Merc crank in a 24 stud pencil test block

[GB SISSON]

I did some searching on this, but my question is fairly specific. I bought 8 'good' blocks last year in one lot. They came from the seller's father in law who left the flathead world. They were stored in a dry shed 2 miles from my home. Story was he junked 14 blocks and these 8 were the keepers, one of which had been tanked, magged and machined. I later bought a std size merc crank on one of the other islands. I dropped it off at the machine shop two weeks ago and I will have it in my hands later today. Looks like the prepared block is a pencil test block bored to 3 3/16 +.040 over. I have a set of merc pistons at .040 so this looks like the block that makes the most sense. It's even drilled for adj lifters. So the question of the day is... Is this bored too far to make sense for this build? The rest of the engine will be stock. I just like the 4" cranks for pulling our island's many hills. The engine in my current project '47 tonner PU is quite tired so it may go in there OR I may put the woodie's 3 1/16" engine in that truck and use the stroker in the woodie. Either way, no hot rod. Once again I have managed to sandwich my simple question into a giant paragraph. I do have nice 59ab and 8ba blocks to choose from, and no I am not interested in selling any of them as I am 65, just starting to look towards some kind of semi-retirement and I love these flathead projects. Thanks in advance. GB

[cas3]

the one i did last i had to go to 80 to get cleaned up, so 40 sounds good to me. sure i would rather have used a better block, but this one was good enough for me, and i hate to tear down another motor hoping to find better, and just end up with more scrap iron. we most likely will never wear one out with better oils today and our limited use. you got the 8ba rods too?

[rotorwrench]

I would get the cylinder walls ultrasonically tested for thickness if it was mine but I know that might be difficult for you. The 221 engines that started life at 3 1/16" on the cylinder bores have relatively thin walls. If it hasn't gone through them yet then it's a good sign. If they are paper thin then that would not be good. You never know on some engines though. It may have enough to get by.

[flatheadmurre]

If it was a 3 1/16 from start and now is 3 3/16 +040 your in the zone where ultrasonic testing is going to tell you what can or can´t b done...

[GB SISSON]

I will call the machine shop that did my crank and see if they can test it. It's quite a large and well equipped shop. What's the limit? Is there a good number for a flathead block? I kind of suspected you guys would say to ultra sonic test it. Once I haul it to the mainland and another trip to pick it up, I might as well just take them a 59ab or an 8ba with a lot of wall and have it bored .040+, but then there is the other testing etc. Maybe I should drop off a couple for tanking and testing while I'm making the trip.

[TomT/Williamsburg]

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My mother always said, " never waste a trip" so yeah, bring some other stuff to have done as well. If your block is no good, having one already there to bore to +.40 will save a trip ....

[leon bee]

What does "pencil test" mean?

[flatheadmurre]

Originally Posted by leon bee

What does "pencil test" mean?

The "lip" above the front cover is wide or narrow depending on how the block was cast so if you are able to put a pencil there and it won´t fall off it passed the pencil test...

[GB SISSON]

And if it passes the pencil test, then that means it started out as a 3 1/16" bore block. Both are 24 stud. If the cleaned and machined block had been an 8ba or 59ab style block bored to .040, there would not be any issues with wall thickness, so I'd just start the build with my new crank and not think about the walls.

[corvette8n]

Cool picture, one can never have too many blocks.

[flatford8]

We have similar plans for retirement Gary....Mark

[GB SISSON]

Here's a picture of the pencil test for those of you that want to know. I called Valley Machine and they don't do ultrasonic testing so I had to build my own apparatus after work to check wall thickness. Expect a very scientific approach from the guy that tests adjustable lifters with a 5lb flour sack, makes teflon buttons with a router jig instead of piston circlips, and hones kingpins with his wife's wooden spoon. All 3 are building up the miles with no hint of failure and I'm having a great time in the process. Would you buy a used car from this man? I would, but that's just me.

[flatheadmurre]

If you´re going to build s few you might want to find yourself a ultrasonic tester.
They are around used and i see some real cheap ones made in waitan...not sure how good though...
It´s not the tool that makes the mechanic...it´s the knowhow...

[marko39]

looks like the parts were well taken care of. i like the cradles for the cranks.

[GB SISSON]

Just like my steering box inquiry last week, I am already gaining new and valuable information on this subject. I made up this caliper just for reference. It showed walls of around 1/8" on all parts I could test on the over-bored 3 1/16 pencil test engine. The pointed end goes down the nearest coolant hole, and the bent side goes into the bore. At zero thickness the two outside points are flush. I later tested an array of 8ba/rt and 59ab type blocks bored .030-.040 and found wall thickness about 3/16". All my measurements I could get were very consistent, showing minimum core shift. There's a part of me that would like to bore some more holes in the deck surface for checking the high side and then tapping and a cast iron plug, but seems too reckless, even for me. I will look into ultrasonic tester. And Cas3 I have a couple boxes of rods, like maybe 75 to pick from. I'd like to make a jig to test for straightness. Is 1/8" ok wall thickness?

[Mart]

I like the improvised caliper gauge. Very interesting.

[rotorwrench]

1/8" should be OK if it is consistent throughout. It's pretty close to where you could bore it for 1/8" thick sleeves as well. Near to an additional +.015 would allow for a 1/8" sleeve to go back to 3.0625" if my math is correct.

[Barn Junk]

Another method to get some idea of the cyl wall core thickness if you have a selection of 24 stud blocks available for comparison, heads off and a variety of longer drill bits of varying diameters.

At the bottom of the cyl head block surface, there is a fairly large hole between each of the two outside cylinders. Drill bits of various sizes can be inserted, in the water jacket at an angle, between the two cyl cores to get some relative measurement of their thickness. 81A blocks should take a larger drill bit than say a 59A block.

[42merc]

'41 '42 Mercury blocks had a 'raised deck' and 3 3/16" bore.
Also would pass 'the pencil test'.

[Step-down]

I'm sorry I,m slow, so if the pencil stays on the lip show on the photo above it 's 3-1/16 "
I have a 40' and haven't take the head off yet but the pencil stays on lip without falling .