DSR

Block casting number 870. I think that is 1962 - 1965. How far can you safely bore out this engine block for a street engine approximately 350 horsepower.

Dave

Mike Smith

You should be able to go .060, probably more, but why? 350 HP is easily attainable with the original displacement.

63Corvette

.030 easily...............according to GM, .060 is the maximum, but some have gone bigger after sonic testing of the block to check for core shift.

DUB

ALSO...let us not forget ..where are you getting the HP rating from...the back of the crankshaft...OR.... the point where the rubber meets the road????

There is a BIG difference in where you are getting it from. Because IF you are wanting it where the tires hit the ground (like on a chassis dyno)...the amount of money increases quite a bit.

I agree. Boring it out to .060" is as far as I would take it IF I had to do that.

DUB

C2Racer

At .60 over that's a 331CI which is what the race motors are. At 12/1 CR and 110 fuel we make a lot more than 350hp so I agree with Garrett that 350hp is easily obtainable with stock or .30 over. Remember, an engine is a pump and most of the power is made in the heads with flow into and out of the pump. Now carb it and cam it correctly, get the exhaust out and you got it.

Steve

DZAUTO

Also, it is super easy to jump up to 350 cubic inches. Simply round up a 350 crank and have the main journals turned down to the size of 327 mains and drop in.

SWCDuke

A 30 over 327 is 331.6; 60 over is 336.6.

A 327 with OE type massaged heads, LT-1 cam, and a true CR of about 10.35 will make at least 350 honest SAE gross HP at 6500, which is about 310 SAE net at the flywheel and about 265 SAE corrected RWHP with the OE manifolds and under the car exhaust.

There are numerous ways to measure and correct HP readings, so you should be more specific on how the power is measured and what correction factor methodology to use.

You should also be concerned about low end torque. If it doesn't make at least 80 percent of peak at no more than 2000, it will feel soggy in normal driving, and the more you cam for top end power, the more low end power you loose.

Duke

Westlotorn

I always go as small as possible on the overbore, just so I have more options should this engine need to be overhauled again some day. The gain from adding 3 cubic inches going from .030 to .060 is not worth it to me. The block supply is drying up.
If your block is already at .060 I understand your question. Have it sonic tested and if all 8 holes are still thick enough you can go to .080 but not all blocks can do that and you will be moving into custom pistons at a higher price.

fyreline

When you get into the water jacket, stop. You've gone too far.

robert miller

But really I wouldn't go over the 331 ci bore. A lot of people back in the day with 60 over in the 327 really had problems.

On my 327 I have gone 30 over with a set of JE forged pistons. Took the rods and main out to 10 over on the crank. Stock rods with ARP bolts thur the motor. A set of rpm heads new carb on top with a TKO 600 5 speed in the car. Drives like a dream with all the new rubber on the mounts and front end... Robert

Easy Rhino

Over boring a 327 to gain the marginally few more cubic inches for horsepower, if that is the goal, is hardly worth it. At most, you can increase engine displacement by about 1%, and at the extreme amount, risking breaking through or creating weak spots or hot spots.

You can find pistons in ten thousands increments from .010 to .060, and can buy .080, if you're daring.

tbarb

I didn't have a choice, my 327 is .060 but if I ever had to go further I would re sleeve all eight and go back to standard. I think a step is left in the lower cylinder area and the head holds the top of the sleeve. I don't think the water jacket is a issue if for some reason the overbore (.250") cuts into it.

I could be wrong but I think the bigger issue is decking the block to ensure the top surface seals properly because of the stamped #'s but decking is really the correct way to do this.

I forgot to add that my engine seems fine with the .060 overbore and flat tops.

R66

Back in the day, racers used to take a 283 and bore it 0.120 to make a 301. This was before the 302 and 327. It was difficult to find a good block that didn't have core shift and could be bored 0.120.
As I remember, they were plagued with heating problems and split cylinder walls.
0.030 and 0.060 pistons are cheaper than the other sizes, I guess due to popularity. Go as small as you can is what I recommend.

DSR

Thanks for all the input. I'm looking for 870 block. mine is cracked at #4 main. So, the horsepower is not an issue. Finding an 870 block that's not bored too far already is. I drive my car a lot (50,000 miles in 7 summers) and cannot have anything that's going to overheat or have any other problems due to being over bored.

Dave

DSR

It sounds like maybe .060" is the max. I would like to find a block with less than .060" if possible.

Dave

csherman

I just had a 283 block bored .100 over
It had been bored .060 and had knurled pistons
.08 didnt clean it up
It can be done - depends on the block and the expertise of the machine shop.

Crunch527

Why not go with a GM 383 crate engine with a warranty?

You can set it up to look like it is a stock 327...it will easily meet/exceed your hp needs, will have great street manners, will pull like a freight train, and it won't overheat. This route might be a little more money but it is a nice piece.

Just a thought...

Frank

DSR

I just noticed the cracked block yesterday so I'm not sure what route I'm going to go yet.

Crunch527

All good...just trying to offer a different perspective/option...

Good luck with your decision.

Cheers,

Frank

Jackfit

My L 79 original engine with five rebuilds (300,000 miles). Last one took it to .60 in 2010. I have put 70,000 miles on engine since then.

We did sonic test the block before going to .60. It put out close to 400 bls torque on dyno...

I have in it a Comp Cam 256 .....great idle , power at 3,600 burns a qt of oil every 1,000 to 1,300 miles.

Better then in 1969 when it burnt or leaked 1 qt every 300......