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jstwkd
06-02-2002, 09:16 AM
I tore the motor down gen 6 502 edel. alum. heads etc. Anyway the knock I heard was a piston starting to sieze. The skirt is scratched to hell as well as the cylinder wall (#3/and #5).My machinest feels that there was a lack of water to the cyclinders.He also found all 4 valves in these chamber mushroomed (over heated) and the head slightly warped.We are thinking I may have ran the wrong head gasket.The head gasket combustion ring also shows heat marks around the same two cylinders.I am running a holley hp 950 by bigs primaries are 80 and secondaries are 86/wiend tunnel ram/msd ign./The carb should have been set up to my specs turning 6000/ the only thing I did kinda strange is turn the scoop foward no air cleaner /machinest said he doesnt think this is the problem. Any Ideas? sorry so long!

flat broke
06-02-2002, 09:39 AM
sorry to hear about your wasted 502. By the sound of things it almost sounds like a hydroloc situation. If you pull a bit of water into a hot cylinder it can expand pretty damn fast, especially with the added compression of the motor. I have heard of valves getting pushed up into the head from the pressure and typically something lets go to relieve the pressure. Perhaps in this case the piston deformed under the stress as well as the valves. That would explain your scored cylinders, mushroomed valves, and deformed pistons. What it doesn't explain is the head gasket. If the gasket was bad it could be the cause of the condition, but that still wouldn't explain the evidence of excessive heat on the gasket.
The other possibility is that it was indeed excessive heat in the #3 and #5 cylinders. Since I believe these are siamesed in the 502 it does make sense that if a water passage was blocked or filled with sand around one, the other might suffer through conduction of heat by the block. While you might see a stout 502 with jetting richer than 86s in the rear, I don't think its lean to the point of being your culprit. When your machinist goes through your block, see how much sand comes out when they flush it. Hopefully that is your answer.
Good luck and keep us posted,
Chris

jstwkd
06-02-2002, 10:25 AM
Thanks flat broke!I looked in the block and see no sand.I am going to check to see if one of the hoses are blocked tonight.

wsm9808
06-02-2002, 11:04 AM
Was your engine a crate motor, or an engine specificaly set up for marine use?In our experiance on car motors used in boats, they typicaly dont have enough piston to wall clearance to be reliable in marine applications (when pushed hard). The cold raw water in the system does not allow the bores to expand the same amount and rate as in a closed cooling system that runs at higher temp. combine that with the heavyer load the marine engines run under (which increases combustion temperatures}, high RPM, and high combustion temperatures caused by the sightly lean condition caused when the motor stops building rpm against a fully loaded pump(vacuum signal drops simular to lugging a motor). It doesnt take long for all those things to add up and have the pistons super heat and expand so much more and faster than the bore that the oil film on the cylinder wall breaks down and and the pistons gall aginst the walls. Usually happens on the 2 center cylinders first.
Mike your damaged pistons, they will measure about .015 to .030 smaller than the undamaged ones. After pistons super heat and gall, they loose their "memery" and shrink when they cool, which in turn results in the piston slap/knock you heard.
Be sure and have your piston to wall clearance correct for your set up when you putt it back together. If your machinest doesnt think you need extra clearance for marine use, educate him or get a written warrantee from him it wont gall again. After he pays for a couple of motors he will believe.
I woulds also bet the valves that where damaged where the intake valves. This also indicates overheated/overpressured combustion. Again, we see it alot in "lugged" motors, where the conditions are WOT,low vacuum and heavey loads. The cylinders lean out, over heat and the intake valves get hotter than they are designed for and appear thave been "sucked" up into the intake port leaving the valve cupped in appearance.
[This message has been edited by wsm9808 (edited June 02, 2002).]

jstwkd
06-02-2002, 11:50 AM
wsm9808 Indeed it was a crate motor.I will let him know about clearances.What should the clearances be?I have read post about marine motors being set up losser but everyone I talked to said it should be ok/ran lots of them.Anyway thanks for the help.

wsm9808
06-02-2002, 02:06 PM
I'll agree, in most cases a crate motor will be fine in a boat, but if the limits are ever pushed do to sudden rise in interal temps for any reason... then you need the extra margin of safty with the extra clearances. Most piston manufactures that I have used recomend .002 to .003 over the stock clearance. If you are running a blower or nitrous, you will need to ad even more clearance on top of that, and the ring gaps will need to be larger also. Too tght of ring gap will cause the rings to break and/or pull chunks loose from the piston top.

jstwkd
06-02-2002, 03:23 PM
Great!Im talking to a friend right now to see if he wants to trade my 2001 custom harley chassis stretched tanks etc.anyway for a 671 blower comp. with dual 750 set up for a BBC. The machinest is a friend of a friend who biulds race motors all the time (from what im told )but I havent even met the guy yet.I do trust my friend comp. .But I figured the motor is apart now the time to make changes!should I make changes to the rod and main bearings etc.?