Exhaust Valve(s) problems

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Jan 20, 2006 | 12:27 PM

skemcis

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From: BC Interior

Exhaust Valve(s) problems

Had a 351W rebuilt 2 months ago. Last 3 weeks I noticed a HUGE increase in fuel consumption. Took it to the local garage, who did a compression test. Ouch! 140-0-70-140 on the 1,2,3,4 cylinders. Removed the head and the exhaust valves in question are seating abput 1/8" below the face of the combustion chamber. Are there no seats in 351W, as I have been told? The rebuild shop says that the timing is right out, fuel is too lean, creating excessive heat and softening the head enough to let the valves pound into the head. The other head was checked, and had 140-150 across the board. Something here does not make sense. Any ideas?

Jan 20, 2006 | 07:43 PM

TigerDan

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From: The hills of No. Calif.

The seats in these heads are simply cut into the casting. They did harden the seat from the factory (or so I've been told), but it doesn't take much to cut through into the softer material underneath.

What year is this engine? Is it pre-unleaded fuel? (About 1976)

A good fix would be to replace the seats with new, hardened seat inserts (probably what they'll have to do to fix it anyway) and it should be done to both heads.

Jan 20, 2006 | 08:59 PM

mdjohnson

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From: Sioux City, Iowa

I know 351W heads have seats in them, I just ground the seats on some 351W heads a month ago.

Jan 20, 2006 | 11:07 PM

TigerDan

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From: The hills of No. Calif.

Depends on what year. I have several sets of older 351W heads ('69-'70) that don't have inserts.

Jan 21, 2006 | 08:19 AM

skemcis

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From: BC Interior

Valves

The block was cast in 1979, so it should have been a post lunleaded fuel engine. However I don't know if the heads are original or not. Is there a casting # on them that can identify the year of pour?

Jan 21, 2006 | 10:56 AM

TigerDan

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From: The hills of No. Calif.

Yes, it's on the bottom of the heads around the pushrod holes. That's the actual casting number which does contain the year of the head design, look for a number like D8OE-AB which was the most typical 351W head from '78-80.

Jan 21, 2006 | 04:25 PM

skemcis

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From: BC Interior

Just talked to the mechanic. He said that he's seen that happen on propane runners. Mines gas. He figures that the timing was retarded too much, and running it under heavy loads for long distances would likely have done the damage. Regardless, there were no valve seat inserts installed at the time of the rebuild. Ah well.....$2500 wiser. Thanks for the input folks. Much appreciated.

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Jan 21, 2006 | 05:05 PM

mvoeller2

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The 351W heads, just like the 302's are only hardened in the ex seat area from the factory. This hardened area varies to about .030 to .060 deep. After grinding the seats in a rebuild, sometimes you can go past the hardening, or the hardening has just "went away". When I reground seats, if there was any doubt that the valves were starting to sink into the head, I replaced them with hardened seats. And like TigerDan said, pre-76 or so didn't have this hardening in the seats, and thus require seats anytime they are reground. This is also mandatory when converting over to propane along with better valves. But like the shop said, if there was a problem with retarded timing, causing excessive cylinder temperatures, this could have caused the valves to sink in the head.