4" Mecury Crank in 36LB Block

Mr. Will

Mr. Will

Well-known member
Hi Guys,
I have a 4" Merc crank I was thinking about using in my 36LB block. Though about sleeving it so it would have a 3-3/16 bore and using 8BA rods. I have the Tex Smith book with the article about using a 4" merc crank in a 37 21 stud block.


Anyone ever done this? Any input would be appreciated.


Thanks Dan
 
You won't be able to just drop the 4" crankshaft in a 36 LB block. The mains size on the later Merc crankshaft is 2.499 while the mains on the 36 are 2.399. The good news is that the mains can be reground down to a STD 2.399 size.

The crankshaft snout on the later cranks would also need to be modified to match the 36 crankshaft if you are going to use the 36 pulley. Also, I don't know if there would be any clearance or fitment issues trying to run the longer snout with a later pulley?

Bores: I am not a big fan of sleeving an otherwise good block in all 8 holes. This tends to weaken the already weak deck surfaces as a lot of material that sort of "holds the decks in place" in removed from the bores. If this is just a street engine, you can get away with it - just make sure you can get head gaskets for a 21 Stud engine for the larger bore size (I don't know this to be true?). It may require custom head gaskets for the larger bore.

You should be able to buy pistons for the stroke you need, though depending on the bore size they may be "custom" for the 4" stroke.

1) Rear Main Seal: I've never worked on a 36 LB block, but I believe you can swap in the later pot-metal "seal plates" from a 59AB to run a good rope style seal in the rear?

2) Rods: You might be able to go with the SCAT 2" rod journal rods (and regrind the rod journals on the crankshaft for 1.999 rods) - though I do not know if they will fit through a 3.0625 type bore. You'd have to check with SCAT to see the minimum bore diameter the big ends will fit through. Another option is to buy a set of 91A or 21A rods (39 - 42 Ford) and run full-floater bearings with a Merc crank that has the rod-journals ground to match.

If I was going to have the rod journals on a Merc crank reduced in size (to the 1.999 221 crankshaft size), I would explore whether or not I could go to a 4.125 stroke and not have any issues with the rods hitting the bores on a small-bore engine? I'm sure this has been done a ton of times - I just don't know the answer to this question?

I think your idea is totally doable (though expensive!) - just need to work out all of the above before you start down the path. Understand the costs for the crank work, rods, custom pistons and who can make them ---> Egge, Ross, etc?
 
Bored&Stroked said:
You won't be able to just drop the 4" crankshaft in a 36 LB block. The mains size on the later Merc crankshaft is 2.499 while the mains on the 36 are 2.399. The good news is that the mains can be reground down to a STD 2.399 size.

The crankshaft snout on the later cranks would also need to be modified to match the 36 crankshaft if you are going to use the 36 pulley. Also, I don't know if there would be any clearance or fitment issues trying to run the longer snout with a later pulley?

Bores: I am not a big fan of sleeving an otherwise good block in all 8 holes. This tends to weaken the already weak deck surfaces as a lot of material that sort of "holds the decks in place" in removed from the bores. If this is just a street engine, you can get away with it - just make sure you can get head gaskets for a 21 Stud engine for the larger bore size (I don't know this to be true?). It may require custom head gaskets for the larger bore.

You should be able to buy pistons for the stroke you need, though depending on the bore size they may be "custom" for the 4" stroke.

1) Rear Main Seal: I've never worked on a 36 LB block, but I believe you can swap in the later pot-metal "seal plates" from a 59AB to run a good rope style seal in the rear?

2) Rods: You might be able to go with the SCAT 2" rod journal rods (and regrind the rod journals on the crankshaft for 1.999 rods) - though I do not know if they will fit through a 3.0625 type bore. You'd have to check with SCAT to see the minimum bore diameter the big ends will fit through. Another option is to buy a set of 91A or 21A rods (39 - 42 Ford) and run full-floater bearings with a Merc crank that has the rod-journals ground to match.

If I was going to have the rod journals on a Merc crank reduced in size (to the 1.999 221 crankshaft size), I would explore whether or not I could go to a 4.125 stroke and not have any issues with the rods hitting the bores on a small-bore engine? I'm sure this has been done a ton of times - I just don't know the answer to this question?

I think your idea is totally doable (though expensive!) - just need to work out all of the above before you start down the path. Understand the costs for the crank work, rods, custom pistons and who can make them ---> Egge, Ross, etc?
Click to expand...

