the 4 inch merc crank dilemma

Ok so heres the deal, a few weeks ago I bought a 1951 ford 8rt flathead for my Av8 model a project and I am in the process of collecting speed parts for it and rebuild parts I found a Merc crank for sale about a half hour away guy wants 200 bucks comes with rods I am waitin on pictures from him if he is able to send them... is there anyway to surely Identify this as a 4 inch merc crank while it is not in a block also are there any markings or stampings on it to find out? any help appreciated and does it sound like a good deal

 

Other than measuring the stroke, measure the large or thick throw. It should be 6 inches. A ford will measure 5 1/2. Google flathead mercury crankshaft id


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thanks 200 sound like a deal to you

 

Here is a good article I had bookmarked.

http://www.flatheadv8.org/boomer/crank.htm

 

Check the size of the knockout plugs on the throws. The ones on a merc crank measure about 5/8" and a Ford measures about 3/8" if memory serves me

 

flatcrazy thats a great article I just found it on google a few minutes before you posted it for all us hambers to see

 

Yep, that article is a wealth of knowledge. As far as 200 bucks for a merc crank, I'd take a look at some of those offered in the aftermarket. Scat has the 4 1/4" and they're others that are brand new. For 200 you'll probably still have to bring it to the machine shop, but maybe not. I picked one at a junk yard two weeks ago for $25, but I was lucky. Just depends on what you want to do with the flathead.


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Merc cranks usually run between $250-400. $200 is a pretty good deal for a good useable crank. I've never seen one of those $25 Merc cranks, some guys get all the luck

 

yeah I usually have 0 luck lol All the yards around me have 1980's and newer and if I find a flathead in one it is freshly pulled...................from the bottom of the nearest lake lol

 

those scat ones are near 700 though too

 

Someone brought in two shoeboxes for scrap, a 49 and a 51. Both were filled with parts that were pretty ugly. But always check the trunk. In the back of one were two cranks wrapped in plastic. One ford one merc. It isn't in the best shape but for a quarter, I can afford to get it checked out at the machine shop. If it is no good, it was 25 bucks.

I would pay 200 for a good crankshaft all day long, as a matter of fact, a month ago I did. It was for a 341 desoto, but a mercury is probably just as rare. Just have a shop NDI it if you can


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Dick's Beaters said it all....measure the throws.....

 

Pretty right answer most of the time but typical ford throws the odd curve ball and I have seen 3 3/4 ford cranks with the large 5/8 clean out plugs too.

 

My Merc crank was $250 and really nice, it was found in an OLD garage with a "fresh" 25 y.o. turn and tag attached stating so. Long story short but there where a few very small pits on a couple of journals and the front seal flange had some rust on it from sitting so long covered in grease which moisture had worked underneath. I decided to go another .010 and it cleaned up perfectly, looks brand new and spins just beautifully. DO NOT rely on the 5/8 clean out as verification as several "Canadian" Mercury cranks I've seen use 3/8 plugs as does mine.

I would assume any crankshaft you're purchasing used needs a turn and factor that into the decision.

Personally I would have been quite happy with the Ford crankshaft that came out of my 59A but since the Merc unit was avail and the Ford piece would have required the same work it was a logical choice and modest expense overall.

 

Greetings! From what I have heard you can't trust the "welsh plug test" and I haven't a clue how you would accurately measure the stroke unless the crank was mounted in a block. The way I figure out which is which is to look at the relationship between the rod throw and the main throw, on a 4" crank, the bottom of the rod throw should be almost even with the top of the crank throw, on a 3-3/4" crank, the rod throw overlaps the crank throw by 1/4", hope this helps.

 

thank you gentlemen

 

To measure actual stroke out of the block...
Lay crank on a couple pieces of 2X4 on its main bearings to get the counterweights off of the ground.
Put this rig against your living room floor against a wall.
Put a board onto a crank throw standing up edgeways, meaning tall edge of board against wall, width across where a rod woud go.
Rotate the crank and mark highest and lowest travel point of the top of that board on the wall. Two people can do this more easily, so have your divorce attorney do the measuring part while you rotate.
Not prcision, but plenty good to tell 4 from 3 3/4...and hey, it might turn out to ba a 4 1/8.

On the measuring at fleam arket:
First, be sure crank is 1949 up with TWO oil holes per throw. This distinction will fail in an expensive direction if measuring a 59A crank!
Next, measure across the biggest front counterweight at its broadest point. Ig greater tha 6", Merc, if a bit less, Ford.

 

As Wideglide sez, Canadian Merc cranks have the 3/8 plugs. I have half a dozen of them.
BTW the rods are the same as 239 rods, it's the pistons that are different.

 

Man o' man I cannot wait to build this one. I should be going to check out the crank this weekend. Then it is time to save up for all the other parts for the rebuild with the 4 inch crank and a .30 over bore what cu.in. Combo will that be like 260?

 

I have a 49-53 merc crank 20 under i was going to use on a stroker motor but went a differnt path. Id let it go for 200.

 

I just used an online calculator which suggested 260 for your 4" x 3.217" combo

 

I have learned some things. That's what I like about the HAMB. And besides, you can't put it on the Internet if it isn't true, right?

 

Yeah the hamb is always helpful and i will keep you in mind with that merc crank if something falls through with this one