The Stovebolt Site Forums The Shop Area Engine Shop My '57 235cui Bored Out , 40 over

Hi guys,
looking for some advice here. I recently brought my 1957 Chevy 235 to the engine rebuilder. The motor was burning oil, and was very tired with 160,000 plus miles. The builder said the motor was run down pretty good. He had to bore it out 40over. I was concerned with overheating issues or other problems at this bore level. He told me I have nothing to worry about, and that 40 over was no problem for this motor.
I will have a Fenton dual intake with Fenton exaust on it as well. Anybody on here have a motor bored out 40 over that can give any advice? Is the builder correct in saying that I wont have issues? THANKS - J.R.

The 235 block was built on the old school principle that "bigger is better", and the cylinder walls will stand overbores of .080" for routine rebuilding, and bit more for hotrodding. The 194/230/250/292 blocks are the thinwall casting type, and .040 over is probably the most they should be overbored. As long as you've got a clean radiator in good condition with enough air moving over it, you won't have a problem with that 235.
Jerry

Yes, got the new radiator. Thanks for the advice.
Anybody know how much this will increase my horsepower numbers?

I have esentially the same thing and it desktop dynos at about 172 HP. I don't recall the RPM or torque numbers. Frankly it is all more for looks than power IMO.

I think my 57 235 was bored .030 or bigger and one cylinder sleeved and it runs 185 all day long in traffic or on the road,I did have a 4 row core installed in the radiator and found a fan shroud from the larger trucks. Jeff

According to the 1955 manual, .040" over was a standard procedure and GM provided pistons for it.

No need to worry about it. Mine's been bored .060, and I don't have any overheating issues at all. Like Jeery put it, a good clean radiator with proper airflow will keep it plenty cool.

The problem was with engines that had seen enough service to require that much overbore, in vehicles that had never had a radiator reworked. The extra heat transfer to the coolant, combined with a dirty, limed-up radiator and a leaky pressure cap would cause overheating problems. Since the engine was the only thing that had been worked on, the rebore had to be the problem, right?

I always reccomended a radiatior job, and new belts and hoses all over along with the cost of the rebuild. The extra few bucks was cheap insurance, and if the customer refused to go the extra mile, that refusal would void my warranty. A few pepole had to burn up the first engine and pay me to redo it, plus the radiator work, before they saw the light.
Jerry

So what's the displacement of a 235 with a 0.040" overbore?

((bore / 2)^2 * pi) * stroke * cylinders = displacement

so...

(((3.5625 / 2) + 0.04)^2 * 3.1415926) * 3.9375 * 6 = 246.18

While you are at it, why don't you consider milling the
head a small amount as well? It will increase the
compression ratio somewhat, give a little boost in
power, and increase gas mileage noticeably.
How much? you ask. Try maybe .040 thousands. Then
tell your machinest that when he seats the intake valves,
be sure that the face of the intake valves are even
or below the new surface of the head. And yes, you
will still be able to use 87octane gas. you go man...

Try:

((bore / 2)^2 * pi) * stroke * cylinders = displacement

so...

((3.5625 / 2) + 0.04/2)^2 * 3.1415926) * 3.9375 * 6 = 240.81 cubic inches

Back in the early fifties, many 235s were bored 3/32" (0.09375") to give 248 cubic inches.

If you mill the head to raise compression, then recess the valves so they don't hit the pistons aren't you taking one step forward and one step backward. Isn't it all about volume?

Stuart

The amount the valves are recessed would't really alter the overall volume enough to make a difference. With the 235, you might not even have to recess them, given the size of the chambers and the angles the valves sit at. I had mine milled .030, the valves weren't recessed in mine.

Oops. Thanks for correcting my math hoyt!

And his reply will be: $$$$$$$$$


As Stuart points out it's all about volume which would make the overbore contribute to compression in the same way. Does anyone know by how much? My .080" over 235 makes 150# on all cylinders and I never touched the head.

The quick & dirty way to check for piston to valve clearance is to get the piston to the top of the stroke on overlap (exhaust closing/intake opening) and pry the rocker arm down until the valve touches the piston. A dial indicator on the spring retainer will show how much the head can be milled without piston/valve contact. I like to keep at least 1/16" of safety margin on street engines, or a bit more, maybe .090" if the engine will be revved close to the valve float point on occasion.
Jerry

I milled my head as much as possible and raised compression to about 9.5:1, and didn't have any piston to valve clearance issues. But that was with a stock cam.
When you upgrade the cam, you really should check.

And for the person asking if you were taking two steps forward and one step back by milling the head, no you aren't.
It's because when you measure displacement of the engine, it is the volume that the piston displaces, determined by the sweep of the piston. No, it's more like taking 3 steps forward and no steps back.

On the formula, I knew how to calculate it, but didn't know the bore and stroke of a 235. But thank you.

Hot Rod Dad,

I thought the question was about compression ratio, not displacement. If you milled .040 off the head, this would change the compression ratio...it would raise it.

If you turned around and sunk the valves in seats so they wouldn't hit the pistons, wouldn't you be doing the reverse of what you did when you milled the head?

Isn't compression ratio the difference (ratio) between the volume of the cylinder at BDC and TDC. If milling reduces the volume then wouldn't sinking the valves increase it again.

Is my thinking defective!

Stuart

Sinking the intake valves by 0.080" (or whatever was milled off of the head) will increase the combustion chamber volume much less than the combustion chamber volume is decreased by milling the head. Thus, there will be a net reduction in combustion chamber volume and increase in compression ratio.

Hoyt

True.
But there is enough meat on that head to shave off quite a bit without having to worry about it.

You can also get a new set of pistons clearanced for valves by a machinist. They just cut the scallops in it. They should be able to handle that.

You should always check for piston to valve clearance though. It's very easy to do. If you deck the block or shave the head, it could be critical. Especially if you put in a cam that has more duration.