Is Pencil Test Bogus for Determining if 961 Block has thick walls. - Chevelle Tech
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post #1 of 18 (permalink) Old May 27th, 11, 9:01 PM Thread Starter
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Is Pencil Test Bogus for Determining if 961 Block has thick walls.

I have read that some of the 65 961 block had extra thick walls and were used for the early 66 427's. I was told that you could check by inserting a pencil though a frost plug hole and if it didn't pass between the cylinders you had a early thick block and it could be bored to 427; but also told to have it sonic tested. Or was it the 962 block that was used to make 427 corvettes.

Well I did this test will 3 different 961 blocks, a G 5 or July 65, a J 5 or Oct 65 and a E 6 or May of 66. All had approximately .375 distance between the cylinder walls. All were standard 4.094 and measured about .750 between the cylinder bores on the deck. How could you bore this to 4.255. If my measurements are correct that would leave the wall thickness at about .107, is that possible?

I had the Oct of 65 sonic tested, since it was at the machine shop. The machinist said all cylinders measure more than .308 And there would be no problem boring it to .090 over. .How is this possible, .308+.308 + .375 is a lot more than .750? According to my calculations, if you bored it .090 over that would leave the cylinder wall thickness at about .145 each.

Something must be wrong with my thinking or the sonic test. Any opinions? Thanks.
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post #2 of 18 (permalink) Old May 27th, 11, 9:17 PM
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Re: Is Pencil Test Bogus for Determining if 961 Block has thick walls.

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Originally Posted by james a larson View Post
I have read that some of the 65 961 block had extra thick walls and were used for the early 66 427's. I was told that you could check by inserting a pencil though a frost plug hole and if it didn't pass between the cylinders you had a early thick block and it could be bored to 427; but also told to have it sonic tested. Or was it the 962 block that was used to make 427 corvettes.

Well I did this test will 3 different 961 blocks, a G 5 or July 65, a J 5 or Oct 65 and a E 6 or May of 66. All had approximately .375 distance between the cylinder walls. All were standard 4.094 and measured about .750 between the cylinder bores on the deck. How could you bore this to 4.255. If my measurements are correct that would leave the wall thickness at about .107, is that possible?

I had the Oct of 65 sonic tested, since it was at the machine shop. The machinist said all cylinders measure more than .308 And there would be no problem boring it to .090 over. .How is this possible, .308+.308 + .375 is a lot more than .750? According to my calculations, if you bored it .090 over that would leave the cylinder wall thickness at about .145 each.

Something must be wrong with my thinking or the sonic test. Any opinions? Thanks.

Personally I think if a GM/Chevy block was intended for 396/402 you won't safely get a 427/454 bore. That "pencil test" is another one of those "folk
lore" stories that I have yet to actually see materialize in my 54 years...........................regardless of books that were written that state otherwise....................I'm surrounded by damn good machine shops here in NASCAR country and this isn't something that was ever done...............good luck
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post #3 of 18 (permalink) Old May 28th, 11, 2:02 AM
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Re: Is Pencil Test Bogus for Determining if 961 Block has thick walls.

dont do it. i been outta this stuff for 20 years and just got back into it, BUT 30 yrs ago something very similiar was attempted with very disappointing results. very. as JC396 stated..."folklore" in nascar country. here it's called bull do-do. from personal experience, i can state, i would not do that again. unless you want to make a boat anchor, conversation piece or look at yourself in the mirror and say, "why did i do that", do NOT DO IT. bozo no no..
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post #4 of 18 (permalink) Old May 28th, 11, 2:15 AM
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Re: Is Pencil Test Bogus for Determining if 961 Block has thick walls.

remember when boring the cylinder in your case with 0.308" cylinder wall that you will only remove 0.045" from the cylinder wall and this will leave 0.263" minimum / the radius of the bore is increased 0.090" because of the 0.045" removed from the circumference of the bore
hope this helps the math error
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post #5 of 18 (permalink) Old May 28th, 11, 4:16 AM
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Re: Is Pencil Test Bogus for Determining if 961 Block has thick walls.

vince, you are correct. just got back an email from a friend who remembered what we did back in '79 with a 402. we went 100 on the bore, but either a main walked, the rod broke or the crank broke in the first 5 minutes. either way, it was a rotating mass problem not a bore problem... never got the chance to find out if the cylinder would have held. back then though when we were 19, tearing an engine out of a car and tearing it down, didnt seem as bad as it does today. was kinda cool. every gear head in the neighborhood came over. sorry about the wrong advise. im gunna shut my mouth now....
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post #6 of 18 (permalink) Old May 28th, 11, 4:17 AM
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Re: Is Pencil Test Bogus for Determining if 961 Block has thick walls.

