"thin" bore block?
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More Turbo
Joined: Dec 1999
Posts: 582
Likes: 5
From: New Orleans United States
"thin" bore block?
I'm starting to wonder about the thin bore plague I keep hearing about on various threads. I guess I have been lucky and got a sound block to begin with without looking. I got the truck, a 77 F150 4X4 as my first vehicle to drive to high school and have since been out of college for 2 years and have the truck set aside indoors for a full resto/ performance build.
I got my 400 from NAPA as a short block with a 0.060 overbore, and the only thing to go wrong on it is the original valve stems becoming worn out and leaking oil into the combustion chamber. When I pulled it back apart after 30k miles to do the heads, the block was in perfect working order even after getting put to almost 6000 rpm more then a few times racing the local rice boy hondas (not even the 6" magaphone coffee can tip on the honda could be heard over one of these things wound out).
Has anyone ever had a block split between the bores? I've seen the cracks in the lifter valley, but never in the bores.
I'll admit, I have tortured this thing over the past 11 years, yet it has never left me stranded, and has been called upon to rescue friends in newer, lesser brand trucks.
I got my 400 from NAPA as a short block with a 0.060 overbore, and the only thing to go wrong on it is the original valve stems becoming worn out and leaking oil into the combustion chamber. When I pulled it back apart after 30k miles to do the heads, the block was in perfect working order even after getting put to almost 6000 rpm more then a few times racing the local rice boy hondas (not even the 6" magaphone coffee can tip on the honda could be heard over one of these things wound out).
Has anyone ever had a block split between the bores? I've seen the cracks in the lifter valley, but never in the bores.
I'll admit, I have tortured this thing over the past 11 years, yet it has never left me stranded, and has been called upon to rescue friends in newer, lesser brand trucks.
Posting Guru
Joined: Aug 2000
Posts: 1,127
Likes: 3
From: (near) Denver USA
The only problems I've heard of with "thin bores" in 335-series engines are related to the amount of overboring the block can tolerate. If in doubt, I always recommend sonic testing to find out if there's enough material in the cylinder walls for the proposed overbore. I have never seen an M-block (351M/400) that couldn't be bored to 0.040" with room to spare.
I have heard from some sources I consider reputable (machinists I know) that 0.060" overbore is pushing the limit on some blocks because of core-shift defects, however, that varies from one block to another.
I have heard from some sources I consider reputable (machinists I know) that 0.060" overbore is pushing the limit on some blocks because of core-shift defects, however, that varies from one block to another.
