There have been plenty of informative threads on here and other sites about the difference between internal and external balance. That's not my query here. My question pertains to whether or not an engine that is externally balanced from the factory could be internally balanced by a shop by either drilling or adding tungsten to the crank, and using a neutral balance dampener and flex plate/flywheel? The thought occurred while thinking in the realm of first-gen Olds, but I'm sure could be just as pertinent in other engines like the 86-up SBC vs the earlier internal balanced variants. The 303 and 324 Olds are internally balanced, while the 371 and 394 are externally balanced. Theoretically, with a neutral balanced dampener and flywheel, and by having the crank balanced to the reciprocating assembly, could you turn the externally balanced 371 and 394 into internally balanced engines like the 303 and 324? Help me out here.
Yup, can be done. Some years back I had a request to change the balance in a cast crank 340 basically doing as you propose. No issues. I am surprised that GM went external as early as '57. .
Yes most engines can be internally balanced but the mallory metal con be expensive.normally only done on race engines
Yes you can. Typically takes a couple of slugs of heavy metal when balancing the crank. Then just use neutral flywheel and dampener.
The downside to doing this is crankshaft weight can go up considerably, which changes how quickly the motor will rev up. Because you're usually adding weight closer to the centerline than the existing balance weight, the some of the 'lever' advantage is lost, requiring more weight for the same balance. As an example, when Ford changed the 302 balance factor from 28 oz to 50 oz, they were able to reduce overall crankshaft weight by something like 9 pounds... no small difference.
Going with a longer ( heavier) connecting rod makes it possible also . My ,3.75" SBC crank uses 6" rods & is internally balanced without heavy metal.
I swapped my externally balanced 350SBC into an internally balanced 383SBC with a kit and a neutral balance flex-plate
That’s an excellent example. I was aware of the difference in the ford balancer weight but had not considered that relative to overall crankshaft weight. That much reduction in the the weight of the crank is certainly significant. I recall back to my college days when my buds and I were daily driving SN95 mustangs, and one guy put an aluminum flywheel in when he did his clutch, and we were all shocked at the difference. I was more considering the situation with the thought of running a lighter piston and rod combination, at which point the assembly would have to be balanced regardless. You make a good point regarding the weight possibly going up, but the same could hold true with the weight going down, no? Sent from my iPhone using H.A.M.B.
Better off getting a new crankshaft from someone like SCAT.. The 408“ in our dirt car now is a stock 400 block with an aftermarket 3-3/4”stroke crank and normal flex plate and balancer.
That isn't because the rod is heavier though....the longer rod allows them to put a bigger counterweight on the crank so that the crank then has enough weight in it to internally balance. That fact often prevents you from using that internally balanced crank with 5.7" or 5.85" rods because the shorter rod puts the pin boss of the piston and the crank counterweights in the same space near bottom dead center. The shorter rods require smaller diameter counterweights, which then need added in weight to internally balance. In the drive to acheive internal balance of an external balance crank, making the connecting rods lighter can be an important facter. Making them heavier is going in the wrong direction since it then requires more weight in the counterweights to offset the heavier rod. i called SCAT once about using 5.85" rods with their internally balanced 3.75" stroke crank which is listed for "6.0" minimum rod length, won't clear 5.7" rods"....just to see of a 5.85" rods could be squeezed in there....the answer was no due to clearance issues, not weight issues. I would have had to turn the crank counterweights to a smaller OD and then turn around and add mallory to get the weight back I had just turned into shavings.
Years ago I had a 394 crankshaft internally balanced to run with an earlier flywheel, as stick flywheels for the 394 can be hard to find. Mallory metal, machine work and balancing was less expensive than a custom flywheel
The thought of a custom crank has certainly crossed my mind. Especially since it would allow me flexibility to perhaps use more common Pontiac or Chevrolet rods and pistons in an alternate application without concern for rod or main journal size.
It would depend on a lot of factors. A lighter rod/piston combo would help, but how much external weight on the original install is involved will probably be the controlling factor. A small weight at the flywheel will translate into much more weight at the crank. I suspect you'd probably need some exotic race pieces to save enough weight to be significant.
Yes as other have previously stated it can be done. I had a 400 3.75 Chevy crank turned down to fit a small journal 327 block. Then had it internally balanced. It wasn’t cheap. I can’t remember how much each slug of Mallory was but it was not cheap! Sent from my iPhone using The H.A.M.B. mobile app
Last time I checked it was $70 an inch for 1" round....gets real expensive in something like you just described where you might need 6 to 8 pieces or more.
Yes you can. But the more important question is, 'Why do you want/need to go to an internal balance?' If you don't have any reason to go to internal balance, there really isn't any reason to spend money on conversion. Two main factor to do so would be; cannot find an external balanced harmonic damper or flywheel/flexplate to match your existing engine. plan on revving to higher rpms and want to reduce the stress on the block.
I remember reading something about crank harmonics being better controlled with internal vs external balance , way above my pay grade ????
My flywheel on my little stroker Ford has exchangeable weights so I can use no weights for internally balance. Add a weight for 28 oz. or swap out that weight and it becomes a 50 oz. Same flywheel and the same for my balancer just swap out the weights.
As been said, it's easily done. As to why, or should you, I think it's application specific. The ones that I have done have been 400 SBCs built to bigger cubes and with longer rods that wanted to use short water pump, small balancer, and a 153 tooth flywheel to fit into a hot rod chassis. They typically took 2 or 3 slugs of heavy metal. So in the overall scheme of things it didn't add that much to the project to get the desired end result.
I built an aluminum Rodeck 427 small block for my roadster and my friend said "you know that Mallory metal is expensive don't you" and your exact words were the first thing out of my mouth, you should have seen his eyes bulge when he saw how much just the bare block cost.
Rumbling around in the back of my mind (and this may be wrong), I thought I saw something about this a long time ago. It seems that the location of the balance weights were different and that converting to internal balance required welding counterweights to the crank and machining metal off the original counterweights of the external balance crank. Here is something I just found. Is this true of all engine cranks.....I don't know. I have heard of finding a different brand crank that was similar to what someone has and being able to custom grind it to fit. Probably the best bet is a custom made crank.
When I bored my 427 tall deck to .030 over, it required mallory metal to balance correctly. It is an internal balanced crank from the factory. I think it was only a couple hundred to re-balance it, including 2 slugs of metal. If I remember correctly, that sucker weighs in at 50 lbs.!!!
Not that I’m going to build another one but any SBC I’d do today would start with a Dart SHP at the minimum. After seen what one takes on a 1/2 dirt track it’s sold me. A little “M” would be better. I guess I just like cast iron.....
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