Cylinder wall thickness after boring a flathead

[JM 35 Sedan]

I was asked this question by an engine builder who's primary business is build 4 cylinder model T, A, and B engines, but also does a few flathead V-8's each year.....

"what is the MINIMUM (average) allowable cylinder wall thickness (before you would begin having performance issues) after boring a flathead V-8 block of any vintage? This would include the early 21 stud engines from '32 to early '38, the early 24 stud engines from late '38 to '42?, the 59 series from '45 to '48 (at least 6 different alloyed cast irons were used here), and finally the 8ba/7rt engines from '48 to '53/'54 Canadian?

Anyone having any knowledge and/or any actual experience on boring any or all of the different series of flatheads mentioned above, please respond here on this thread, or if that's not to your liking, send me a pm.

thank you in advance for any information you may have on this question.

[Walt Dupont--Me.]

This is just me. I don't recommend over .060 on the 32=36 they seem to have pretty thin walls, the 37 38---21 stud I've gone .125 but it's pretty close to lower head bolt hole, the 39-41 I remove the tin cans sleeves and bore .045 to 3-3/16 I've done a lot of them, makes a good little engine with the A heads, 46-53 blocks I've bored .187 to 3-3/8 many times, some have bored to 3-7/16 but I haven't had the guts. I'm sure you'll find a core shift once in awhile but I've been lucky. I've seen a lot of those 32-36 that was so rusty in the block they have rusted right through wall. Just Me Walt

[chuck stevens]

John, I don't know what the cylinder wall thickness turned out to be but, I have a 8BA bored out to 3 3/8" and have driven it many many miles with no issues. My 39 that we took on the blue ridge prkw with you is out to 3 3/16", it has stock 239 pistons, it was done in the 1970's no troubles yet, Chuck S.

[rotorwrench]

Every block is just a little different than the next so each one has to be scrutinized for what condition it's in. Cylinder walls can be tested with ultrasonic systems that can tell a person what the exact thickness is at each station checked but corrosion doesn't have a predictable nature. I would say that an engine has to have an average thickness that is near to the thinnest workable cast iron sleeve that is available for good strength and cooling capacity.

The engine blocks of the 239 CID era have the thickest walls to start with and they are a lot thicker than more modern engines like the Ford 302 V8.

[deuce_roadster]

My machinist who bores my blocks insists after THOROUGH sonic testing a minimum .100 wall thickness when finished.

[flatjack9]

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I think .100 is usually accepted as a minimum wall thickness.

[Pete]

I use .090.
I usually have to go through 5 or 6 blocks to find a good one.

[JM 35 Sedan]

All good information guys, really appreciate the responses so far. Wondering if ol' Ron and JWL have anything to add here?

[JWL]

In asking about "average" wall thickness the theoretical could be .,010 on one side and .400 at the other side which would be near, a very desirable, .200 average and sure to fail.

In reality, if there is a general area, at the upper 2 inches of bore length, which shows less than .100 it would be cause for concern. Ford cast the upper(approx.) 1 1/2" of each wall thicker than the areas below which helps meet this criteria. A pin-point thinner spot is less troublesome and some have probably been considerably less than .100. The long term flexing cycles of an engine built for 20+ years of service deserve more consideration than short-use engines. Although some short term engines are built to produce higher cylinder pressures and this, too, requires individual judgement.

Those who have read my book learned the use of sonic micrometers is no guarantee of cylinder wall viability. The area being measured in the focal point of the transducer is very small so a thorough examination of a cylinder wall is very time consuming and probably only done by those who own a sonic checking device.

[flatheadmurre]

Thrust side needing a bit more strenght the pinside to if its the pure force that is considered.
What comes first flexing giving leaks stress related failure of walls or...

[Bored&Stroked]

I've heard some guys go as low as .080, but I sure like to be not less than .100. I have a sonic tester and last year I tested three blocks that had been bored to 3 3/8. They were all between .090 and .110. And the old wives tales about certain '59x' blocks or 41-42 Keystone blocks being thicker . . . I don't believe it as it is not what my sonic tester has shown me. Also, rust can surely take it's toll on these old blocks, so who knows how thick some of them were when they started out.

With that said, the thinner you get them, the more chances for a problem spot due to corrosion, core shift, whatever. One thing nice about a sonic tester is that you can check all bores in detail (before you bore), see if there is any core shift and make decisions as to whether or not to "move" certain bores a bit (to have more even material around the bore).

Lastly, the final application can come into play. If you're building competition engines and looking for max cubic inches, you also have to think about whether or not it is supercharged, running nitro, etc.. Cylinder pressure is cylinder pressure - the more you have, the more chance of splitting a cylinder wall.

[scooder]

Originally Posted by Pete

I use .090.
I usually have to go through 5 or 6 blocks to find a good one.

What are you boring too to have this "rejection" rate?
Martin.

