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Hey everyone! New here, I have spent the past 3 days trying to identify my engine. I just bought a 1979 f150 custom 4x4 4 speed manual. I have found so much misinformation and just back and forth stuff online, I really wanna be sure what I have here. As far as I understand, I think I have a 351m. The valve covers have 8 bolts, the cylinder heads are marked with “m” and the decal on the driver side valve cover shows engine family “5.8m/6.6 6GR” as far as I can make out. And for displacement is shows “5.8L 351cid” it also reads part number “D9TE-9C485-8TA” as far as I have found, that’s the designation for the head type but unsure of what that tells me. also I read somewhere that the thermostat being located on the block determines it is a 351m or a 400. Just trying to be sure what I have before I buy parts. Also not totally sure on parts compatibility from the 351m to the 400? My understanding is, crank and pistons are the only difference? And I think different heads and intake manifolds too? I’ll attach some pictures just in case I’m missing something here. Thanks for any help! Shows the engine family+displacement markings Shows the cylinder head part number “M” casting on cylinder head Thermostat location Thermostat location
I don't know of any reliable way to tell if one of these engines is a 351M or 400 from the outside especially one that has been worked on over the years. There might be some differences in the style of dampers originally used between the 351 and the 400 but the amount of balance weight is the same and they can be interchanged from one engine to the other. You could check the VIN and know what engine the truck originally came with but that could have been changed over time.
You have a 351M
The valve cover was the only way to tell the difference back then
Thank God, yours is still there
360-390 was the same way in the 70s You had to check the valve cover
You can double check yours with a drinking straw and pulling one of the plugs out.
Put the straw in the sparkplug hole and turn the 15/16 crankshaft damper bolt
The straw will move either up or down
Go until it hits the bottom and make a mark with a Sharpie magic marker or something
Then go until it tops out and make another mark
Those 400s had a 4 inch stroke
Your 351 has a 3.5 inch stroke
You will not be able to measure it perfectly, but you should not see anything over 3,5 inches of movement of the straw
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