Pointhunter75
New Member
I can stock my finger through the exhaust port in the block and wiggle the pistons a little.is that normal? Engine has about 50 hours since total rebuild
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Should pistons have play in them?
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Not really.
The pistons should fit into the cylinder near perfect. Once you add the rings a piston should show no sigh of movement.
Of you have movement the piston will move each time it goes up and down and wipe out the cylinder over a short period. Not to mention how fast the pistons themselves will wear.
I agree that you should do a compression test and post your findings here.
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Sportster-2001-951C-Stock
Well-Known Member
Too much clearance will allow piston slap and can cause the skirts to crack, check the skirts while you're in there for signs of this.
I'm surprised you can actually feel a looseness, I've not seen this before on a healthy engine, if you can "wiggle" the piston, then the clearance is suspect. This bears watching closely, IMO.
I'm surprised you can actually feel a looseness, I've not seen this before on a healthy engine, if you can "wiggle" the piston, then the clearance is suspect. This bears watching closely, IMO.
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Pointhunter75
New Member
Great, they do make a clank. I have only ran it a couple times for a few secs on premix in the carbs other day. Sounded fine like that. I dont see how it could be worn out with 50ish hours. I have to buy or borrow a compression gauge.
Clank and clunks are not normally a good thing from the inside of an engine.
Once you get a compression test and post the results we can likely help you with better info...
Pointhunter75
New Member
Gonna go try to feel the side of the pistons. My brother said it couldnt have more than 30 hours at the absolute max on that rebuild. Half a summer and it ran top notch, then he bought a new ski.
Pointhunter75
New Member
That shouldnt even be enough to feel....
Pointhunter75
New Member
That shouldnt even be enough to feel i dont suppose
Pointhunter75
New Member
Oops didnt think that went through first time.
Pointhunter75
New Member
Wel compression was like 120 on each cylinder.... All that time and about $8-900 wasted i guess. I still dont see how 30 hours or less could be worn out like that. Disgusts me to death.
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Pointhunter75
New Member
Guess i could throw some pistons and rings in it. What are the chances the bottom is bad? The whole motor was rebuilt because it blew up and chewed everything up before. With that few hours since, shouldnt the lower prob be good? Please say yes lol. Oh and it was bored and has wiseco pistons.
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Sportster-2001-951C-Stock
Well-Known Member
I think if it's running good with brown colored BR8 plugs, the skirts aren't cracked, and the compression is decent, I would continue running it and not worry.
Cracked piston skirts are a result of too much clearance so if they are cracked, it confirms there's too much. If the pistons don't look damaged and the compression is decent then run it, especially if the plugs are brown, not black.
I can hear my 951 clunk slightly at TDC and BDC if I turn the crank back and forth a small amount, there's some play in the bearings. Not sure if it's the large or small end, probably both, but that clearance probably decreases with heat.
So yeah, I wouldn't condemn this engine based on a little more than anticipated bore clearance if all else was normal. I've run plenty of worn out engines, even with lower than normal compression, can even put a hotter plug in them to compensate for the colder than necessary BR8 plugs as long as close inspection for erosion of the center electrode doesn't show overheating, then it won't cause detonation.
An overheated plug can cause detonation, a hotter plug (700~1000F degrees is target operating range) will burn off soot from an overly rich mixture. This is why cold plugs have to be tuned leaner, which can cause detonation if ignition advance is too aggressive. Often, the correct solution can be a hotter plug and less ignition advance. Both of these together steer the engine away from potential for detonation.
Detonation occurs when the fuel burn ends abruptly.
Cracked piston skirts are a result of too much clearance so if they are cracked, it confirms there's too much. If the pistons don't look damaged and the compression is decent then run it, especially if the plugs are brown, not black.
I can hear my 951 clunk slightly at TDC and BDC if I turn the crank back and forth a small amount, there's some play in the bearings. Not sure if it's the large or small end, probably both, but that clearance probably decreases with heat.
So yeah, I wouldn't condemn this engine based on a little more than anticipated bore clearance if all else was normal. I've run plenty of worn out engines, even with lower than normal compression, can even put a hotter plug in them to compensate for the colder than necessary BR8 plugs as long as close inspection for erosion of the center electrode doesn't show overheating, then it won't cause detonation.
An overheated plug can cause detonation, a hotter plug (700~1000F degrees is target operating range) will burn off soot from an overly rich mixture. This is why cold plugs have to be tuned leaner, which can cause detonation if ignition advance is too aggressive. Often, the correct solution can be a hotter plug and less ignition advance. Both of these together steer the engine away from potential for detonation.
Detonation occurs when the fuel burn ends abruptly.
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Pointhunter75
New Member
I should have added more info. The engine hasnt been ran in a couple years but ran fine then. I poured some premix in it right before i started the work and it would run. Tank was out getting cleaned and new fuel lines. The compression test was done with a no name gauge and with no carbs or exhaust manifold. Plug boots were on ground block. I'll buy a decent gauge this week. I havent put the carbs or exhaust back on it yet, or the pipe. I guess i should go ahead and put it all back together and gas and oil it up and bleed everything and put it in the water.
Sportster-2001-951C-Stock
Well-Known Member
Yeah, get a compression reading on that thing, I'd still be tempted to run it. Any idea of what the plugs looked like, does it look like they were going to foul? The reason I ask is b/c if the compression was low then it may not have been combusting all the fuel and the plugs would've had soot on them. In that case I'd try a hotter plug, like BR7's or something, but it might not even be worth putting back together just to find out it won't run.
Sounds like a judgment call on this one, I would go by the compression reading you get before making the leap to reassembling. Also, consider the reasons for why it was disassembled, was it just a mistake or what was the motivation? If the pistons didn't match the bore then it could've just been an assembly issue and the correct pistons might be the way to go.
I just checked my spare 787 which is a good low hour motor and I too can rock the piston back and forth through the exhaust port. So you need some clearance for expansion, I believe some brands require even more than stock. So I would recheck the compression and go by that. If it is within spec, just run it.
Sportster-2001-951C-Stock
Well-Known Member
Ok I have thought about this for a bit. Piston clearance is measured on the diameter, straight across, so say it is .005. Now through an open exhaust port, if you were to measure the play it would be greater than .005, why because part of the cylinder is missing. Also at that point, when the exhaust port is open so there is minimal load on the piston anyways. All I am trying to say is don't worry about it.
Pointhunter75
New Member
Gonna try to get the full story in here. My brother bought it new in 95. He took it to fl. In about 03 and his buddies flipped it in the ocean and killed battery trying to crank it, got towed in. It didnt get cranked till next day due to dead batt. Only time ever in saltwater. Whenever later it blew up. When the guy rebuilt it the bearings were rusty. The metal eat things up pretty good. Full rebuild and road it fir the second half of summer and bought a new kawi 1500f in 04. I took it to test ride a few years later and the motor mounts broke with me so i didnt buy it, he wouldnt let me pay to fix it. It sat and was cranked and fogged 3 or 4 times a year till two summers ago when he gave it to me. A few weeks ago i finally started on it. Thinking i had to take the engine out i stripped the pipe, exh manifold, intake man/carbs off, chenged all fuel lines cleaned selector etc. rebuilt pump, crabs, bought used vts. Its almost new now mechanically cept the engine. Gonna put it all together and ride it. I did put some premix in it before stripping it and it fired right up till the bowls ran dry. Sounded fine. Never revved it up
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