resurfacing a head, maxium compression

[boltbreaker]

I'm attempting a rebuild on an early 80's(i think it's 84, would have to double check) 60 HP Evinrude 2 cyl outboard. <br /> The head has pitting/damage on one of the surface's. My machine shop says they can resurface it and get the bad out but we are unsure on a minium head thickness. My Clymer doesn't touch the subject. I was also wondering if anyone new what a MAXIMUM compression reading for one of this engines would be. Thanks in advance,looking forward to the discussion!

 

[ledgefinder]

Re: resurfacing a head, maxium compression

In re-contouring the head, you may run into problems if the squish areas of the head work differently. The squish areas are where the piston & head are close together (gap typically 0.025 to 0.100 inch) at top dead center. Some engine manufacturers work with the squish areas quite a bit to get the highest compression they can, without causing pre-ignition and detonation. Basically, the fuel-air mixture that gets squeezed out of the squish areas is supposed to cause a lot of turbulence in the combustion chamber (the non-squish area), increasing the flame speed. Higher flame speed means combustion can take place before the chemical changes that lead to pre-ignition/detonation can occur.<br /><br />Anyway, a long-winded way of saying that, unless you & your shop are reasonably good with two-stroke squish design, you may want to just find a used head that isn't pitted. Try eBay - shouldn't be more than $20 or so tops.

 

[rabidfish]

Re: resurfacing a head, maxium compression

Your cylinder head should be flat in the "squish" area anyway, so squish clearance shouldn't be a problem. If your piston to cylinder wall fit is good, you can take .030 off without a problem.<br /><br />If your piston to cylinder wall is sloppy, the increased compression will only compound the problem.<br /><br />We did alot of work with heads on th "Formula E" engines, they were the 45cu-in based OMCs.