351M crank converted to 351C crank - is it possible?

I have a couple of 351M cranks laying around and I was wondering if it is possible to convert them into 351C cranks. The big ends are the right diameter already and I plan on turning the mains down to the cleveland size of 2.749".

Now the counterweights are big enough to balance the 351M rod and piston so they will be too large (heavy) for 351C piston and rod. So a balance job will be needed, but I'm thinking it may be possible to internally balance the rotating assembly due to these larger counterweights.

This sounds like a good way to build in the neutral balance the factory failed to do when they designed the cleveland.

Now has anybody done this before? Or have any opinions on the matter?

Thanks
Tristram

Mmm..Interesting question.. Thinking about it don't see why it wouldn't work.. As long as the conterweights are the same and will work with the shorter 351C rod.. I'd like to know if it works out.

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On 2007-02-16 00:23, xafalcon wrote:
I have a couple of 351M cranks laying around and I was wondering if it is possible to convert them into 351C cranks. The big ends are the right diameter already and I plan on turning the mains down to the cleveland size of 2.749".

Now the counterweights are big enough to balance the 351M rod and piston so they will be too large (heavy) for 351C piston and rod. So a balance job will be needed, but I'm thinking it may be possible to internally balance the rotating assembly due to these larger counterweights.

This sounds like a good way to build in the neutral balance the factory failed to do when they designed the cleveland.

Now has anybody done this before? Or have any opinions on the matter?

Thanks
Tristram
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I don't know of anyone having tried this as the machining costs in my neck of the woods makes it a money lossing option. The counterweights will need to be cut down to avoid interference with the piston skirt. The 351M/400 has a 10.297-10.302 deck height which means longer rods (6.58" vs 5.78" ). Once you cut down the counterweights, you will need to neutral balance your flywheel/flexplate and get a neutral balancer. There is a lot more weight in the 351M rotating assembly than a 351C, which means about 12 - 15 lbs (5.5 - 6.2 kg) extra iron in the crank to balance that tall piston and long rods. BTW, how did you get a bunch of M cranks in NZ? Never heard that Ford officially exported the 351M/400.

--J

<font size=-1>[ This Message was edited by: xafalcongs on 2/16/07 9:29pm ]</font>

<font size=-1>[ This Message was edited by: xafalcongs on 2/16/07 9:29pm ]</font>

Isn't the snout a little different too? Seems there is an extral spacer machined into the C crank if I'm not mistaken. You'd have to machine a spacer to go behind the harmonic I believe.
[addsig]

Question:

The main bearing oiling holes come in at an angle. So, turning down the main's would move the oiling holes more to one side of the bearing instead of a dead center inlet.

So, would that restrict how much oil is getting into the Crank main bearings, then to the rods?

Also if there is a partial blockage of those oiling holes, would that raise the oil pressure in the rest of the engine block?



<font size=-1>[ This Message was edited by: Mikes66 on 2/16/07 10:20pm ]</font>

Likely by the time you turn the counterweights down to fit you'll be back to adding heavy metal to go neutral. You should price it out - you'll probably find that unless you own a crank grinder and a $25K balance machine that it's waaaay cheaper to buy a stroker kit than fix some old 351M crank.

Figure If I did it here and paid for it - I just did a neutral 351C crank. It took 4 slugs of heavy metal and 10 hours labor. At the local shop prices that's $150 for the base balance, $35 hr shop time plus materials. Comes to $660 to neutral the stock crank.
[addsig]

Hey, thanks for the replies. It's always great to get some critical and knowledgable feedback before I leap into another project. In answer to some of the questions above I can say.

Crank snouts are the same

Years ago I bought a couple of 400M cranks to build a stroker, but I never did it. So I've now bought a couple of 351M's to turn into 400's. This of course leaves me some spare 351M cranks which would otherwise go to the scrap metal dealer. The 351M/400 was never available in any domestic NZ vehicles so they must be US imports.

The crank counterweights aren't much longer in radius from the crankshaft centreline (1/4" from memory). They seem to add the extra weight through some additional counterweights that aren't present on 351C crankshafts.

I'm going to balance the motor regardless of if I use a 351C crank or a modified 351M crankshaft, so that sort of cancels out alot of the balance bill. It's only the extra work that actually adds to the cost.

A neutral balance flywheel came with a 300I6 engine I got a while back. If I choose to go auto, it's a simple job to remove the counterweight from a flexplate. So I'm only going to need a neutral balancer.

But, I'll be sure to check out prices first. Like you say, a stroker crank may be a cheaper alternative. But living in NZ they are about $600, then I need rods ($500) and special pistons ($600).

Thanks again, any more comments greatly appreciated

Cheers
Tristram

If you're going to run it hard, you'd better spend bucks for the rods and pistons anyway. My stock stroke motor that's in work has $500 rods and $500 pistons. I intend to beat the hell out of it and those parts will pay back. The downside was the cost to get the crank lined out. If you're spinning it any RPM, you'll want an SFI balancer and most of those have removable weights. So some of your costs may be built in already.
[addsig]

Try fitting a 351C rod an piston first. When you realize how much has to be removed from the counterweights you'll throw it in the scrap bin and forget about it. I can't believe a regular 351c crank could be that expensive compared to the amount of work you're suggesting.

<font size=-1>[ This Message was edited by: BrianS on 2/17/07 7:08am ]</font>