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Old 06-29-2014, 01:59 PM   #1
flatheadkid1
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Default Merc crank in 36LB block

Can someone outline what all is required to install a 4" merc crank into a 36LB block? All I know for certain about this conversion is that the counterweight O.D. needs to be turned down to clear the block and the snout needs re-machined to the early configuration. Any help is greatly appreciated.
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Old 06-29-2014, 07:30 PM   #2
Bubsyouruncle
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Default Re: Merc crank in 36LB block

I would think a really good engine shop could stroke your 36lb crank to 4 inches with a lot less fuss than modifying a mercury crank. But maybe you have been down that road.
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Old 06-29-2014, 07:42 PM   #3
Ken/Alabama
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Default Re: Merc crank in 36LB block

Im not sure that the larger journal rods like the 29A or 99A or the 8BA will pass through the 3 1/16" bore. The rod journals may need to be ground to the early 2 inch diameter so the early rods can be used. You will need a set of 21A or 91A rods for this.
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Old 06-29-2014, 07:50 PM   #4
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Default Re: Merc crank in 36LB block

A source told me that the 4" Merc. journals need to be ground to accept the 21A/91A connecting rods. I read on the barn somewhere that 8BA rods can be used in 37 and later 3-1/16" bore engines by installing the pistons before installing the crank but I'm not sure that will work with the 36LB block.
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Old 06-29-2014, 09:00 PM   #5
Walt Dupont--Me.
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Default Re: Merc crank in 36LB block

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Originally Posted by flatheadkid1 View Post
Can someone outline what all is required to install a 4" merc crank into a 36LB block? All I know for certain about this conversion is that the counterweight O.D. needs to be turned down to clear the block and the snout needs re-machined to the early configuration. Any help is greatly appreciated.
Well, the crank needs some work. The mains ground down .100 to 2.400, the rods must be ground down to 2.000 so the LB rods will go through the bore, I wouldn't bore the LB block .125, Max .060, The block is thinner than the 37 up blocks. Get an aluminum rear seal retainer for the cap and block. Looks like you have the rest of the machine work figured out. It's a lot of work. If I was going use a 4in. in a 21 stud I'd use a 37-38 block. You can bore them .125 and use the 36 heads. Walt
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Old 06-29-2014, 10:09 PM   #6
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Default Re: Merc crank in 36LB block

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Further to the above, not only do the main journals need to be ground smaller, the centre main will need to be widened too I think...
I THINK the centre main bearing is longer with the early crank.
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Old 06-30-2014, 07:03 AM   #7
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Default Re: Merc crank in 36LB block

This was done in a "Great Race" car a few years ago and there was a write up in one of the flathead engine books about it. While interesting, it did not detail all of the necessary mods. If you do this, please keep all of us posted with pics if possible.
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Old 06-30-2014, 08:56 AM   #8
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Default Re: Merc crank in 36LB block

Read, Tex Smith's flathead book as there is a complete engine Buildup for one lap of America if needs be I can get it scanned for you The blokes who racd it did very well
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Old 06-30-2014, 01:29 PM   #9
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Default Re: Merc crank in 36LB block

I put an 8BA crank in a 37 block and the only change was themains had to be turned down. After that it dropped right in. However Walts right it was a 37 block and we used 8ba rods as well. on the 3 1/16 bore I ubderstand you can put the 8ba rods up from the bottom and install the wrist pin to install the piston. Never tried it, but somebody did.
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Old 06-30-2014, 01:44 PM   #10
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Default Re: Merc crank in 36LB block

The GAR 35 coupe in the Tex Smith book did use a 37 engine with block offs so it would be a 21 stud with head mounted pumps. They bored all 8 cylinders for big sleeves. They wanted a 4 inch stroke 255 with thick walls so they spent a lot of time & money to get there. They did mention that the reliability was what they wanted in the mountain country and I guess it worked out OK.
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Old 06-30-2014, 04:27 PM   #11
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Default Re: Merc crank in 36LB block

Wonder if the rod throws on a 59AB wer cut down to 85 size offset if the throws would be hard enough?
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Old 06-30-2014, 04:43 PM   #12
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Default Re: Merc crank in 36LB block

From an article on interchanging."The small block bore (81)will accept the large big end crank but the con rods will not fit through the bore,this can be overcome by inserting the pistons first then fitting the con rods from the bottom of the block, this must be done before the crank is fitted. The crank is the lowered down and the rods fitted.On 4 ring pistons the bottom ring must be fitted after the con rod is fitted to the piston.The above procedure is achievable but not recommended as it is a difficult job."
Might be easier to over bore it. Cheers
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Old 06-30-2014, 09:32 PM   #13
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Default Re: Merc crank in 36LB block



Its been done a few times
This one I know for a fact has a 4 inch crank … but I wasn’t there when it was buttoned up
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Old 07-03-2014, 02:44 AM   #14
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Default Re: Merc crank in 36LB block

I installed a four inch crank into one of my 21 stud motors and can honestly say the performance increase did not offset the cost. I wouldn't do it again.
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Old 07-03-2014, 12:01 PM   #15
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Default Re: Merc crank in 36LB block

I am wondering if you could order a stroked crankshaft directly from SCAT that would drop right in? Might be worth checking with them.
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Old 07-03-2014, 02:16 PM   #16
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Default Re: Merc crank in 36LB block

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Wonder if the rod throws on a 59AB wer cut down to 85 size offset if the throws would be hard enough?
Hard enough? Of course they will. This is how they got the 1/8" strokers before the 4" came out. Same applies to grinding the 4" to 4 1/8" the rod throws arnt a hard shell on a soft crank.
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Old 07-03-2014, 02:22 PM   #17
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Default Re: Merc crank in 36LB block

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I installed a four inch crank into one of my 21 stud motors and can honestly say the performance increase did not offset the cost. I wouldn't do it again.
Interesting, I have pondered this. Merc crank in a 21stud. I know the increase in seat of pants power is very noticeable when you bore a 21stud to 3 3/16". Often thunk is it bore increase or stroke increase that makes more difference? Both is obviously good. But if you had to choose, what would it be?
Just idle thinking really.
any thoughts?
Martin.
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Old 07-03-2014, 02:54 PM   #18
Ol' Ron
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Default Re: Merc crank in 36LB block

Torque is what the seat of the pants feels. Doesn't make any difference how you get it. Cubes work and so does compression. IF you could get 8.5 CR in a 221 engine you'd feel the differance as well. It'd hard to get much over 7:1 You still have to open the valves
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Old 07-05-2014, 09:31 AM   #19
flatheadkid1
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Default Re: Merc crank in 36LB block

Thanks for all of the replies on this topic. I just tore down the 36LB engine in question. It is .040" over with no cylinder ridge to speak of. If it has no cracks I'm going to leave it alone.
I have another cracked LB block to mess with. If I can locate a 4" merc crank that is way undersized or has journal damage on the mains I'll grind the mains to the LB size and find out for sure what works and what doesn't.
Thanks again.
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Old 07-05-2014, 10:15 AM   #20
Russ/40
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Default Re: Merc crank in 36LB block

I have a Merc Crank set up for the 21stud with a 4 1/8" stroke and shortened snout if your interested. Expensive, PM me.
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