Never heard of the pencil trick but I do know that some early 396 and 427 blocks had the same casting number. I know a few 427`s that started life as 396`s so it can be done but you need to sonic check the block for wall thickness.
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post #7 of 18 (permalink) Old May 28th, 11, 6:59 AM
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Re: Is Pencil Test Bogus for Determining if 961 Block has thick walls.

The wider it is between the OUTSIDE of the cylinder barrels (as seen through the middle core hole), the less you can bore one. I've had several with well under 1/4", just like a stock 454 block, and several with more like 1/2" between them.
Sonic check the block, don't rely on a pencil to guess how far a block can be bored.

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The correct spelling is
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post #8 of 18 (permalink) Old May 28th, 11, 7:04 AM
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Re: Is Pencil Test Bogus for Determining if 961 Block has thick walls.

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Originally Posted by sschevellefan View Post
Never heard of the pencil trick but I do know that some early 396 and 427 blocks had the same casting number. I know a few 427`s that started life as 396`s so it can be done but you need to sonic check the block for wall thickness.
If there were "early" 396s that could accept the 427/454 bore I'd bet that run of blocks was short lived at best.

I've heard this tale for a long time....know of a guy that had about 14 Chevelles (granted allot was junk) and a barn full of every 396/402 made back around 1974. He never found one that would accept the big bore...he finally broke down and bought a Corvette 427 to build for his drag motor.....
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post #9 of 18 (permalink) Old May 28th, 11, 10:43 AM
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Re: Is Pencil Test Bogus for Determining if 961 Block has thick walls.

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Originally Posted by JC396 View Post
If there were "early" 396s that could accept the 427/454 bore I'd bet that run of blocks was short lived at best.

I've heard this tale for a long time....know of a guy that had about 14 Chevelles (granted allot was junk) and a barn full of every 396/402 made back around 1974. He never found one that would accept the big bore...he finally broke down and bought a Corvette 427 to build for his drag motor.....
Looks like mortec is no longer but i found this link. If you go down a few numbers and find the last 3 numbers of 961 it shows the same casting number for 65-66` 396 blocks and 66` 427 blocks.

http://www.strokerengine.com/BBCcasting.html

I`ve been hearing about this for almost 20 yrs and like I said I know of a few that were done and lived a long time so unless everyone I knew that did it lied, including the machine shop I worked at as a kid, it can happen but you need to sonic check the block to make sure the walls are thick enough.
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post #10 of 18 (permalink) Old May 28th, 11, 10:55 AM
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Re: Is Pencil Test Bogus for Determining if 961 Block has thick walls.

When the casting thickness is iffy, sonic checking is good to check for core shift. You can have lots of meat on one side of the bores and not nearly enough on the other side. Not all castings are perfect. Years ago I read about the pencil check but have had no experience with block differences. It probably would have made sense to have one casting for both applications other than they would be wasting a little cast iron on every 396 block made. The little bits add up when you are making a lot of them.
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post #11 of 18 (permalink) Old May 29th, 11, 12:31 AM
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Re: Is Pencil Test Bogus for Determining if 961 Block has thick walls.

This thread reminds me of a Zora Duntov story. Either Road & Track or Car & Driver were complaining about the 1965 396 Corvette having such a heavy powerful engine in a sports car. When the 1966 Corvette was introduced with the larger 427 engine they complained even louder. Duntovs response was, we lightened the engine block by boring it out just to please you guys and you still complain!!!

Thanks
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post #12 of 18 (permalink) Old May 29th, 11, 3:09 PM Thread Starter
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Re: Is Pencil Test Bogus for Determining if 961 Block has thick walls.

I did this post just for knowledge and clarifications. Not intending to do such.

According to my information, in Aug and Sep of 65 there were 2 396 blocks, the 961 2 bolt block and the 962 4 bolt block, with the 4 bolt used for the Higher HP applications. My information from the Colvin book and other sources, states clearly that the cylinder walls of the 962 were thicker than the 961.

So if the 427 corvette block 3869942 was not yet in production or they could not keep up with the demand, then they could very easily have used the 962 block (thicker walls?)
to make a 427.

I think you will find that the distance between the cylinder walls of a 961 are approximately .375; but the distance between the walls of a 962 is approximately .250; thus making the cylinder walls approximately ..062 thicker.

