3 3/8 bore on a 8ba ?

Ok I am building a 8ba . I have a 4" merc crank, finally rounded up a good block, well pretty good anyway its gonna need a healthy bore - was thinking .125 but I wanted hypermax pistons, all I can find is 3 3/8 when running a 4" crank. The water passages look very good with no piting. The motor will have 5-6 lbs of boost from the blower max. Is that bore just asking for trouble ? I assume any future rebuild would include a set of sleves. Just fishing for some advise I tried the search function but didn't come up with what I was looking for, thanks

 

I'm not the flattie expert, but I think that once you go 3 3/8, well, that's it-anything else and your block is dust...(okay, not counting sleeves...) And while the flattie has enough meat to go 3 3/8", your cylinder walls will still be thin enough to enhance any heat issues you might have with a flattie. It's just the way it is (thinner walls-higher temps). Lots of guys have done this (even with a blower with 5-6 lbs of boost, I believe) with no real issues...

 

Have it sonic tested

 

So is sonic testing commonlly avaiable?

 

I always thought a 3 3/8 bore was plenty safe in a Flathead until: I bought a 3 3/8 shortblock at a swapmeet many years ago; refreshed it and installed in my '32 coupe; drove it for a long time (not many miles); decided to tear it down due to heavy smoke out of one side; discovered a scored cylinder from broken ring; elected to bore another .030 across the board (all the cylinders had wear in them); went thru to water on the opposite side of engine. At + .030 on the bore, that means that removing .015 went thru-that's all the cylinder wall I'd had all that time. Yet, Rumbleseat (Old Ford Barn) has run a 3 7/16 for tens of thousands of miles and last I knew thought he could rebuild it one more time.

 

I ran a 3 3/8 X 4 8 BA, it has since been sold. The old guys say its OK, like you, I had the pistons so I took the chance. Go for it, cut back the boost to 4. Mine cooled fine NA. In a light car it will blow off 350 chevs before breakfast.

 

Could be very well wrong, but have always been told that .125 is about max on boring the Henry Flathead as general rule of thumb by the guy's that rebuild.

 

mastergin,
For sonic testing you may have to go to Tulsa or Okie City. Ya know who would know for sure is Roothog. You may want to drop him a note.

For a bog bore it is common to sonic test first, even if it is a popular bore. Lots of flatties have been hawged out that much but I think sonic testing is a good idea.

 

My last sonic test cost $60...cheap insurance up front. Not all blocks are created equal (core shifts, etc) and 60 years adds a few more unknowns

 

My avatar has a 3 3/8 X 4 merc and stays plenty cool.

 

As difficult (and expensive) as it can be to find a good block anymore, if it were mine, I'd want to make sure that particular block would stand it before I fired up the boring bar. Remember, back in the heyday of 3 3/8 inch flatheads, every junkyard had 100 more if one didn't work out.

 

I work on a lot of flat heads I sonic test every one that is to be bored and about 40 % are too thin to go 1\8". You can roll one across the floor and get a quart of sand from inside water jackets so all I am saying is get it sonic checked first just my 2 cents worth.