Dale absolutely nailed it. I have never built this combination but intend to in the near future. I see big $$$$$$ if you can’t do it yourself.
 
I've built a 255 cu inch 21 stud engine. Not only do you need to reduce the dia of the mains as described above, the centre main needs to be widened, and I think from memory [it's been a while] also the front journal. Cometic can supply big bore 21 stud head gaskets. I used 21A rods, no clearance issues.
 
Hey Brian: Did you sleeve the motor to get to a 3 3/16 bore? My guess is "Yes". As long as he has a 36LB crank and block a good machine shop and crank grinder should be able to handle all of the crankshaft modifications. If it was mine, I'd go for a 4 1/8 stroke - no reason not to - if you're already at 3 3/16 bore.
 
Nah Dale, It's a 37 block with pump block off plates. They have thicker cylinder walls than a 36 LB. You'd not be able to bore any 36 LB 1/8" oversize, cylinder walls much too thin on those.
 
Thanks Brian - I knew I was missing something in your build!

This particular situation is one where the builder might find that after all the custom crankshaft work and then custom pistons, etc -- it might just get a bit pricey!

The key will be finding a machine shop and/or crank grinder that can and will do the work . . . mighty hard to find these days! One has to be very selective and should give them the block, with a set of main bearings to work with . . . otherwise what you get back may or may not work.
 
Thanks everyone for your comments.

I just got my block back today from clean and mag and it was perfect, no cracks, valve seats are perfect but, the block is .060 over so, the plan is to sleeve all cylinders. One cylinder was sleeved but not stepped at the bottom so that sleeve will be redone as well.
As many here have said it would be to expensive to have the 4" crank that I have machined so I'm going to buy a scat crank and go 4" stroke and use 8BA rods.
 
Mr. Will said:
Thanks everyone for your comments.

I just got my block back today from clean and mag and it was perfect, no cracks, valve seats are perfect but, the block is .060 over so, the plan is to sleeve all cylinders. One cylinder was sleeved but not stepped at the bottom so that sleeve will be redone as well.
As many here have said it would be to expensive to have the 4" crank that I have machined so I'm going to buy a scat crank and go 4" stroke and use 8BA rods.
Click to expand...
I don’t know what size bore you plan on and I maybe wrong without checking but I don’t think 8BA rods big end will fit down through a 3 1/16” bore. I believe you may need 3 3/16” bore.
Me I’d stroke that Scat 4” crank to 4 1/8” and use 21A or 99A rods in a heartbeat.
 
Scat actually make a 4" stroke crank that is supposed to fit a 36-38 LB engine...it doesn't!! I went that route, bought said Scat crank, it didn't fit!! I had to do considerable machining on it to get it to fit. Scat didn't want to know when I notified them. You'd be better off modifying the Merc crankshaft, it would be superior. Plus, using a Merc crank, you've got to grind the rod journals down to 2", so offset grind and get 4 1/8 stroke!
 
Hey Brian: When you say "it didn't fit" and a lot of machine work was needed - do you happen to remember the details? This might save Mr. Will some drama, time and a lot of money.

Thanks as always!
Dale
 
KiWinUS said:
I don’t know what size bore you plan on and I maybe wrong without checking but I don’t think 8BA rods big end will fit down through a 3 1/16” bore. I believe you may need 3 3/16” bore.
Me I’d stroke that Scat 4” crank to 4 1/8” and use 21A or 99A rods in a heartbeat.
Click to expand...
Yes the plan is 3-3/16" bore. I still have to do a bit of research but, if I can go 4-1/8" I will, that would put up to 263ci.
 
Brian said:
Scat actually make a 4" stroke crank that is supposed to fit a 36-38 LB engine...it doesn't!! I went that route, bought said Scat crank, it didn't fit!! I had to do considerable machining on it to get it to fit. Scat didn't want to know when I notified them. You'd be better off modifying the Merc crankshaft, it would be superior. Plus, using a Merc crank, you've got to grind the rod journals down to 2", so offset grind and get 4 1/8 stroke!
Click to expand...
How long ago did you do this?

Thanks for that information, I will do some research.
 