[Ol' Ron]

Having bored several blocks for sleeves I find the brake through comes near the bottom. In the past I've built 2 engines of 3 7/16 bore one is still running after over 20 years and I have no Idea where the other is, but it ran in the stock car for a season, before we switched to a belly button engine.

[Pete]

Originally Posted by scooder

what are you boring too to have this "rejection" rate?
Martin.

3.438

[scooder]

Thanks Pete, that's fairly large 3 7/16" (rounded up) in old money. I spose your rejects could probably go 3 3/8" and still have over 0.090" wall?
Martin.

[Pete]

Yes, if wall thickness was the only flaw.

[scooder]

Originally Posted by Bored&Stroked

I've heard some guys go as low as .080, but I sure like to be not less than .100. I have a sonic tester and last year I tested three blocks that had been bored to 3 3/8. They were all between .090 and .110. And the old wives tales about certain '59x' blocks or 41-42 Keystone blocks being thicker . . . I don't believe it as it is not what my sonic tester has shown me. Also, rust can surely take it's toll on these old blocks, so who knows how thick some of them were when they started out.

With that said, the thinner you get them, the more chances for a problem spot due to corrosion, core shift, whatever. One thing nice about a sonic tester is that you can check all bores in detail (before you bore), see if there is any core shift and make decisions as to whether or not to "move" certain bores a bit (to have more even material around the bore).

Lastly, the final application can come into play. If you're building competition engines and looking for max cubic inches, you also have to think about whether or not it is supercharged, running nitro, etc.. Cylinder pressure is cylinder pressure - the more you have, the more chance of splitting a cylinder wall.

I don't know about the 41-42 "key stone" blocks, but I got a couple of 99A blocks here and the walls are considerably thicker, almost stupid thick, than the 59 (any letter) and 8BA blocks I've sniffed inside. The 99A's here also don't have the wall that gets thinner half way down like I've seen in the others.
The 99A also has the key stone water hole but is a different casting than the 41-42 blocks. The cylinder wall is only a snige short of that key stone hole.
Just adding my observations.
Martin.

[rotorwrench]

They did change the casting for the 19A blocks with the stepped intake deck surface and they did away with the plugs on the pan rails. I don't know about wall thickness though. The 29A stuff is kind of rare and the Military V8 stuff from 43 & 44 is an unknown quantity.

[Bored&Stroked]

Originally Posted by scooder

I don't know about the 41-42 "key stone" blocks, but I got a couple of 99A blocks here and the walls are considerably thicker, almost stupid thick, than the 59 (any letter) and 8BA blocks I've sniffed inside. The 99A's here also don't have the wall that gets thinner half way down like I've seen in the others.
The 99A also has the key stone water hole but is a different casting than the 41-42 blocks. The cylinder wall is only a snige short of that key stone hole.
Just adding my observations.
Martin.

So that is a 39 Merc block - must still have the soft plugs on the pan rails? I've never had one of those, would love to get my 'sonic tester' into it! Have you ever sonic tested one - with a STD bore?

Anybody got a 99A block for me to acquire???

D

[scooder]

Yes Dale, got the pan rail plugs.
One of them is still standard bore, I haven't sonic tested one yet. When I have some spare "messing" time I'll dig the standard bore one out and see what it test out too. I'll shout when I got some numbers.
Martin.

[Bored&Stroked]

Originally Posted by scooder

Yes Dale, got the pan rail plugs.
One of them is still standard bore, I haven't sonic tested one yet. When I have some spare "messing" time I'll dig the standard bore one out and see what it test out too. I'll shout when I got some numbers.
Martin.

Awesome!

[deuce_roadster]

I am finish assembling a NOS 59 series block 3-3/16 std bore (no way I'm cutting life out of it for a few more cubic inches). Pistons are in but can probably sonic test the cylinders above the pistons in a few spots. I also have a NOS 41 style block that seems a little odd. It has the "raised" deck, 3 3/16 std bore, but has other earlier type features like the pan rail bulges and lip above the timing cover. This isn't just a bored out 221. I was with the previous owner who was hoarding 41 Merc parts when he bought it 35 years ago. When I dig that out of storage I can sonic test that one too, it is a bare block.

[Bored&Stroked]

Originally Posted by deuce_roadster

I am finish assembling a NOS 59 series block 3-3/16 std bore (no way I'm cutting life out of it for a few more cubic inches). Pistons are in but can probably sonic test the cylinders above the pistons in a few spots. I also have a NOS 41 style block that seems a little odd. It has the "raised" deck, 3 3/16 std bore, but has other earlier type features like the pan rail bulges and lip above the timing cover. This isn't just a bored out 221. I was with the previous owner who was hoarding 41 Merc parts when he bought it 35 years ago. When I dig that out of storage I can sonic test that one too, it is a bare block.

That is how my 42 Merc engine is - has the bulges in the pan rail - just no plugs - along with the 'lip' above the timing gear. If you ever want to part with it, let me know!

B&S