As seen in Speedboat Magazine’s
Nov/Dec 2024 Issue.

Valve Seat Issues

Dear Bob:
I am having issues that keep me out of the driver’s seat of my boat. The engines are 454 cubic inch Mark IV big block Chevrolets. The exhaust is CMI E-tops with the water mixing in the tailpipes just past the transom. The cam is a flat tappet hydraulic with a lift of .540 on the intake, and .565 on the exhaust. Duration is a mild 234 degrees on the intake, and 262 degrees on the exhaust (@ .050 lift). The lobe separation on the cam is 112 degrees. The cylinder heads are GM iron rectangular port (GM049) with larger valves installed. I am using a Holley 800 CFM marine carburetor (list number 9022) with an Air Gap intake manifold. The ignition is MSD Marine with mechanical advance in the distributor set at 34 degrees total advance at 3,000 rpm. The compression ratio is 8.8:1.

I am having a problem with valve seats in stock GM rectangle port heads falling out or being crystallized. In most cases, the seats are falling out almost fully intact with a very small chunk of seat missing. The problem usually occurs on start up after the engines have been run throughout the day. After running hard, I reduce the rpm down to a slow cruise speed of about 3,000 rpm for a cool down period before returning to idle which is about 750 to 800 rpm. I have run this set up in various engines for years with no seat issues until the past couple of summers.

Are there any fuel additives available that help (lead substitute) cushion the valve seats? Is there a better seat that I should be installing other than stock ductile? Are there better seat installation practices that can be done to help this situation? Would richening up the high speed air fuel ratio increase valve seat longevity? Could the 91 octane fuels that are available today be causing valve seats to fall out?

Andy Noble
Boulder City, NV

I have written about an ongoing problem with exhaust valve seats quite a lot in the past few years. It is apparent that combustion temperatures are increased resulting from using gasoline that is reformulated with ethanol and other oxygenating components. The GM Mark IV heads were designed a long time before gasoline octane levels were reduced and the lack of availability of leaded gasoline. The version of the GM head that was before yours did not even have hardened valve seat inserts. The valve seating surfaces were ground in the bare castings.

As evidenced by the fact that your seats are breaking into pieces indicates that the valve seat material is possibly too hard and brittle. If you inspect the seats and there is evidence of them being hammered (appears like the valve is sinking into the seat surface), then the indication would be that the seat material is too soft. This is also an indication that your valve springs are too weak which results in the valve hammering the seat. That is probably not the case with your heads, because the seats are likely Stellite material, which is very hard. Most aftermarket heads are now built with ductile iron seats. Traditionally, the hardness of these seats should be around 30 Rockwell. More recently, we have seen these seats being damaged in higher performance applications. In these cases, we are installing seats that are at least 40 on the Rockwell scale. And in very severe duty situations, we are installing valve seats that are closer to 50 on the Rockwell scale. One source for these custom valve seats is Tucker Valve Seat Company in Odessa, TX. Tucker can make any seat for any head. Tucker valve seats are available from quality distributors and engine machine shops.

The reason that your problem seems to occur after the engine has been run hard and then cooled down is because that’s when the head is hot, the casting expands at a rate greater than the valve seat which then allows the seat to come loose. Once the seat loosens slightly, continued use along with heating and cooling eventually allows the seat to fall out. The solution to this problem is to install the seats with more of an interference fit. This will require obtaining seats that have a larger outside diameter.

Your heads likely have valve seats that are only 1/4 inch in depth. Most high performance seats are now a depth of 3/8 inch. This results in 50 percent more material to hold the seat in the casting. The problem with heads that were designed for the thinner valve seats is that there is probably not enough casting material to accommodate the more desirable thicker seats.

If you have a head that has had a seat fall out, it is also likely that close inspection will reveal a crack in the casting that passes between the exhaust and intake seats and ports. Heads that are cracked in the seat area should be considered as junk.

Being that your engines are Mark IV, replacing the heads with GM Generation V or VI heads is not possible because of problems with the head gaskets sealing the water pas-sages. The GM Gen V and VI heads also have the same valve seat problems as yours. If you desire to continue to use cast iron heads, many aftermarket heads are available that will fit on both Mark IV and Generation V and VI blocks. World Products offers a couple of designs that would work well depending on the size of inlet runner you desire. For your smaller cubic inch engines, I recommend an inlet runner of around 320 CC which will improve the flow velocity and performance. The World Castings part number for the bare casting is 030620. Dart Machine and others also has a similar head. If you use one of these aftermarket heads, be careful to buy new castings because the later versions have had the thicker (taller 3/8″) valve seats installed with an increased interference fit. As always, the highest quality valves must be used for the rigors of the marine environment. Exhaust valves must be Inconel (Manley and others), or Super Alloy (Ferrea).

If you are in a position where you must replace the head castings, I think you should consider going to aluminum heads. A lot of effort has been dedicated to aluminum head development in the last several years. A real benefit for the harsh marine performance environment is that aluminum heads dissipate heat more effectively. Aluminum heads designed for marine use are available in hard anodized castings for increased corrosion resistance. It is important that you buy marine versions of these heads and not just head assemblies for automotive use from performance parts warehouses. In some cases, you will have to purchase the heads and parts to assemble them separately to make sure you have the correct valve material and valve springs for your cam requirements.

In looking at your camshaft specifications, it seems to me that there is too much duration on the exhaust compared to the intake. The result of a cam that has too much exhaust duration is lower cylinder pressure and a corresponding loss of power. Flat tappet cams are a thing of the past for many reasons. Lower quality flat tappet camshaft and lifter materials combined with engine oils that do not have zinc and other additives have resulted in reduced reliability and more instances of camshafts going flat. I would not hesitate at this point to convert the engines to hydraulic roller cam set ups with the higher on-the-seat pressure valve springs when replacing the heads. Proper lift and duration depends on other factors, but I think somewhere between 230 and 236 degrees duration at .050″ for the intake and between 236 and 244 degrees duration at .050″ lift on the exhaust. Lift should be around .610″ or a little more. Make sure the cam core material is 8620 with an iron cam gear installed.

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