I posted the issues on here back when I had these troubles, but who knows how to find that stuff now? Basically those later 21 stud blocks have 2.399' dia main bearing journals, whereas from 39, all engines, 221,239 and 255 had 2.499" dia mains. Scat use the same casting as their 24 stud cranks for their 221/ 4" cranks but grind the main journals to 2.399". BUT...whilst the 21 stud cranks are 2.399 dia, they are longer on the front and centre journals. Scat are unaware of this and didn't want to acknowledge it when I drew their attention to it. This necessitates widening both sides of the centre main and the rear portion of the front main. Then, I discovered the crank timing gear was too far back and didn't line up with the cam gear. So, I had to make a spacer to locate the crank gear in correct location.
If you used a genuine Merc crank, you could attend to the widening beforehand in a lathe, and finish grind the mains to the required dia and width in one operation, rather than trying to widen the journals when the dia is already ground to finish size. [bit trickier!!]
Once I had the piece of **** Scat crank able to rotate in the block, I discovered that some of the rod journals were not located centrally in their respective bores. The rear main thrust faces become the datum point from which to locate the crankpin centres. Scat screwed that up too! As a result, some of the rods in my 255 21 stud engine, are not located exactly centrally in the pistons! When I drew this matter to Scats attention, with photos, all they could ask is 'what conrods are those?' [21A rods, not Scat crap!] If I ever attempted to do this again, I'd use genuine Fomoco crank of any description and be miles ahead from the get go!
Incidently, I had this Scat crank sitting on a wooden bench in my shed for a coupla months before I got around to using it, and it developed flash rust on the throws, whereas I had old Ford crankshafts lying on the concrete floor under the bench which remained blemish free. Chinese iron ain't as good as Henry's iron!
 
Brian said:
I posted the issues on here back when I had these troubles, but who knows how to find that stuff now? Basically those later 21 stud blocks have 2.399' dia main bearing journals, whereas from 39, all engines, 221,239 and 255 had 2.499" dia mains. Scat use the same casting as their 24 stud cranks for their 221/ 4" cranks but grind the main journals to 2.399". BUT...whilst the 21 stud cranks are 2.399 dia, they are longer on the front and centre journals. Scat are unaware of this and didn't want to acknowledge it when I drew their attention to it. This necessitates widening both sides of the centre main and the rear portion of the front main. Then, I discovered the crank timing gear was too far back and didn't line up with the cam gear. So, I had to make a spacer to locate the crank gear in correct location.
If you used a genuine Merc crank, you could attend to the widening beforehand in a lathe, and finish grind the mains to the required dia and width in one operation, rather than trying to widen the journals when the dia is already ground to finish size. [bit trickier!!]
Once I had the piece of **** Scat crank able to rotate in the block, I discovered that some of the rod journals were not located centrally in their respective bores. The rear main thrust faces become the datum point from which to locate the crankpin centres. Scat screwed that up too! As a result, some of the rods in my 255 21 stud engine, are not located exactly centrally in the pistons! When I drew this matter to Scats attention, with photos, all they could ask is 'what conrods are those?' [21A rods, not Scat crap!] If I ever attempted to do this again, I'd use genuine Fomoco crank of any description and be miles ahead from the get go!
Incidently, I had this Scat crank sitting on a wooden bench in my shed for a coupla months before I got around to using it, and it developed flash rust on the throws, whereas I had old Ford crankshafts lying on the concrete floor under the bench which remained blemish free. Chinese iron ain't as good as Henry's iron!
Click to expand...
Brian,
Thanks for posting that information.
 
Mr. Will said:
Glen,
That link won't work for me for some reason.
Click to expand...


I did an advanced search of Early V8s for user "Brian" and "36LB"



555361



... AND GOT THESE RESULTS:


555362

For some reason, The URL of the search won't work but you can do the same and find the relevant posts from Brian.


Glenn
 
I did a search and came up with this article I posted way back;


A genuine Merc crank is far superior to a Chinese Scat crank. I have run both versions. The crap Scat I had went rusty just sitting on the wooden workbench, whereas I've got old Ford cranks kicking around on a concrete floor that stay good, however, the bigger problem is the quality control, rather the lack of it, exhibited by Scat. The Scat crank I had wouldn't even fit into the block!- I had to machine it to suit, then throws two and three were incorrectly positioned , putting the rods off cylinder centrelines. Upon informing Scat, they didn't give a toss! All communication with Scat was through some dickhead salesman, not engineer, who, when looking at photos I sent him, had to ask "What rods are those? they must be the problem"!!! [NOS genuine 21A rods] yeah right, must be the problem!!
Thing is, in my case at least, I would have been better off, both financially and quality wise, if I had've used a Gennie Fomoco crank and modified to suit.


scat
noun (1) Definition of scat (Entry 2 of 5)
: an animal fecal dropping


Quite true; ****!!
 
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