So if you can bore a 961 @ .090 over (402 @ .060 over), then you should be able to bore a 962 @.161 over (makeing 4.255), If the walls are indeed approximately .062 thicker.


I think the pencil test would only confirm that you have a 962 bock, which the casting # does. And also if you had a 961 block that was a 4 bolt, then someone or the fractory many have coverted it to a 4 bolt.

Last edited by james a larson; May 29th, 11 at 3:39 PM.
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post #13 of 18 (permalink) Old May 29th, 11, 3:27 PM Thread Starter
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Re: Is Pencil Test Bogus for Determining if 961 Block has thick walls.

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Originally Posted by VinceS427bb View Post
remember when boring the cylinder in your case with 0.308" cylinder wall that you will only remove 0.045" from the cylinder wall and this will leave 0.263" minimum / the radius of the bore is increased 0.090" because of the 0.045" removed from the circumference of the bore
hope this helps the math error
Maybe I am wrong. But if you remove .045 from the cylinder wall, you ae also removing .045 from the ajoining cylinder wall or a total of .090 from the distance between the bores.

So on a standard bore, if the distance between the cylinder bores is .750 and you have a gap of .375 between cylinders and you remove .045 from each, thats a total of .090 from the thickness of the 2 cylinder walls. So each cylinder wall would be .143 thick at the frost plug area.

I must be all goofed up as where where they are taking the measurment when they sonic test the cylinders. Can some explain where this measurment is taken?
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post #14 of 18 (permalink) Old May 29th, 11, 4:20 PM
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Re: Is Pencil Test Bogus for Determining if 961 Block has thick walls.

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Originally Posted by james a larson View Post
Maybe I am wrong. But if you remove .045 from the cylinder wall, you ae also removing .045 from the ajoining cylinder wall or a total of .090 from the distance between the bores.

So on a standard bore, if the distance between the cylinder bores is .750 and you have a gap of .375 between cylinders and you remove .045 from each, thats a total of .090 from the thickness of the 2 cylinder walls. So each cylinder wall would be .143 thick at the frost plug area.

I must be all goofed up as where where they are taking the measurment when they sonic test the cylinders. Can some explain where this measurment is taken?
the distance between the outside of the cylinder walls does not change.
you are removing a small amount of the inside of the cylinder wall during boring.
the way the cylinder walls are sonic checked (ultra-sound) is with electronic thickness measuring tool called an UT or ultra-sound tester.
several measurements are taken in a line up and down the cylinder bore from the inside.
the basic method is to test both sides along with the top side and bottom side of the cylinder wall (as you are looking down into the cylinder bore).
documentation of the measurements will map out the minimum/maximum thickness of each cylinder walls thickness in the areas measured.
this way if you need a minimum of 0.250" of cylinder wall thickness to safely run the engine, and the minimum cylinder wall thickness detected is shown to be say 0.310" then you could safely bore the block to remove a maximum of 0.060 total from the inside of the cylinder wall.
as the cylinder bore is measured as a diameter from one side to the other = if you took 0.050" from the bore it will actually be bored 0.100" larger...

i made an error in my previous post using radius when i meant diameter.
when you take 0.045" from the inside of the cylinder bore, this measurement is added to the radius, this increased radius measurement is 1/2 of the bore diameter= radius X2=diameter which then shows a total increase of 0.090" in bore diameter
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post #15 of 18 (permalink) Old May 29th, 11, 8:33 PM Thread Starter
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Re: Is Pencil Test Bogus for Determining if 961 Block has thick walls.

Thanks for being so patient with me. Thats how the machinest expained it to me. The only thing confusing to me is if you took 0.050 from the bore, it will actually be bored 0.100 larger.

I am kind of a novice here and what perpetuated this whole thing, is I have two 961 block that I took to two different machine shops to have checked, etc. Both machinist said they could be bored 0.090 (0.045 taken off each side) over since they were the same as 402 blocks for which they have pistons at 4.185 (the same as a 396, 4.094 +0.090). Maybe I am confusing the terminology.

Now I check with a micrometer the distance between the cylinder bores of both blocks (which are standard @ 4.094) and that distance is approximately 0.750 between 1&3, and 3&5, and 5&7. A 3/8" rod or 0.375 fits exactly between the cylinder walls when inserted between the frost plug hole.. That leaves approximately 0.375 for the thickness of the two adjoining cylinder walls.

This tells me that each wall is approximately 0.1875 thick. I must be thinking incorrectly. Or messing up the terminology. How can a wall that I measure as 0.185 meet the mininum of 0.250?
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