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A simple question as I am bench racing in my head - how big can a '37 Packard six be bored out? I rebuilt the engine in my 115 and went 1/16 over to run 38 stock pistons, but it seems like there is a lot more room. How much room is there?

1/16th comes out to .060 over, being .60 over is about the max usually....so id say not much more. but I'm no expert. ultrasonic testing would be needed of the bores.

then again a 55 block can be bored to a 374....can it go over 374...maybe... as I'm sure they did in 56 with oversize for the 374? I went from 352 to 374 on my 55 with stock 374 std pistons. no issues.

Yes, No, Maybe.

This is old junk we're discussing, so how it was treated over the years is equally important to how thick it was originally.

We recently had a 374" open a hole in a rear cylinder at .060". That block had the gnastiest water passages ever seen; the surmise is a previous owner put lots of stop-leak compound in there. Whatever, there was a weak spot. A Packard V8 is always safe to bore .125", except when it isn't.

Sonic testing is a must if large overbores are contemplated, but from experience, it is impossible to get a valid reading for every square inch of every cylinder. Sometimes, even with best practices, shit happens.

Bottom line, your 6-cylinder would probably be safe with another .060", except when it isn't.

(Henry, remember there were also 320"s in '55 ;>)

jack vines

Yup sorry I had 352 on the brain. I went .030 over on the 37 because one piston was already .020 over.

For a little background, I went 1/16 over because the only change between the block in '37 and '38 was the bore. Same casting, but they changed the bore from 3-7/16 to 3-1/2. I guess using the .060 logic, I could go 0.060 from 3-1/2". That should take the motor from 237 ci to 253 ci, or about a 7% increase. The goal is to make maximum power, just not sure if this would be worth it.

I am running 3 9/16 International truck (1600 Series) pistons in my 38 Six, together with 0.125 from the head. Have done so since 1976 without any problems. Goes very well with the R6 overdrive. Also changed out the rear axle from 4.54 to 4.36 ( 120 ). I wouldn't know if the 115 has the same block but it would be getting pretty thin with an 0.125 inch overbore. Good luck. Peter Toet

On engines made before the early sixties it was typical to bore up to 1/8". They were designed with thick cylinders. It was assumed the engine might be rebuilt once or twice in its working life. Some Oldsmobile V8 blocks could be bored 1/4" that is .250 oversize.

That was on a fairly new block that did not have corrosion problems or core shift. You could be a bit chancy because there were enough engines around that if you ruined a block you could easily get another one.

Today I would suggest keeping any overbore to the minimum necessary to true up the cylinders especially on an engine from the thirties.

If 60 thousandths over is the fractional HP gain you're after, you might want to reconsider or have very low HP goals. There are better ways.
Boring is for compression integrity (ring seal / wear / damage).

Quote:

As a very old hot rodder, I'd respectfully beg to differ. Boring increases displacement and compression ratio. Both increase horsepower, as long as cylinder integrity is not compromised thereby.

But yes, agree, any flathead has very low horsepower goals; the horsepower increase from a .060" overbore would in the low single digits.

But, by all means, share with us the better ways. AFAIK, there are no easy, inexpensive means for substantial horsepower increases in seventy-year-old flatheads.

jack vines

Do the math.
60 thou is like 3% on a 4" bore. Maybe 3 HP at most. It's actually less do to friction and flow disturbances. That's take the air cleaner off HP.
Not intending to offend anyone or change the discussion, just bringing the numbers.