318 .40 Over..?

-
nodemon

nodemon

FABO Gold Member
FABO Gold Member
Joined
Mar 21, 2015
Messages
1,138
Reaction score
602
Location
St.Louis
I'm not much of a gear head, horsepower guy.. I just don't have the knowledge.. I bought a 71 318 a couple years ago.
I have not heard it run, and it's been on a stand since purchase.
I was "told" it ran good / strong.. No details given on the rebuild, but it does have a double roller chain and j heads. I'm putting a LD4B intake on it. No idea on cam specs.
I recently took a look inside the cylinders and found the .40 stamped pistons.
I guess my main question is, what is the reason someone chooses .40 over .30 or .20..? Why not .60 over..? May be a dumb, rookie type question, but just wondering.

Thanks.
 
Perhaps it was ether;

It needed to go that much because it was screwed up
Previously bored over and that was the next size up
It was the only piston available
They were free pistons

Oh! The less you bore it out the stronger the piston walls are which promotes good sealing and provides more power because of.
 
Rumble covered it.

But agree, bore as little as possible. .040 wouldn't bother me, for a driver....as long as it doesn't overheat. The .040 over shouldn't cause overheating on its own.
 
Have a few 318's that are .060 and one with a purple cam will not run under 200 degrees and heaven forbid you hit traffic or a stop light. The others run 170s and haven't had an issue and I do tend to beat the tar out of everything I drive because my kids love burnouts... So do I. .040 aint nothin
 
.010 is not going to make it over heat. My stroker is .040 over and it doesn't over heat. You should be looking at an LD 340 intake for those heads, LD4B is a 273/318 head intake.
 
CRUZE 418 said:
.010 is not going to make it over heat. My stroker is .040 over and it doesn't over heat. You should be looking at an LD 340 intake for those heads, LD4B is a 273/318 head intake.
Click to expand...
Good call on the intake. I bought the LD4B for a different 318.
 
nodemon said:
I'm not much of a gear head, horsepower guy.. I just don't have the knowledge.. I bought a 71 318 a couple years ago.
I have not heard it run, and it's been on a stand since purchase.
I was "told" it ran good / strong.. No details given on the rebuild, but it does have a double roller chain and j heads. I'm putting a LD4B intake on it. No idea on cam specs.
I recently took a look inside the cylinders and found the .40 stamped pistons.
I guess my main question is, what is the reason someone chooses .40 over .30 or .20..? Why not .60 over..? May be a dumb, rookie type question, but just wondering.

Thanks.
Click to expand...
Machinists will generally encourage a customer to only bore as much as an engine needs to clean it up. Going from standard bore to even .100 over shows so little addition in power you'd never feel it, so boring is not the way to gain power, despite what the Chevy guys brag about.
 
nodemon said:
I'm not much of a gear head, horsepower guy.. I just don't have the knowledge.. I bought a 71 318 a couple years ago.
I have not heard it run, and it's been on a stand since purchase.
I was "told" it ran good / strong.. No details given on the rebuild, but it does have a double roller chain and j heads. I'm putting a LD4B intake on it. No idea on cam specs.
I recently took a look inside the cylinders and found the .40 stamped pistons.
I guess my main question is, what is the reason someone chooses .40 over .30 or .20..? Why not .60 over..? May be a dumb, rookie type question, but just wondering.

Thanks.
Click to expand...
Bores wear...
They machine/bore to the next size they can safely make them round again to.

Simple as that.

Next...
 
To answer your question;
I chose 40 over, making the engine 4.04 x 3.58, thus making it equivalent to a stroked 340. Cuz everybody at the car-shows knows that a 360 is a dog, and a stroker-340 is the cat's meow.(NOT) So then, eventually, I stuck my one and only track-ticket in the window, showing 93mph in the Eighth, and that was the end of the trash-talk.
---------------------------------------------------------------
additionally;
I once had a seemingly unsolvable overheat issue, that turned out to be tight ring-gaps. All I did was tear the engine down, had the machine shop hone out the cylinders another one-half thou, open the ring-gaps, and reassemble it. Instantly it normalized. This on an 360 at 40-over.
The first trip around the block with the bigger ring-gaps, I knew my troubles were over.
Over the following weeks I increased the minimum coolant temp to near 200*F. Right soon after that I added a Thermostatic fan clutch to a 7-Blade factory A/C fan, and the engine has run 205>207 by IR gun, ever since.
I mean I had tried all the usual stuff.............. Hi Flow pump and stat, 7-blade direct drive fan, straight water, lots of timing, a 7-qt deep-sump oilpan, ventilating the hood, all molded hoses with the lower one having the anti-collapse spring in it. The only thing I didn't try was water wetter.
Prior to that;
she would heat up in traffic, run hot on the hiway, couldn't idle it without the temp-gauge rising, and EVERY TIME I shut it off, it would lock up for 15>20 minutes, until, with the hood open, it had cooled down enough so that the brand new Dakota starter could crank it. then as soon as it was running, I got it up to speed, for ram-air assist.
Afterwards;
the bonus was that the car gained a bit of power, and the tune stabilized, I was able to idle it way down, and the fuel-economy went UP!.
It was awful hectic that summer, but the next spring, it was all worth it.

And then, I cut a hole in the hood, and sealed it to the carb....................
 
Last edited:
You must log in or register to reply here.
-

Back
Top