Crank Balancing DIY

[KiwiGF]

Thanks for the feedback guys. I'll try to get some pictures later. Regardless, what would be the best approach to getting  the BF higher?

If you use your figure of 1310g for “counterweight (CW) in the calculation for balance factor, what % balance factor do you get?

Note: I used the std counterweight of 1056g in my calcs on the basis that was more likely to be the correct figure than what I actually measured (1050g), but in your case you need to use 1310g.

EDIT: my calcs indicate that an A10 crank with a CW of 1310g with std weight BSA rods etc would have a balance factor in the region of 75%.

[Seabee]

Hi Kiwi,

I didn't use the figure of 1310 for CW. It is the actual weight the crank required to maintain position.

I used 1113 (RR large journal) as CW. Should I have used the 1310 actual instead of the 1113 standard as the CW?

My formula ended up looking like this:

RF = 100 (1113-665)/900 = 49.77%. 665 is my actual big end weights, including 25g for oil. 900 is my actual reciprocating weight totals. If I substitute 1310 for 1113, I get a BF of 71.66%! Yikes!

Here's the interesting part, my crank has 3 fairly deep 1/2" diameter drillings. The 3 holes total 2.34" in depth! I measured another large journal crank I have. It has 3 shallower 1/2" drillings totaling only 1.26" in depth. I'm guessing the crank I'm using has been played with before.

If my math is correct, I think the 49.77% will be okay, but I'd like to raise it to 55-60%. Would the best course of action be to drill opposite the crank pins in the flywheel?

I have added some pics of the flywheel drillings.

Thanks for the help,

Seabee

[Seabee]

Well, it only let me attach one. Here's another:

[Seabee]

And one more

[KiwiGF]

Hi Seabee, yes I think you will have to use a measured value for CW as opposed to a stock value, given you have measured it at 1310g.

I don’t think the drillings in the pics would cause a 200g difference from stock but I have not calculated the weight removed. They might be stock drillings by BSA, might not.

Did you accurately measure the CW of your crank? Maybe using either of the (2) methods I used? Or even the BSA method in the service sheet?

[worntorn]

If my math is correct, I think the 49.77% will be okay, but I'd like to raise it to 55-60%. Would the best course of action be to drill opposite the crank pins in the flywheel?

I have added some pics of the flywheel drillings.

Thanks for the help,

Seabee

I'm no expert on this, but after getting involved in a couple of Dynamic crank balancings and asking lots of questions, here's what I think I know-

Raising the balance factor of any crank can't be done by drilling opposite the crankpin. Removing weight from that area lowers the bf. The only way to raise the bf by working on the Bob weight side is to drill the crank metal away and replace it with Mallory metal which is quite a lot heavier than the crank Bob weight steel.
Sometimes, at a later date and very high rpm, the Mallory metal flies out and destroys the engine.
I had a look at the remains of a racing V8 engine that had lost its Mallory metal and that really put me off that plan!

Glen

[Seabee]

Kiwi,

I used your method with the bottle of water. It worked very well.

Worntorn,

Logically thinking, I agree with you. I think drilling any more would make things worse.

I am building a bit of a "hot rod" version with this spare engine. It will have a 357 cam, aluminum A7 head with detachable intake manifold (dual carb?!) re-chambered to match a10 bores, and hi-comp pistons. I guess I could just accept it as a higher RPM engine and go run the snot out of it! I could build a Musky imitator, but I'm probably shy in the cahone dept!

I guess I'm going to just have to take a chance and see if I end up with a Tri-hard foot and hand number....

Thanks guys,

Seabee

[KiwiGF]

Kiwi,

I used your method with the bottle of water. It worked very well.

Worntorn,

Logically thinking, I agree with you. I think drilling any more would make things worse.

I am building a bit of a "hot rod" version with this spare engine. It will have a 357 cam, aluminum A7 head with detachable intake manifold (dual carb?!) re-chambered to match a10 bores, and hi-comp pistons. I guess I could just accept it as a higher RPM engine and go run the snot out of it! I could build a Musky imitator, but I'm probably shy in the cahone dept!

I guess I'm going to just have to take a chance and see if I end up with a Tri-hard foot and hand number....

Thanks guys,

Seabee

I think we would all be interested in knowing how the crank has been modified to have a CW of 1310g, steel weighs about 8g/cm3, so something like 25 cm3 of steel has been removed and/added? If mallory metal plugs were used they would be in the 2 counterweights I guess.

Maybe the “74” in the first pic of the crank is an indication of the % BF by the person who modified it? (Unlikely but you never know eh).

[muskrat]

G'day Seabee.
I'm flattered.
The only problem with using an A7 head is the valve size. I ended up with a bit of a mish mash. SR valves but had to marry the A7SS collars to the SR collets and use SS springs. Using the SR gear the springs will bind, especially with a 357 cam.
Cheers

[JulianS]

I think the 74 is a factory marking - similar seen on other cranks.

The photos show one with 79 stamped and one with 77 stamped both plus a scribed line. Timing side on the left both photos.

[Seabee]

Kiwi,

I used your method with the bottle of water. It worked very well.

Worntorn,

Logically thinking, I agree with you. I think drilling any more would make things worse.

I am building a bit of a "hot rod" version with this spare engine. It will have a 357 cam, aluminum A7 head with detachable intake

I think the 74 is a factory marking - similar seen on other cranks.

The photos show one with 79 stamped and one with 77 stamped both plus a scribed line. Timing side on the left both photos.

manifold (dual carb?!) re-chambered to match a10 bores, and hi-comp pistons. I guess I could just accept it as a higher RPM engine and go run the snot out of it! I could build a Musky imitator, but I'm probably shy in the cahone dept!

I guess I'm going to just have to take a chance and see if I end up with a Tri-hard foot and hand number....

Thanks guys,

Seabee

I think we would all be interested in knowing how the crank has been modified to have a CW of 1310g, steel weighs about 8g/cm3, so something like 25 cm3 of steel has been removed and/added? If mallory metal plugs were used they would be in the 2 counterweights I guess.

Maybe the “74” in the first pic of the crank is an indication of the % BF by the person who modified it? (Unlikely but you never know eh).

Kiwi,

I didn't find any mallory plugs. It's interesting that the total drillings are twice what my other crank has.........

[Seabee]

G'day Seabee.
I'm flattered.
The only problem with using an A7 head is the valve size. I ended up with a bit of a mish mash. SR valves but had to marry the A7SS collars to the SR collets and use SS springs. Using the SR gear the springs will bind, especially with a 357 cam.
Cheers

Thanks for that Musky! I'll have to take a very close look see!

[Seabee]

I think the 74 is a factory marking - similar seen on other cranks.

The photos show one with 79 stamped and one with 77 stamped both plus a scribed line. Timing side on the left both photos.

Wow Julian, that crank has been drilled to death! I wonder what the story is there?

[JulianS]

Static balancing by a company in Swanage, Dorset back in 1975 or 1976. Balanced to 65 %. Not convinced there was any noticsble improvement.

it grew some extra holes when dynamically ballanced by SRM 10 years ago. 54%. See photos below of the extra holes on the flywheel edge. That job did reduce the vibs.

[muskrat]

G'day Fellas.
I wish I had pics of my race crank. There wasn't much left of the flywheel. 1/4" off each side and drilled like swiss cheese. Can't remember how much lighter it was (around 1.5kg). Balanced to 72% she revved like a two stroke and would rattle my teeth out at anything under 6 grand but smooth as silk at 7.5 to 8
Cheers