346 Counterweights

[Paul Phillips]

Can someone please clarify when the crank counterweight style changed on the 346?  In rebuilding my engine (47 block), I found what I think is the earlier counterweight style with slotted mounting screws.  I was expecting cast weights with hex bolts.  Parts book seems to only list as a crank with counterweight as assembly, so it did not help.

Thanks!

PS: anybody have 3 of these bolts or know of a source?  Please see my WTB post.

[Steve Passmore]

My guess would be that it was sometime during wartime Paul, my 41 had the slots but the 47 I had did not, two wartime engines I have use the bolts.  I dont think you will be able to just change them as the weights are turned out more to allow a socket on the bolt head, you could have them turned out but I dont know what that would do to the balancing?

[markl]

Paul,
I wonder if you want to give a brief tutorial on how to get these bolts loose.  From your photo and my recollection, they are "staked" once tightened down.  And this must be what crossed up your machinist.  So maybe some finessed work w/ a Dremel tool can get the deformed material out of the slot. 

And then if you don't know the best tool to remove them, perhaps another Forum user has recent experience using a specific drag link (Snap-on, Mac etc.) to tightly fit the slot.  I though I could lay my hands on a derelict crank, but no luck.  How could a guy lose a crank?  But anyhow, I'm sure others have same laying around.  I've never seen new ones (slotted bolts) for sale, but they must have existed at one time.  As I recall, the counter weights need to be removed to turn down or polish the crank.  So how did they do this "back in the day"?

Mark Lowery, CLC #25216

[Paul Phillips]

Mark & Steve
Thanks for the replies.

It is necessary to remove these counterweights to machine and polish the crank.  Not sure if there is an undiscovered trick for back in the day, but here is what I found.  These screws are quite interesting to remove, so there is probably no perfect tutorial. 

Mark, you are correct that the counterweight is staked in the slot area to the screw, that material must be ground away.  The thread is 'oversize' at a full 0.625 diameter for a 5/8 - 18 thread, so it is an interference fit in the crank.  I did not measure the counterbore diameter, but it is nearly an interference fit to the screw head, which measured 0.874 dia.  These were torqued to a high rate originally, and have had +/- 70 years to find micro points of corrosion to bond together.  Of the 4, only 1 came out intact.  That took a bit custom made of tool steel to fit the slot, using a 3/4 drive and a 4 ft breaker bar. Not sure if there is an off the shelf drag link bit as an option.  The other three positions broke several bits, did not respond to heating, impact on the head, etc, all the usual machinist tricks.  Finally the heads were burned out, the counterweights removed, and bar stock welded to the remaining shank to create leverage points.

My first quest was to find replacement OEM bolts, but so far that has failed.  Plan B is to take a stock grade 8 socket head cap screw and modify to be a suitable replacement.  I have selected a McMaster Carr item #91251A487 as the basis.  This will be done with the principle of no machining on the counterweights and trying to match weight on the screws with the OEM sample.  The head will be turned down from 15/16 to 0.874- Dia and from 0.625 to 0.500 height to match the original.  The shank will be turned down slightly, at least to the thread minor diameter of 0.5568 (OEM was 0.539) to a measured depth that matches the start of threads in the crank.  The original thread engagement was at some depth in the crank, so this will give both strength potential and extra mass.  The OEM screw was 3.323 long, so I selected the 3.5 long replacement instead of the 3.25 option. There is extra depth in the crank to allow for more thread engagement, so final length will be chosen to get the weight match.  Red Loctite will be used as final insurance.

The work should be complete in the next week, so I will update on exactly where we end up.

Paul