24 stud crankshaft in a 21 stud engine

Russ/40

Russ/40

Well-known member
I have come up with a merc crankshaft 1CM. The rod journals have been ground for the smaller 1.998" rods. I have zero experience with 21 stud engines, but have heard there are a few problems that need to be dealt with. The stroke is at 4 1/8". The snout has been substantually shortened, which is what leads me to believe it was for a 21 stud.

Would those in the know please educate me as to what needs to be done for use in a 21 stud application. Also what problems will the short snout present in a 24 stud application.

I have a very nice '49 8BA block I have been saving for something a little special, but may end up selling it.
 
Russ, What sizes are the main journals? If the larger [2.499" Std] size, I don't see why you couldn't simply drop this crank into a 24 stud engine, and use an early 221 crank pulley. The crank snout would need to be not only shorter, but also smaller dia to suit the early pulley.
Obviously, you'd need to run the 37-48 water pumps and suitable generator setup in order for the belts to line up.
 
What Brian said is the key to know if it's made to fit a 21 stud.

If the mains measure 2.399 or less, then it's been ground to fit the 21 stud and won't fit a 24.
 
Guys, the mains are 2.479, so it looks like a 24 stud application with .020" under mains.
So, am I correct as to rod bearings? It needs floaters at crank standard and whatever the rods come out to be. Will these floaters be flanged bearings?

Back to mains for a moment. Are 21 stud mains the same width as 24 stud mains?

I think that's it. Cant think of any other questions to ask. Anything else relavant?

I happen to have a set of .125" over Merc pistons I'm considering using as slight pop ups with this crank.

More comments please and thank you.
 
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If you´re going to put it in a 21-stud with 68 bearings that crank has to have a date with the lathe first and then the crankshaft grinder...
You stated it´s ground to 2" on the rods...if it´s stroked you will have to check for clearance rod to block to...
 
flatheadmurre said:
If you´re going to put it in a 21-stud with 68 bearings that crank has to have a date with the lathe first and then the crankshaft grinder...
You stated it´s ground to 2" on the rods...if it´s stroked you will have to check for clearance rod to block to...
Click to expand...

As I mentioned above; I have no experience, or interest in, a 21 stud job. My 21 stud inquiry is only to be knowledgeable on that application should I decide to sell it. I would not feel right if I cant represent it properly.

From what I'm hearing, its ready to go in a 24 stud application. All that machine work is done.
 
The floaters you'll require will be 81A prefix [non flanged], in conjunction with 91A or 21A rods. Or aftermarket rods that take Buick bearings possibly [I have no immediate experience with those things; I've always used genuine Ford parts].
As Murre stated, the widths of #1 and 2 main bearings needs to be increased along with dia reduced on all three mains, to fit a 21 stud block. Also a spacer made to fit between the front of #1 main and the timing gear, in order to position said gear correctly
 
Great responses guys, you have given me a lot of info. Thank you for taking the time to help me out.

Left to explore would be the pop-ups. I have seen this done before with short lived results with Ross forged pistons. Just playing a mental venture into something i have never done before.
 
Brian , we use the aftermarket rods in the dragster that has the buick bearings, they work OK , but look puny compared with the floaters,and when you see the bearing area of the 2.139 floaters,they would never wear out,
Lawrie
 
Russ/40 said:
I happen to have a set of .125" over Merc pistons I'm considering using as slight pop ups with this crank.
Click to expand...

Won't there be an issue with some of the bottom studs with pistons that large in a 21 stud block? I thought the bore limit was about 3.0625" plus 0.060" on the early engines to avoid getting too close to the cylinder head studs in the bottom row.
 
He is not wanting to use a 21 stud block, he was just attempting to ID the crankshaft intended application.
 
Zeke, I've currently got a 21 stud engine with 3 3/16 bore x 4" stroke = 255. No clearances issues with that set up. Uses 21A con rods.
 
The seal conversion that Ron is talking about;
All 21 stud and some early 24 stud engines had slinger style rear mains.The later 21 stud and the 24 stud engines so equipped had a die cast insert top and bottom, which could easily be substituted for die cast inserts that hold a rope seal.
The earlier 21 stud engines only had the upper die cast piece; the cap just had a big space. Therefore, in order to run a positive seal one needs to make a suitable seal holder. I make my own [pictured], but I understand Reds makes something probably similar.
 

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Thanks Brian, good to know it is possible to go to 3 3/16" bore on a 21 stud block. I thought the OP was thinking of using some +.125" Merc pistons, which would be 3 5/16" wouldn't they? I have no experience in that area, but it sounded too big to me.

My engine has the labyrinth rear seal in the top with nothing in the rear main cap. I was planning to get the added die cast piece for the cap and not modify the crankshaft.
 
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To My knowledge only the 37 early 38 block can be safely bored to 3 3/16. I was wondering why the pop ups? are you talking about the dome pistons?
 
Hi Ron

The reason for the pop-up-pistons is i want to use what i have, and with the 4 1/8" crank stroke , and using the .125" over Merc pistons, i plan to accomodate the difference in the heads. As to what heads to use, that i need to explore - iron or aluminum.

I'm thinking the small amount I would have to machine, could be safely accomodated.
What say you?
 
Anyone with the experience of how much meat can be cut from the combustion chamber to accommodate pop-up pistons? I know the offenhauser heads have a lot of thickness to do them, but how about stock iron heads? The 4 1/8" stroke means we're only dealing with 1/16" proud pistons.
 
To use any 4.125" crank you'll need the pistons to have a 1.374"/1.375" C.H.

The stock Ford has a 1.560" hgt, the stock Merc has a 1.436" hgt, and all are based on the 7.000" rod length and OEM deck hgts.

Not sure the C.H. on the pistons you have on hand??

Thanks, Gary in N.Y.

P.S. Am going to try to keep my posts a bit short, still in the middle of some major health issues, half-way out of the woods so far!
 
Have you testfitted them in the block yet ?
Hopefully you don´t even need to machine the full 1/16...
Any shop with a CNC Lathe around ?...then you can machine the pistons...maybe easier then machining the heads.
 
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