Flathead Bore Size ?

Whats your favorite bore size using a 4 inch merc crankshaft

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    62
  • Poll closed .
I'd probably go for +.080 unless I was building for performance. It's a decent overbore over stock, would clean up most bores (except maybe a +.060) and is the biggest size you can use with stock rather than big bore gaskets.

You've still got +.100, +.125 left to go after that, but as said, that option would probably be a consideration for someone else.

Mart.
 
For most flathead REBUILDS (meaning after hot rodding) the block will clean at .010 or .020 at the most so I go big the first time.
The customer is always more pleased with the hp a 315 ci engine puts out than one under 280 ci.
A big bore can be left with .020 and get 2 sometimes 3 rebuilds with nothing but honing. I have found that 3 3/8+.030 is a safe bore with many 8ba blocks.
I always sonic check them to be sure.

Unless they are racing, most people could never wear out 3 flathead rebuilds.
I tell the customer, they are paying for the engine so THEY should enjoy it to the maximum and don't worry about the next owner.
Going conservative is like putting seat covers on a new car so the next owner can enjoy the new seats.
 
Pete said:
For most flathead REBUILDS (meaning after hot rodding) the block will clean at .010 or .020 at the most so I go big the first time.
The customer is always more pleased with the hp a 315 ci engine puts out than one under 280 ci.
A big bore can be left with .020 and get 2 sometimes 3 rebuilds with nothing but honing. I have found that 3 3/8+.030 is a safe bore with many 8ba blocks.
I always sonic check them to be sure.

Unless they are racing, most people could never wear out 3 flathead rebuilds.
I tell the customer, they are paying for the engine so THEY should enjoy it to the maximum and don't worry about the next owner.
Going conservative is like putting seat covers on a new car so the next owner can enjoy the new seats.
Click to expand...
Or more to the point - like not having sex with your girlfriend, so the next guy can enjoy it!
 
I guess it is good to be in America where opinions can be expressed no matter the quality of content.
 
We run a 99 engine in vintage competition with a 3 5/16 bore. Seems to have been the preferred performance capacity bore out here in NZ for many years.Cheers.
 
How did that 3 5/16 bore with the stock stroke turned out ?
Cam ? Ive been thinking abiut doing that using an L100 on my 8BA


tubman said:
I did my first a 3 5/16 (276 with a Merc crank) and it worked well. I just bought another set of 3 5/16 pistons (this time for a stock stroke). Pistons, rings, and pins were $100, which is a heck of a lot less than a merc crank (if you can find one) for a comparable increase in displacement.
Click to expand...
 
mercjoe said:
Thanks tube

You kept it single carb ? How does it feel driving it though ?
How about compression drop because of the over bore without stroking ?

I got myself an L100 already and I dont think I will be able to use a Merc crank
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The single carb was for the initial start only; it now has a Navarro Universal dual (like a super with exhaust heat). You will notice the Edmunds heads; they were individually fitted to this engine to optimize quench and compression. After initial run-in, the engine had 150-155 lbs compression on all cylinders.

Unfortunately, advancing age has caught up with me, and progress on the car this is going in has come to almost a complete stop. Spending 6-7 months a year in Florida away from my shop doesn't help either. When I get back this spring I'm going to try to finish the rear suspension and get to final assembly.
 
Sound really good. Hope you can finish the car soon

I just found out there are Ross specific pistons for 3" 5/16
Bore and 3.75 stroke :D


tubman said:
The single carb was for the initial start only; it now has a Navarro Universal dual (like a super with exhaust heat). You will notice the Edmunds heads; they were individually fitted to this engine to optimize quench and compression. After initial run-in, the engine had 150-155 lbs compression on all cylinders.

Unfortunately, advancing age has caught up with me, and progress on the car this is going in has come to almost a complete stop. Spending 6-7 months a year in Florida away from my shop doesn't help either. When I get back this spring I'm going to try to finish the rear suspension and get to final assembly.
Click to expand...
 
On this one, I used pistons from Speedway Motors I got from their "Garage Sale" section about 10 years ago. Less than $100 for pistons, rings, and pins. I was a little leery about them at that price, but bought them anyway. I took them to my engine builder, and he checked them out and said they were fine. They were 4 ring pistons (an extra oil ring at the bottom of the piston), but after several discussions with the gurus here and on the H.A.M.B., decided to run them without the bottom ring. That meant I had to have the rotating assembly balanced, but you should do that anyway. Although it has minimum hours, the engine started immediately the first time and it runs, well you can see for yourself. After replacing the Speedway oil pump with a used genuine Ford pump with a new relief spring, oil pressure is factory spec (25 psi hot idle and 57 psi hot at 2000 rpm). The compression and vacuum numbers are where they should be and I am completely satisfied with the build.

In my opinion, a 258" engine like this is actually superior to a stock bore and stroke Mercury as it has 3 more cubic inches, and lower piston speed (which is always good). If I ever decide to rebuild the 255 merc in my '51, you can bet it'll get 1/8" over pistons, even if it doesn't need them.
 
Seth Swoboda said:
I agree. I never bore more than is needed to clean up the cylinder walls.
Click to expand...

Sure, but Seth, Aren;t you still in your twenties? :)

For us 70+ characters, I say

" Gimme one two five on the overbore,
Then stomp the ol' go-pedal hard to the floor.
Might not win the race
But you'll never lose face
And you'll bury that dude with your Roar."

Go big or go home. And Seth, I once felt as you do, but I have ten good blocks, and see many more coming into the stream. Don't make me write a poem about ' The block hoarder, who had a disorder.'....
 
Another thing. The way we use and maintain these engines means that they would probably get rebuilt as soon as they started to blow a puff of smoke or lose a little compression. This can be solved with a hone and a new set of rings. That means another rebuild or two before another bore job is needed. Experience with my car tells me that they will go 75-80 thousand miles before they need any work if maintained properly. That's a lot of miles on one bore job.

Having replaced a tired engine with a healthy one, I know it really increases your enjoyment in a car.
 
Ive always thought the 0.125 overbore went hand in hand with a Merc crank swap. Specially when using a performance cam like the L100 due to the low end drop, etc.


tubman said:
On this one, I used pistons from Speedway Motors I got from their "Garage Sale" section about 10 years ago. Less than $100 for pistons, rings, and pins. I was a little leery about them at that price, but bought them anyway. I took them to my engine builder, and he checked them out and said they were fine. They were 4 ring pistons (an extra oil ring at the bottom of the piston), but after several discussions with the gurus here and on the H.A.M.B., decided to run them without the bottom ring. That meant I had to have the rotating assembly balanced, but you should do that anyway. Although it has minimum hours, the engine started immediately the first time and it runs, well you can see for yourself. After replacing the Speedway oil pump with a used genuine Ford pump with a new relief spring, oil pressure is factory spec (25 psi hot idle and 57 psi hot at 2000 rpm). The compression and vacuum numbers are where they should be and I am completely satisfied with the build.

In my opinion, a 258" engine like this is actually superior to a stock bore and stroke Mercury as it has 3 more cubic inches, and lower piston speed (which is always good). If I ever decide to rebuild the 255 merc in my '51, you can bet it'll get 1/8" over pistons, even if it doesn't need them.
Click to expand...
 
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