Causes of tight wrist pins and scuffed pistons ?
12-11-2012, 03:15 PM
#42
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.004 is way too tight on an open cooled marine SC engine in my .02. That would be the root of the problem. Easy fix though with some new pistons. If you are going with a bigger blower, than the new pistons give you a chance to lower compression a bit anyway. The little blower needs the squeeze though, so if you are staying with 174 I'd go up in compression. And of course you need to set the quench to .040 
12-11-2012, 08:10 PM
#44
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Originally Posted by [email protected]
I had very bad luck with the 250 B&M blowers and I NEVER will run a blower without duel carbs. Ever. Period! Always leaned out Cyl. 5 and 7. Too much heat.
I also had issue with my B&M 250 blower on 454ci with single carb setup and offset carb plate, all 4 rear cylinder was lean. I will run twin carb setup now.
12-11-2012, 08:42 PM
#45
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Has for the wrist pin i have notice on my engine that i had a tight pin in the melt #6 piston, that is normal after a melt piston, but if lil red have 7 on 8 tight piston wrist pin and had only 3 scuffed piston (surely due to a piston wall clearance issue), no sing of detonation what can make that tight wrist pin???
Can be not enought clearance between wrist pin to clearance in piston during the assembly??
Can be not enought clearance between wrist pin to clearance in piston during the assembly??
12-11-2012, 09:20 PM
#46
That is one thing I'm still wondering, I did not have a chance to call dean yet, speaking with one engine builder, they had found too smooth of a cylinder wall finish to not retain enough oil and cause issues. This was on high ductile dart blocks that clog up typical fine honing stones and should not be an issue with an oem block
Last edited by Unlimited jd; 12-11-2012 at 10:02 PM.
12-11-2012, 09:23 PM
#47
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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??
12-12-2012, 01:04 AM
#50
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If you notice those pistons have the small grooves machined in the skirts to retain oil between the piston and cylinder. Those really aren't that great of pistons but I don't think that was the problem. In my opinion it was all due to poor machine work. If you have the ability to double check the specs of the machine shops work before assembly than you know everything should be fine. Most people don't think twice about doing this. Here's the piston clearance specs from the merc manual.
