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Old 09-03-2020, 08:28 PM   #1
alanwoodieman
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Default 255 merc engine

how much bore out can a street engine take--looking at 100 to 120 over

overheating?

wall thickness?

sonic test?
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Old 09-03-2020, 08:31 PM   #2
JSeery
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Default Re: 255 merc engine

It will always depend on a sonic test because of core shift and possible corrosion inside the water jackets.
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Old 09-03-2020, 08:43 PM   #3
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Default Re: 255 merc engine

Years ago I bored my 255 to 3.375in. Ran fine up north with cooler ambient temperatures. Ran hotter in in higher ambient trmps. Wall thickness has a lot to do with heat dissapation and cavitation. 3 5/16 bore seems better.

Opinions will vary as you are about to discover.
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Old 09-03-2020, 09:32 PM   #4
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Default Re: 255 merc engine

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Originally Posted by 19Fordy View Post
Opinions will vary as you are about to discover.
Ain't that the truth! Now here's my opinion, LOL!

I'm by no means an expert, but I want as much meat in the cylinder walls as possible. Rust and corrosion plus possible core shift are issues I'd rather not deal with. And don't forget, the cylinders are a structural part of the block, they basically keep the decks in place.

A flathead is never going to make very much power unless you spend HUGE money and do loads of work. At that point, you may as well go with an overhead-valve engine and save all the anguish. Don't get me wrong, I LOVE the flathead V8 and appreciate its smoothness, torquiness, reliability, historical significance and nostalgia, but I don't love 'em for their brute power.

Anecdotally, I've also read that post-war Merc engines aren't that much more potent than the early 221s. I have not experienced this myself, but it seems plausible. So, is a huge overbore worth it? Not for me.
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Old 09-03-2020, 09:47 PM   #5
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Default Re: 255 merc engine

My 276 is over 25 years old and still going strong.
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Old 09-03-2020, 09:54 PM   #6
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Default Re: 255 merc engine

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Admiral makes excellent points. You really don't feel an improvement in performance unless you make other enhanced modifications to the engine. You are better off keeping the bore to the minimum.

Bore a 255 to 296 without making any other engine modifications and you won't be able to tell the difference between the two when driving down the road.
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Old 09-03-2020, 10:38 PM   #7
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Default Re: 255 merc engine

I know all there is to know about flatheads and if you bore it any more than need be, the world will come to an end at midnight.
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Old 09-03-2020, 11:28 PM   #8
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Default Re: 255 merc engine

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I know all there is to know about flatheads and if you bore it any more than need be, the world will come to an end at midnight.
...
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Old 09-04-2020, 06:43 AM   #9
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Default Re: 255 merc engine

In my opinion, performance is gained by sacrificing reliability. but Now that is not a hard rulw but think for a moment what you do. If you stroke it you increase piston speed which promotes wear. A high lift cam requires the valves move further which increases wear. Also consider ramp angles, time on seat to cool. How much of a change it makes I cannot say but you cannot deny it. A five second dragster turning 10,000 rpm gets torn down after every run in spite of the crank making fewer than 1000 turns on the run.
I'm not against sacrifices to the gods of speed just figure out what you want it to do.
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Old 09-04-2020, 06:51 AM   #10
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Default Re: 255 merc engine

what is happening is the usable cores for 239/255 8ba, 8rt, 8cm we are finding all are just worn out, been rebuilt in the past to 60/80 over and we trying to avoid the cost of sleeving 8 cylinders. I defer to the older timers than me to use their knowledge about what could be considered a good usable street engine
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Old 09-04-2020, 07:34 AM   #11
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Default Re: 255 merc engine

Quote:
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I know all there is to know about flatheads and if you bore it any more than need be, the world will come to an end at midnight.
Shit! My world came to an end in 1965.
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Old 09-04-2020, 08:54 AM   #12
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Default Re: 255 merc engine

I bore all my engines .125" over, using Ehhe 3 ring pistons. I did this for over 10 years while running "Nostalgia Engines". Several were bored .187" over. When I raced I had several that were .250" Over.
Now for the next question? Will just boring the engine improve performance? The engine in my truck is bone stock, except for the pistons. Egge makes .020/.030" over 3 5/16. It was an old '"276 street" motor bored .020, Thus I have 279.4 CI. I call it My 280, Engine has EAB heads& cam. Trans is 3sp W/OD rear is 3.73.
She climbs a 14% grade in OD AT 43 MPH. Fun engine
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Old 09-04-2020, 08:59 AM   #13
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Default Re: 255 merc engine

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Shit! My world came to an end in 1965.
What happened?
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Old 09-04-2020, 10:17 AM   #14
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Default Re: 255 merc engine

Is there any appreciable difference in power from boring .060-.080 versus.125 or more ?
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Old 09-04-2020, 10:35 AM   #15
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Default Re: 255 merc engine

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Is there any appreciable difference in power from boring .060-.080 versus.125 or more ?
Cubic inches equals torque and torque x rpm equals horsepower. It's not totally true, but the old saying was "you can't beat cubic inches". Cubic inches relates to the volume of air that can be moved per rpm. The rest of the engine has to contribute to the air volume movement, but you need the cubic inches there. At higher rpms many factors come into play, but at low rpms cubic inches is the big one.
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Old 09-04-2020, 11:47 AM   #16
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Default Re: 255 merc engine

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Originally Posted by old Tom View Post
What happened?
I had my first flathead bored .125" (obviously).
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Old 09-04-2020, 12:31 PM   #17
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Default Re: 255 merc engine

A sonic check will tell how much will be left if you want to bore to a certain size. A person can always bore it way out there and sleeve is for specific wall thickness but that stuff is a lot more expensive than just boring it far enough to keep the walls a safe thickness.

I hope Tod casts some new blocks one of these days so a person can feel a bit safer about corrosion. You ever know what some folks ran for coolant in these old engine blocks. Some could have some pretty deep corrosion pits in there.
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Old 09-04-2020, 03:56 PM   #18
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Default Re: 255 merc engine

What most people here don't understand is the fact that the stock compression is very low. Don't be mislead by the factory 6.8 & 7.2 numbers. Back when I wrote my book I checked many heads and found the average Cr was less than 6:1 angle milling .070" from an8ba head will reduce the chamber volume to less than 70cc, and a stock head will still clear a cam with .350 lift. the 1/8 bore will add another point in cR. But your still at only 8:1, but better than 6
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Old 09-04-2020, 07:09 PM   #19
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Default Re: 255 merc engine

I'm with "Ol' Ron"; the most important thing with a flathead is to get the combustion chamber right.
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Old 09-04-2020, 09:30 PM   #20
alanwoodieman
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Default Re: 255 merc engine

what about milling a merc head?
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