Thread: How reliable is the flathead?

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I am just getting started into the hotrod scene. I have yet to decide what car I am going to end up with (wanted a highboy for a while but my state says fenders and hood). I think that right now I would like a '40 coupe with a flathead. What I would like to know is can the flathead be reliable? I would probably put a couple thousand miles a year on a rod if I built one (there to drive right?) and I don't want to be stuck all the time. Any stories about yours?
CHAZ

Can they? Yeah!

Unless you get lucky and stumble across one that's already a proven good one though, you'll have to spend a steep chunk of dough. The blocks are very prone to cracking, particularly between valve seats. They have a reputation for overheating because the exhaust passages go between the cylinders. If you get a good block (or spend a young ton to make one so), make sure you get all the core sand out of it, and don't short cut your cooling system you could avoid those issues. The other cause of overheating is guys try to use the original low ratio rear gear. Many '40's have a 4:11, some a 3:78, but at anything over 45 you're winding it pretty tight. A 3:54 makes it happier.

A full rebuild with the usual stuff will run $5-8k depending on machine work needed and if you stroke it.

A good trick on any used block, especially a flattie, is to bump the empty block around on a sheet of plywood. This loosens up all the years' accumulation of crud and makes 'em run cooler. Takes some elbow grease, but well worth it.
Flatheads will run 80K miles in stock form, hopping them up can shorten their lives, just like any other engine.

Had no problems with my 53 merc flattie in my 41 ford pickup but what Bob says about the lower gears is true. I went from 4:11 to 3:23, and it made alot of difference in the temperature of the coolent.

5-8k for a rebuild????
Oh My God, I had no idea you would get that into rebuilding a flattie. That really changes my ideas. You can make a lot of horsepower in a modern small block for that.
CHAZ

Yeah, no kidding, you could screw together a 500 hp chevy for less than that.

Completely reliable !, I have never heard of one dropping a valve

One of the main stream magazines just did a big article on flatheads, I'm at work and can't remember which one. I'm sure somene can tell you which one it was It was a great article. Carlg

5K to 8K for a rebuild? Maybe if you are doing an all out full race motor with blower, etc. I would say you can do it for less than 3K. Mine ran just over 5k and that was with all the goodies minus a blower set up. Also included polishing all the aluminum. It's a very reliable motor if you don't over abuse it. And it keeps up with small block Chevys too. John

Flathead4d, that is B-yutiful! However, I would have used a 3 5/16" bore instead of the all out 3 3/8" bore, that may be the last time you can bore that block now, although I have seen them bored to 3 7/16", but that assumes a perfect positioning of the cylinder cores in the block which is usually very iffy. Anyway if you take care of it and use a stiffer rear gear like a 3.54 you may get a lot of reliable miles from it. Where did I read about a guy who was going to replace a flathead anyway so he put a brick on the floor pedal and let it run at WOT until it overheated and stopped; then an hour later he found it started right up again, so they can be tough but overheating will eventually lead to cracks somewhere in the block. Still that picture makes it look easy to get a strong flathead setup but as Tech1 said you can easily get a SBC set up for close to 300 H.P. for under $2000.

Don Shillady
Retired Scientist/teen rodder

Originally posted by southerner

Completely reliable !, I have never heard of one dropping a valve

It was last month's (I believe) Hot Rod that just did a 12 page article on the L-Heads

my bone stock falthead in my rambler has gone 120k for me since rebuild and i still haven't had a problem.... but you cant take flatties up to high or you'll be asking for problems.... when taking to hard they tend to overheat, crack valve seats, burn valves, break rods..

i can hook you up with some people around here that could help u be more like 2-4 k lol 5-8 LOL

Speaking of rebuilding a flathead, my father told me that there was a local motor exchange in the Detroit area that would remove your old flathead & install a rebuilt for $29.95. This was just after WWll