04:45:59pm, Saturday, May 11, 2024
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Piston seizing
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Against the wall
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I have a strange problem with pistons sticking to the cylinders. 4.⅞ bore x 7.5 stroke gasser. Idles for exactly 2 minutes then they start to grab. Originally had .008 clearance and some builders suggested .015 so that is where it is now but still does the exact same thing. Scuffing in both piston skirts. I have oil squirters. 12:1 comp ratio,fresh 117 octane gas. Scuffing on the sleeves as well but not all the way to the top so I am sure the rings aren't butting up (.036 end gap). Any ideas? Also the piston skirts only come out of the bottom of the sleeve less than ¼ inch. |
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Re: Piston seizing
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Idea
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Your timing ok? Too far advanced? How the fuel mixture? Too lean at idle?
What are your coolant temps? Water pump circulating water? What material is the piston made out of? |
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Re: Piston seizing
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Against the wall
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Timing is @ 2btdc, the temperature gauge never moves, you can put your hand anyplace on the engine. Had an electric fan and water pump that are both working plus new aluminum radiator. Has the big zenith carb. I tried making the fuel mixture overly rich but made no difference. It runs smoothly at start up, but only for exactly 1 minute then you can hear it start to labor. |
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Re: Piston seizing
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plum krazy
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ring gap to small |
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Re: Piston seizing
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Been there
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I've had this happen where the bore wasn't true and actually was tapered causing similar scuffing. Not saying that's 100% your problem but worth checking out. |
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Re: Piston seizing
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Registered: 03/04/2009 Posts: 35 |
check your pistons for being out of round |
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Re: Piston seizing
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Registered: 12/01/2015 Posts: 31 |
Are you kidding me? Got to check for piston out of round? Do you know piston aren't truly round? |
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Re: Piston seizing
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Registered: 03/04/2009 Posts: 35 |
duh |
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Re: Piston seizing
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Machinist
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It sounds like the bores are tapered from top to bottom. It's pretty common, you need to find someone with the proper honing equipment that can dwell at the bottom of the cylinder to prevent this. |
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Re: Piston seizing
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for what it is worth after you clean up the scuffs and check bores trueness I would try glass beading the skirts of the pistons and this will make lots of little dents that will hold oil better we do this on alky supercharged and turbocharged engines to help with this problem and some of the NHRA prostockers also did this on brand new pistons before running them you will need to use a parts kleening brush and solvent on them after blasting to remove glass dust before installing them |
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Re: Piston seizing
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Against the wall
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The bores were perfect less than .0005 taper and out of round on a 10.25 inch sleeve. It was all checked prior to assembly. I checked the sleeves before and after I put them in. They are wet sleeves by power bore. I am really leaning towards junk pistons. |
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Re: Piston seizing
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Registered: 10/28/2010 Posts: 42 |
How about the wrist pin bushings? too tight or not getting lube? can get tight and rock piston sideways in cylinder. |
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Re: Piston seizing
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Ring groove depth
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You may also want to check to be sure piston ring groove depth is sufficient for the rings. Ran into this once where pistons were shipped with wrong rings. As piston expanded it pushed rings into cylinder walls. Cool it would start fine but would start to seize just as you describe in a few minutes. |
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Re: Piston seizing
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Against the wall
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The pin bushings look fine, I leave them a little looser on a puller than a stock rebuild. I will definitely look the ring grooves a little closer, but the rings look good and the scuffing is down lower concentrated in the skirt area. I will try to post a picture. |
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Re: Piston seizing
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LEWIS
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Don't lean towards junk pistons thy couldn't be none made that couldn't idle for two minutes how much oil pressure are you making and what weight oil are you running in just two minutes of running 50 weight oil wont be warm enough to squirt very high besides therye just piston coolers you crank needs to be slinging oil against the sleeves to lubricate 0r enough needs to be coming from the sides of your Rod journels Whoever stroked your crank could have the clearance to tight on each side of the Rod to loose you get no oil pressure to tight no oil on the sleves |
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Re: Piston seizing
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Melvin
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You need to hone the sleeves. I run 25 on mine |
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Re: Piston seizing
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Against the wall
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I did the crank myself, there is .013 to.015 side clearance rod to crank. I was using 10 40 with some zinc additive. I usually run it for 5 to 10 minutes then drain it. Thanks for all of the helpful suggestions |
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Re: Piston seizing
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Against the wall
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Also have 50 lbs of oil pressure. |
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Re: Piston seizing
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Take one of the pistons to your wife's oven and measure how much it grows as it heats up. I had some BRC pistons that would expand more than a stock piston. MACK |
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Re: Piston seizing
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Registered: 04/06/2008 Posts: 974 |
I have had this issue on atv's where the piston WAS out of round and the banshee didn't live long at all and stuck the exhaust side. Got different pistons and he problem was gone. Good luck. Pretty sure there are guys that know more about this than I would ever know
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Re: Piston seizing
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Registered: 01/20/2011 Posts: 446 |
Maybe your stroke is too long for your sleeve and the piston is coming too far out of the bottom of the sleeve at the bottom of the stroke then it is trying to cock in the bore to shove back up and this causes excessive heat and failure. I have seen that before. |
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Re: Piston seizing
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Against the wall
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Kta, can I get your contact info? The pistons come out of the sleeve about .250. |
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Re: Piston seizing
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Registered: 01/20/2011 Posts: 446 |
PM me and I can send you my number. |
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