#8 heads on 455 block

Old April 30th, 2018, 03:08 AM
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#8 heads on 455 block


our gliding club just acquired a used winch to pull our gliders into the air. This winch came with an Oldsmobile 455 engine, coupled to a TH400 transmission which is blocked in 3rd gear. What confuses me is that the block has the marking 396021F stamped on it (the F without serifs), but the heads are #8 heads. Apparently they come from a smaller engine. Is there anything completely wrong with this combination and what kind of performance can we expect from it?

The engine has been rebuilt a couple more than 10 years ago and has only few hours on it. After fixing some initial issues (it was running too fat) it basically works. In a glider winch, we need an engine capable of 5000rpm and sufficient torque in the range between 2000 and 3500 rpm.

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Old April 30th, 2018, 06:31 AM
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The 455 block appears to be an early (1968-70) block. The heads are 1973-77 Olds 350 smog motor heads. These heads have the same 80-ish CC combustion chambers as do most 455 heads, so CR will be unchanged, but the No 8 heads have small valves and small ports as compared to the correct 455 heads. This will both limit higher RPM breathing and will make for a horrendous mismatch where the intake ports meet the heads. This flow disruption will further handicap this motor. I'd source a set of correct BBO heads (pretty much anything but J heads) and swap them out.

Of course, since this is obviously a cobbled-together motor, we have no idea what was done to those No 8 heads before they were bolted on. It is POSSIBLE (though highly unlikely) that whoever did this installed larger valves and did some porting work. You won't know without tearing down the motor. I wouldn't hold my breath.
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Old May 1st, 2018, 11:24 AM
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Thanks for your response. The intake manifold has the casting number 412753 which apparently belongs to a 455 engine, too.

The motor was sold as part of the winch by the German winch company Tost. They were still selling the 455 with their winches until few years back, most of them had the J heads. From the manual, they have 250hp and 375ft lb of torque. Experience from other operators says these motors are quite powerful but don't live too long when driven at rpms higher than 4500.

Our engine was remanufactured somewhere in the Netherlands during the late 90s. So far it serves its purpose and it definitely is more powerful than the Chevy 350 we use in another winch. But whenever there's a need for an overhaul, we will look for the right heads to put on. It might become a bit difficult to source theme over here in Europe, but we'll see.

Is there anything else we should look for in order to get a nice tow-motor with good torque and capable to sustain up to 5500rpm?

Best regards from Berlin
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Old May 1st, 2018, 11:57 AM
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It may be productive to check with the boating crowd to see what they do during rebuilds. Many jet boats are powered by Olds 455 engines and those fellas typically have two settings - idle and full throttle.
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Old May 3rd, 2018, 06:32 AM
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Precise machining, which helps a lot, along with proper clearances, which is an argument in itself. The 455 has very heavy rotating parts, combine that with heavy pistons and factory, heavy, soft forged rods with tight clearances, problems can show up quickly. The #8 heads can be made to flow, the exhaust port is better than the J head, just needs a lip removed with a bowl hog and already has an adequate 1.625" exhaust valve. The intake port will require the most work, larger 2.07" intake valve should be used with the bowl opened and the port roof raised. Both sides should be ported, the intake side especially. This is why stock BBO heads are a no brainer. The J's have a better intake port and valve size compared to the #8's but an awful exhaust, the #8's exhaust port require a grinder or bowl hog to have a decent exhaust port, the J's, hours of porting.
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