Late Merc Crank--4"

qmdv

qmdv

Well-known member
I traded an 8BA engine for a 4" Merc crank today. I checked the Vanpelt site to see the crankshaft specs to determine how good of deal I got. I always thought that the main journals were the same from 1939 to 1953 and the rod journals were the same from 1946 to 1953.

I was correct on the rod journal but I noticed that in the 1952 and 1953 the mains are .0005 smaller. I wonder why that is the case.

Oh and the crank is pretty good so far. It has never been turned. Worn standard. Need to get it maged next.

Tim
 
That .0005 is one half of .001... Most bearings call for .001 to .003 clearance. So, I think you could ignore that small difference...

I've favored .001 clearance for rods and mains. .002 is OK, too. I've also used valve grinding compound between the main caps and the block, just to close up the clearance a bit...

Karl
 
What is the going rate these days for a Merc Crank that has never been turned? Years ago when there were lots of flathead engines ending up in the local metal recycling yards, I used to go and scrounge the 4-inchers at 15 cents a pound. Of course, I had to disassemble the engine blocks to get them out, sometimes having to torch the rods and break the pistons to do it, so I had a fair bit of labor, blood, sweat etc into each of them. I probably have 4 or 5 of them stashed somewhere in my sheds or barn (if I can find them).

One can buy a SCAT crank (and kit) today in perfect shape, so salvaged Merc cranks aren't the only way to get that added displacement nowadays, but i'll bet the SCAT stroker cranks aren't especially inexpensive.
 
The original Merc crank is better quality than the scat cranks, balance is much better.
I've also heard that a Merc crank will take a lot more nitro fueled passes than the scat cranks from a drag racer. If I recall correctly, he had two scat cranks let go on the launch.
I know we don't all run them that damn hard, but it highlights the strength/quality difference.
Martin.
 
Another thing about un-turnedness...a Merc crank that close to standard, assuming wear isn't unusually deep, can be cut off-center enough to make a 4 1/8 crank, meaning it might be worth more to a rodder or racer than to a calmer sort of person.
A common thread I've seen in articles on prepping iron cranks (like most SBC) for lower racing classes is that journal undersize is just about meaningless in its effect on strength. An engineer/writer I respect a great deal, Vizard, in fact suggests turning Chevy cranks down to smallest available rod journal size in order to allow cutting a substantial radius at the fillet area, much more important in strengthening a crank.
Just saying...application of a Merc crank that can be used with only a one size cut should be considered carefully.
 
Bruce Lancaster said:
Another thing about un-turnedness...a Merc crank that close to standard, assuming wear isn't unusually deep, can be cut off-center enough to make a 4 1/8 crank, meaning it might be worth more to a rodder or racer than to a calmer sort of person.
A common thread I've seen in articles on prepping iron cranks (like most SBC) for lower racing classes is that journal undersize is just about meaningless in its effect on strength. An engineer/writer I respect a great deal, Vizard, in fact suggests turning Chevy cranks down to smallest available rod journal size in order to allow cutting a substantial radius at the fillet area, much more important in strengthening a crank.
Just saying...application of a Merc crank that can be used with only a one size cut should be considered carefully.
Click to expand...

Hi Bruce; Are you saying that a crank already undersized on the rod journals can't be cut for an eighth inch increase in stroke and just use oversized bearings? Thanks, Craig.
 
Thinking about it hurts my head this early in the day, but there is a limit to what can be made up by the rod bearings. You can of course go to odd stroke increases less than 1/8 with special pistons, but that seems like expensive limited returns...
If remaining journal size can't be cut to exact standard size of the 21A bearings, then it can be cut smaller on same center...but difference has to be made up in undersize bearing there, and you run out of sizes quickly.
 
My cost of the crank is as follows: Bought late flat head complete with everything for $100 that was removed from dump truck for engine swap (chev). Stored it indoors for 20 years. Removed 94 carb and traded it for merc crank.

It is worn standard so I am pretty stoked

Do they use the same rods on 49 to 53 Fords as 49 to 53 Mercs

Tim
 
GMDV: Good for you. I'd say you did pretty good.

I also found a 4" in the local scrap yard that was standard. $100. Needed to go .010/.010 to be cleaned up. Needless to say, I was happy.

I also have two others. One is a from a Canadian bus (I think). It has the dimple/kidney bean in one of the throws. That one is standard. I was also able to secure a standard French 4" crank.

From what I've read, they are really decent quality.
 
A fragment of history...some time around 1950 or '51 Hot Rod reported on a local speedshop that had gotten hold of new genuine Merc cranks with unfinished rod throws... obviously some bootlegging going on there!
As I recall, many were capable of going to 1/4" over stroke with 21A rods, and some could be finished to 4 1/8 with full-size early or late Merc rods.
The next issue reported that the last few were available, and that there would be no more...Clearly, Ford had discovered the hot rod mole who had been slipping half-finished cranks out the back door!
Somehow, I imagine a Rouge plant machinist carrying and extremely odd, extremely heavy 3-foot long lunchbox home every day...and probably being buried in it the day after he was caught.
 
The flavor is this thread, I guess, is that our 4 inch Mercury cranks are no less loved today, than they were about 60 years ago.
 
They make ne Navarro heads and manifolds. New 97's. New pistons. But nothing like finding a really good Merc Crank to start off the new year.

Got a really nice 59A engine from same guy a few month back. No cracks. I think I can think of a nice place to put the crank.

Tim
 
If I understand the post you traded a good block for a good merc crank???It's your call but I would much rather have a good block, you can buy a new crank with various strokes any day of the week. Kind of tough finding blocks without cracks.

Harry
 
HP Hunter said:
If I understand the post you traded a good block for a good merc crank???It's your call but I would much rather have a good block, you can buy a new crank with various strokes any day of the week. Kind of tough finding blocks without cracks.

Harry
Click to expand...
I did not pull the 8BA apart. I did not know the condition of the block. I do know that it was pulled from a dump truck cus it was giving them problems.

What I got in the deal was a 59AB disassembled (no crack block)(studs removed no strip threads) and a standard merc crank. What I gave was 100 bucks and an 8BA that I did not know much about.

I am happy with the deal.

Tim
 
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