a UFO in there when it was put together. The Rod bearing shows that either dirt got in there when building, or the clearance wasn't right on that journal. If that was the case, 2 would be the same way. More than likely it was dirt in the cylinder.

Todd83C
Dirt Full Roller

Total posts: 53
posted September 12, 2005 05:35 PM  
Hey dirt did not cause this it looks to be a bad pin in the piston. The bearing would proble be that the guy i bought it from never had the rods resized.


Xtreme12x
Dirt Forum Champ
Total posts: 874
posted September 12, 2005 05:56 PM  
The pin would have to come out of the rod in order to scuff it up, and if it did it would likely pull the pin completely out and grenade. I'd still bet money on dirt down in that cylinder. The Rod still could have been dirt, or the clearances were too tight on that journal. If they are stock rods, then that could be why too. Like you said, that one might have been a couple thousands out of round. My JR motor that I bought new was like that. More than likely it was from turning a lot of rpm for many races.

Todd83C
Dirt Full Roller

Total posts: 53
posted September 12, 2005 07:38 PM  
The skirt is what scuffet the wall. Talked to a guy that said the pin could be egg shaped and cause it. As far as the rod bearing they are all worn to the cooper on the top or push side of the rod in the center. Would a set of good 4340 h-beams help to keep this from happening?

Xtreme12x
Dirt Forum Champ
Total posts: 874
posted September 12, 2005 11:37 PM  
The main thing is making sure the clearances are right. I have helped assemble some real cheap claim motors for IMCA mods that had Stock Re-built cheap rods from Midwest, a Stock GM Crank, and Speed pro's with a claimer balance on everything.. 13.5-1 Compression, guy ran it all year, won some races with it. They pulled it down to have the rods re-sized for roundness and new rings/bearings. The bearings were like new. No Copper, nothing. Turned 7300 Nightly. Could be a freak deal, but proper clearancing is a must.


For your application. if your going to keep racing Indee, unless they go Open, I'd build something to these sorts

Callies Dragonslayer Crank- $600 and well worth it
Eagle SIR 6" I beams
Wiseco Pro Tru Flat Tops
Find a set of Used 50cc heads, have them ported and polished... Order a good custom cam and you won't have any problems with power.

Main thing... Proper machine work, have it all balanced. Balancing a motor will make EVERYTHING live longer. Use a good bearing, good rings and you'll be happy. You shouldn't wrap up more than $3500 if you do it right. When you have everything balanced, have them check your rod and main clearances. Shoot for .00225 on the Rods and .00275 on the mains. If everything is kosher there, they'll let you know what bearing to run to get those clearances.

H Beams are Kind of overkill for IMCA, but if you can get them cheap enough on sale, they are cheap insurance, but like everything else... If not put together right, won't last.

zeroracing
Dirt Forum Champ
Total posts: 1876
posted September 13, 2005 12:34 PM  
The main thing is proper clearences.
Sometimes a machine shop does not bore them correctly then when the piston heats up it will begin to rub a little bit at one point on the piston or more.
Not all of the 8 pistons are exactly the same size.
As far as putting togethere a motor, if your putting a 600 dollar crank in it then i would not trust i beam eagle rods. I personally dont care for any of the eagle rods. Look for an american made set.
My backup has eagle i beams, stock crank no balance, we hold our breath when its on the track and up in the high 7 range.

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