239 bore.
 

If a standard piston is 3.185, what is the optimum bore size at bottom of cylinder ?

How much wear at bottom and how much taper is allowable at top of bore for just a reasonable runnning engine that will never see a lot of miles.

ie, fitting new pistons into an existing bore.

Re: 239 bore.
 

I don't know the specs off the top of my head, but there are several things to consider. Taper, how egg shaped/out of round the bore is, ridge at top of the cylinder, etc. Ridge must be removed or there is a good chance of breaking the top ring.

Per Repair Manual, Ford, Mercury & Truck, V8 Engines, 1937 to 1947, Ford Motor Company, July 7, 1947:

Ware limits

Bore out of round - .003 max
Bore taper - .006 max
Bore ware - .006 max
Piston to bore - .005 thickness gauge 1/2 wide 6 to 10 lbs pull

Re: 239 bore.
 

Side note: I thought the 3.187 bore was for a 221 cube flattie? A 239 flattie would have 3.375 bore? Correct?

Re: 239 bore.
 

OOPS...I was waaay wrong - sorry!

Re: 239 bore.
 

a 239 stock engine is 3 3/16" inch bore (3 /.187") using hasting steel vent rings I have gone way past the books engine tolerance specs, ran strong and no smoke, good for around 40,000 miles

Re: 239 bore.
 

The auto-thermic pistons can be set to .0015/.002 skirt to wall clearance. This style piston was commonly used from 45/53, and most replacement pistons for the 39 up are of this style. Not sure of the early pistons. .006 is alittle much.

Re: 239 bore.
 

You need to use a bore gauge and check the taper in the bores frist If you have 6th taper all you are doing by putting new pistons and rings in is adding a band aid to the problem.

Re: 239 bore.
 

As a quick inspection, see how much ridge there is at the top. As someone said you need to remove it to prevent breaking rings. Wear typically occurs more at the top due to the ring pressure. Someone said with just new rings you're putting a band aid on it but if funds are lacking and you just want it to run reasonably well, new rings may do what you need. If this is an 8ba and this far apart, new bearings might also be a good idea.

Re: 239 bore.
 

To get a close idea of your taper all that is required is square up one of your existing rings in the bottom of the bore and measure the end gap.Do that three times starting at the bottom then half way up and last at the top under the ridge compare your findings as to end gap and you can calculate the taper as the end gap increases with the formula of PI. Every .001" increase in bore will give an increase of .00314" end gap.

R

Re: 239 bore.
 

Interesting approach Ronnie.

Re: 239 bore.
 

Not my first rodeo :) It does work and you know where I learned that don't you "lol"

R

Re: 239 bore.
 

Why didn't I think of that. Trying to get my big hands, and an inside mic just right is a chore I can do without.
Thanks Ronnie
Bruce

Re: 239 bore.
 

I use the same technique as Ronnie. Probably obvious but use a piston to push the ring into position to make certain it's square in the bore.

Re: 239 bore.
 

I'm spoiled, I always get someone to loan me a bore gauge! I don't like telescope gauges myself, to hard to get consistent readings.

Re: 239 bore.
 

Exactly! And a good 3 point bore gauge - not one of the el-cheapo ones . . . thy typically cost over $1000 new!

Re: 239 bore.
 

The setting gauge for the bore gauge isn´t a to fun investment either...but if you can´t trust what you measure it´s not much good either.

Re: 239 bore.
 

First you need to be consistent.I have grown up using measuring equipment.Telescope measuring is a good way to get a measurement but then you need a micrometer to read the gauge.You now have 2 investments.Most non machinist people don't know how to use a telescoping gauge.I have seen some struggling and not liking the results.It takes 10 seconds to get the size done.If you are fiddling around you are doing it wrong.

R

Re: 239 bore.
 

Yes - and I have used them a plenty. But when you're trying to measure the bore of something - especially a LONG cylinder, it is sure nice to have a precision dial bore gauge . . . that you can slide up/down the bore - then switch to a 90 degree location and do it again . . . just can't beat it. (Now that is just one mans opinion!). Same goes for measuring the mains on an engine - using a LONG dial bore gauge that makes it easy to start on #1 and work your way back.

Re: 239 bore.
 

Ronnie, thanks for good trick.

Frank, absolutely zero ridge , I cannot understand it on a standard bore.

Block is in shop now to get a guy to measure bore.

Have another Q, are (mushroom) intake and exhaust valves identical ?

I have a box of stainless valves that look almost new here.

Are these Ok to use in a standard block ? ie no hardened seats .

Re: 239 bore.
 

I've always seen different valves for intakes versus exhausts from the factory in all the 32-48 engines (from the factory). Now, I'm sure there are many cases where this doesn't hold up - but generally it was true.

With that said - I've taken apart many rebuilt flatheads where all sorts of intake versus exhaust combinations were used - so I don't know if it actually makes any difference. I don't know if there were different alloys or material properties between the two different valves from Ford.

Anybody know the details?

B&S