Hi guys

I have just bought a pair of 336781 oval port heads really cheap and wondered if they could be fitted to my engine to give any power gains without changing anything else?

Would they cause any issues?

My engine is a 454 Mk4 peanut port 330hp with an edelbrock performer rpm inlet manifold at the moment.

They will definitely help, but if you really want to wake that motor up you'll have to change the cam and exh. If your performer RPM intake was for peanut port heads (small oval), you should change that too (to a large oval intake).

Adding 781s shouldn't cause any issues, as long as you get later model castings with hardened valve seats, and you use quality valves.

How do I tell what year the castings are? And do I have to change the valves or just regrind the ones that are in it?

Intake was for large oval anyway so will match as far as that is concerned.

There should be a casting date code inside the head that will indicate what factory it was cast at and the year. It will say something like "F 71" under the valve cover. You can take that date code and look up online when and where they were made. Check each head. They may be different.

The heads alone will make no difference. You will need to change the cam and exhaust and spool up to 5000 or so RPM to get any gain. If you decide to use the heads have 2.19/1.88 valves installed, do a little cleanup/port work on them. I use the 820's very similar to those but a smaller chamber.

My budget will not stretch to new exhausts and cam at the moment so is there any harm in fitting these as they are?

Casting date is H11 73 and H13 73 I think. Have had a search online and can not find out exactly what this means.
Would these have hardened seats?

They put these heads on big blocks all the way from 1973 up to the late 80's. I believe unleaded fuel did not become mandatory on new vehicles until the mid 70's. Based on the 1973 date, these probably do not have the hardened seats.

With stock valve sizes and everything else remaining equal, you will probably see little if any gain for the trouble of switching the heads. I found a website that had an exhaustive list of flow numbers for cylinder heads, and the stock 781's flowed about the same at lifts of .500 and below as the peanut port heads. Maybe just a little higher, but these heads do not really "wake up" until you add the larger valves and do some cleanup work in the valve bowl area. Then you would be looking at some healthy gains.

As a side note, you would be surprised at the flow numbers that the peanut port heads can achieve with larger valves and a little port and valve bowl work done to them. A setup like this would run pretty well on a 454.

The old heads have to come off anyway as I suspect they may be cracked as am getting a lot of water in the oil. So I figured as they were off I could simply swap them for the large ovals, bit if I have to spend money on new valve seats this boat might never see the water again!! Living in the UK and loving powerboats is painful!!!

If the old heads have to come off, then i'd say ya, put the larger ovals on. But, before bolting any used head on my motors, i would want to go thru them anyhow.

Water in the oil could be several things. Leaking exhaust manifolds, intake manifold gasket or manifold leaking, oil cooler leaking, camshaft reversion, etc. I wouldnt go pulling heads until you trace down the source.