Uneven Squish Bands?

[Pro-pulsion]

I was at a friend's house last night helping him out with his jet boat engine. He has the head off for a new gasket replacement and was showing me the machining marks on the gasket surface. It looks awful, almost as if the machinist that did it ran one insert at about 400 rpm and a feed rate of about 60 inches a minute. Very large cut paths, terrible looking finish and more importantly, uneven squish bands. I know the proper method to measuring squish is with the squish putty inside the combustion chamber with the head on, but that's not possible at this point. So I did a rough measurement from the gasket surface to where the dome starts. The squish band is not even all the way around on all 4 domes, and as you progress toward dome #4, there is a 0.035" difference in height from the dome to the gasket surface. So dome one has a rough estimate squish of 0.055", dome #4 is 0.085", and not balanced all the way around either, the opposite side is a little less than the 0.085" and it varies on all domes.

I can't help but think the "machinist" that did the work just threw the head into a vice with no regard to indicating it flat, and just milled as it sat. Cylinder #4 was about 5psi lower on compression than cylinder #1. I figure that is due to the uneven squish. Has anyone ever seen varying squish bands purposely used or am I right and this was a hack of a machinist at work? He's going to send me a few pics tomorrow to post but I really think this was just shotty work. The work was all done well before he ever owned the boat so taking it back is also a no go ordeal, all that is left now is for me to take the part into work when the time permits and try to indicate the thing just so to get a balanced squish band back. Any thoughts? The engine is a 120hp Force 2 stroke that was bought out by Mercury. I was also wondering if anyone has ever seen anything like little rubber stems with what appears to be an epoxy resin core used as a water diverting method? These little plug things sit inside the water jacket and seem to prevent water from flowing around each cylinder and direct it to form a circuit all the way down and back up the water jacket. It's a little bit of a difficult thing to explain but I will try to get pics of those too. I wonder if these little diverting things in the water jacket are really that important or is there benefit in tossing them. They have melted down anyway making the core liquify and slime out, just interested to know what, if any experience anyone here has had with this kind of thing.

 

[badvinny]

Most boat motor heads have a 'cast' dome, unmachined, they usually only machine the head gasket area and that is if the head is even removable, many are not.

 

[CARBONX2]

Cast heads are never accurate on squish

 

[Pro-pulsion]

I agree about the cast heads, but this machining job was clearly done well after the original time. The cylinder deck has a much more finished and appropriate surface. I will try to get some pics of what I'm talking about tonight. In this situation, pics really do speak for themselves. The guys in the machining industry will know what I mean by a slow rpm with a high feed rate. It just shakes the mind to actually see such a significant difference that it raises questions of should it actually be like this. As you guys have mentioned, production casting will always be marginally accurate, this however is just nowhere near what I would consider acceptable. Thanks for the thoughts so far, I hope there are more to turn up such as the little rubber water jacket flow diverting plugs.

 

[DAG]

piston rock could be a culprit for uneven squish readings.

PS: what the hell is squish putty? I've lead checked squish

 

[Pro-pulsion]

Squish putty? It's kinda like play dough but with more plasticity. I have seen it come in pencil sized styles and in a ball. There used to be a bouncy putty for kids, it was basically the same thing. I know the stuff you're talking about, solid core solder is very similar to it.