Overheated this weekend - 0 compression in 5 and 7

As for the flappers and fogging. The flappers are still in tact, but thinking about it now I vaguely remember having a flapper get sucked back into the exhaust a while back. I can't remember now if that was when I pulled it out of the water or on the hose.

I never thought of fogging before this. I always figured it was only need for winterizing up north, but I'm definitely going to start doing it more often now.

The overheating may have just drawn my attention to a problem I was overlooking or could have helped take the metal over the edge. Either way I know the heads need some work so I will get that taken care of and do another compression test to make sure I'm not having another issue.


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I've been sitting here thinking. We now know the source of water was through the bad exhaust valves, but what has me puzzled is where the oil came from. If the valve guides are totally shot, it could be from them, or from a bad head gasket after the overheat.

I'm worried that your rings in 5 and 7 may be as corroded as the valves, and are not sealing. I've been trying to think of a way to check them before you get everything back together. It may be worth the trouble to put a good head back on the engine and compression check those cylinders again to make sure the rings are sealing. I'm probably over thinking this...but I'd hate for you to be surprised later...

Check my post right above. We posted similar things around the same time :)


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Originally Posted by pridekit

Thanks! I'll get in contact with the shop then. I was doing a quick Google search and noticed that there are some z28 kits that come with new springs and all. Is it worth spending the extra for the springs or just get the stainless valves?


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Springs are easy to check. Ideally, you want to measure the length they will be on the head, by installing the valve and spring retainer without the spring, and measure the installed height of the spring. It will be slightly different for each valve. Then use a scale and compress each valve to that height and measure the force it takes. I found an old bathroom scale I use by placing it on a cheap hydraulic press for checking.

You can then compress the springs the additional amount corresponding to the valve lift for the camshaft you have. This should be listed in the engine manual, or measure a cam lobe lift and multly by 1.7 (the rocker arm ratio).

These 2 numbers...the installed pressure and the open pressure...should be close for all the valves. If they are close, but one or two are a little weak, you can place a shim under those springs to bring them up to spec. Just be sure not to shim so much that the spring binds on itself from being compressed too much.

Now...I know this is long winded...but here is the answer, finally!?! If you measure and get springs that are much weaker than others, then it's time to replace your springs. Boats don't rev nearly as high rpm as a Z28, so those springs are likely much stiffer than you need. Stiffer isn't bad, but it can cause the valve train to wear faster.

If I go with the 2.02 intake valves, should I go with the 1.5 or 1.6 exhaust valves?

I'm going to give the shop a call tomorrow and discuss the plans. I remember him saying for $330 each, that the castings were new, and he was going to use as many parts as he could from my heads and whatever he has collected over the years. I'm going to insist on the SS valves for sure.


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i would shop around for a machine shop , the valve s are cheap , seats are cheap , and guides are cheap .. get a 3 angle cut on the valve / valve seat .. at $330 each is that just labor or new valves and such ?

here in tx they do heads for $220.00 for rebuilds plus parts .. most of the time valves are salvageable but in your case its time for new for sure .. :cool:

Labor, new head casting, and any valves that can't be reused from my old heads.


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have you searched for new valve kits ? i mean if hes gonna do a valve job might as well just put new back in there .. but thats just me being picky i guess ... i just would hate to do all that work and have something else mess up and have to pull it back apart again ya know ?

is that assault a SBC ?

Yes, I'm trying to decide between 1.5 or 1.6 exhaust valves.


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On a high rpm engine I would definitely recommend the 2.02 intakes and 1.6 exhaust. Remember that most of the time we want these big boats to come cleanly out of the hole, and never get over 5k rpm. So low rpm torque is what we need. The 1.94 intake/1.5 exhaust valves will be fine. They tend to be more durable, as you have more "meat" between the valves in the chamber. The only difference is you will get a bit more top end out of the larger valves, but could lose a bit of grunt on the bottom end. So I guess if it were my decision, I'd go with the size that is the best price...and never look back!

I did think of something else...your heads are large chamber, often referred to as "open chamber". This is a rather low compression head, and I'd recommend going with the same chamber size so you don't have to worry about the gas octane. The marina here in Texas only sells 86 octane...so 8:1 compression is about all I could run without bringing my own gas to the lake. Just something else to look at. Most later heads are large chamber...but some of the early street heads (before 1971) and aftermarket may have the smaller "closed" chambers. Interestingly, although the closed chamber heads are considered the high performance heads, the open chamber heads actually breather better, since the valves are not shrouded by the tight clearances.