Causes of tight wrist pins and scuffed pistons ?
12-09-2012, 04:40 PM
#31
Not sure on the quench I'll have to check upon reassembly, the rings and ring lands look fine.
Maybe I just don't want to believe it, but I've run a lot of big power motors, granted not many in boats, but there are no signs of detonation anywhere on these. I've detonated pistons into a million pieces, and I've made some last that should have blown up right away. How could it have beat the wrist pin bore out of the piston without a single sign on the plugs, bearings or domes??
Maybe I just don't want to believe it, but I've run a lot of big power motors, granted not many in boats, but there are no signs of detonation anywhere on these. I've detonated pistons into a million pieces, and I've made some last that should have blown up right away. How could it have beat the wrist pin bore out of the piston without a single sign on the plugs, bearings or domes??
12-10-2012, 06:16 AM
#32
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the pics of the bearings don't look good to me...now might just be them on my end...also, not sure if merc does anything diff on a HP500 oil lines, but std 502 stuff is junk, search here couple or great articles, but line ends, fittings etc are terrible, are you running a oil temp gauge ??? need to with a blower set-up, also did you have any carb flooding/super rich ?? does not take much fuel in the oil to cause issues like these...I can tell you that from experience on a small blower engine I killed a few years ago....
12-11-2012, 09:07 AM
#34
Platinum Member
What is the quench distance? Are you at zero deck? negative deck? Makes a big difference when your putting timing in a motor. Also, piston to wall is a little tight but doesnt look like that was your issue. Detonation will open up the ring lands and you will loose your lubrication on the pistons. It will also take the molly right off the top ring as soon as it detonates. should run a tooled steel top ring in a boosted motor it's worth the extra money. I've put timing in boosted motors without any issue but I've always payed attention to the quench area. Guys that are slamming motors out the door usually don't do this cause it takes extra time...
12-11-2012, 09:37 AM
#36
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12-11-2012, 10:23 AM
#39
Tight wrist pins would cause the skirt to skuff, piston could not rock freely and would cause side load on them. If you had deonation to the point of knocking a wrist pin hole out of round the upper rod bearing would show copper. Post pics of the rod bearings. sound like poor machine work, and bad assymbly. Side clearance sounds a little tight for a boosted motor, but the piston manufactuer would have the determaining specs for their product.
12-11-2012, 12:09 PM
#40
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mfg specs are a recommended minimum, a boosted marine engine is often much greater, not uncommon to see .008 piston to wall on a 4.500 bore a tad less on a 4.250 must remember that controlling the expansion rate of a block at 140 degrees is nearly impossible, so err on the side of caution and add some clearance impo
