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Author Topic:   Piston question
Eljojo
Dirt Forum Champ
Total posts: 968
posted August 20, 2005 04:58 AM  
Last year I installed a set of Speedpro pistons that had some black coating on the skirts. When I removed them, most or all of the coating was gone. Is this normal? If not, what happened and do I need to have some machine work done to the block before I install new pistons?

leapinlizard
Dirt Freak

Total posts: 402
posted August 22, 2005 09:00 AM  
How old are the pistons? Are the skirts scuffed?

As far as remachining, you need to make sure the Cylinders are round and true, no scuffs and pitting or other flaws. I can't say without seeing it but you need not to worry about the coating coming off. It makes a better visual que than a performannce enhancer on those pistons.

Eljojo
Dirt Forum Champ
Total posts: 968
posted August 22, 2005 04:39 PM  
Thanks for the reply. I was beginning to think I had asked one of those questions that couldn't be answered!
They came out of a motor that wasn't a season old. Either I had run too much timing, too lean on fuel, or insufficient octane, because the motor developed horrendous blow-by. Upon removing the pistons I found that the ring lands had broken between the top and second ring on three pistons. I was running .100 domes with 58cc heads and I guess the compression gremlin caught up with me. The cylinder walls look great, with no scuffs and scratches, therefore I plan to take a ball hone to it and go back in with some new flat tops.
The guys who make the rules at our track and most others here in the south don't allow dome pistons. They don't care if you spend hunderds of dollars anglemilling 40 year old double humps, but they sure don't want to see any dome pistons! Go figger!

leapinlizard
Dirt Freak

Total posts: 402
posted August 23, 2005 08:46 AM  
Broken rings and ring lands indicate the rings butted. Insuffiecient ring gap. If you were running them tight and it got hot it might explain it. How the piston to wall clearance? Should be .004-.006. double check with the piston manufacturer though.

Detonation usually beats the plug, the crown, and the top ring land but not usually the second ring land. You might check the rod bearings for signs of heat damage. **** , it's cheap enough to change them anyway.

JohnG
Dirt Maniac

Total posts: 137
posted August 23, 2005 03:50 PM  
the black coating is to aid in less cylinder scuffing while new.

it is only on there temporarily...


Xtreme12x
Dirt Forum Champ
Total posts: 874
posted August 24, 2005 01:58 PM  
Actually, the coating is to aid in less cylinder and piston scuffing along with a little bit of performance enhancement, but really It doesn't do much for performance. If your wearing all of the coating off, then there was a problem somewhere when you put the motor together.

What were you running for Cylinder Wall Clearance? What kind of ring gaps? What were you using for an air filter also...

Eljojo
Dirt Forum Champ
Total posts: 968
posted August 25, 2005 07:39 PM  
I run an Amsoil oiled foam filter with an outerwear. I filed the rings and, altough I have slept since then, I believe I cut them to .018 and .020. I do not know the piston to wall clearance. How can I check that? I only have basic tools such as feeler guages and a dial indicator. Would I need to borrow a bore guage or inside/outside micrometer?

juliaferrell
Dirt Freak

Total posts: 370
posted August 25, 2005 08:09 PM  
90% your problem lies at the bottom of the cylinder bores. Stick your finger down there and feel that sharp edge? chamfer the bottom of the bore and more than likely that coating will be on there the next time you take it apart.

Xtreme12x
Dirt Forum Champ
Total posts: 874
posted August 26, 2005 06:53 AM  
Easy and fairly accurate way is to buy yourself a set of the 12" feeler gauges... start with an .003 feeler gauge, place it down in the cylinder, and then stick the piston in rod up.. go by feel... then try a .004, .005.. I'm guessing your close to .003 if the coating is wore off. I run upwards of .005 on my speed pros, but this is my last year using them.

The machine shop should do all the clearancing for you though if you had the block bored to fit those pistons.

leapinlizard
Dirt Freak

Total posts: 402
posted August 26, 2005 11:14 AM  
Simple. Stick the piston that belongs in the hole without the rings. Stick a feeler guage inbetween the piston and wall. Find the one that gives you a snug feel. example, if you try a .005 and it fit's tightly, but a .004 is snug, call it .004.

To much clearance comprimises ring seal and piston drag by cocking side ways in the cylinder. To tight and scuffing with undue wear will result.

juliaferrell
Dirt Freak

Total posts: 370
posted August 26, 2005 12:33 PM  
TOO tight in the bore will increase scuff but that chamfer on the bottom of the bore will probably take care of it.

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