DIY crank balance?

I am building a 2.9L stroker (M50 block, S50 crank + rods) for my E30.

I have to send the block and rotating assy out to be cleaned & checked. I expected to do all the final assembly myself but do any of you fine gentlefolk use a scale and a file to balance the rods yourself? How would you balance a crank? Maybe with a lathe... and drill out the lobes that rise to the top until it won't spin by itself?

I'm all for DIY stuff, but the race shop that bored the hell out of my mini block ( .114 over :twisted: ) also balanced and assembled my rods & pistons and dynamically balanced my crank for an extra $40.

Well... that certainly makes it a no-brainer. I was quoted "A couple hundred bucks" for a crank and rod balance with me doing final assy.

A couple hundred?? ouch...

My guy needed the pistons when he was doing the overbore,. When he called to tell me it was done, he asked if I wanted the rods/ pistons balanced and assy'd for an extra $20. Then I asked about crank balancing. "Sure, another $20, bring the crank and rods when you come for the block."

Yup, it's when you start adding magnaflux, shot peening, cyro-treating, etc. that the number start adding up faster.

jack

Jack wrote: Yup, it's when you start adding magnaflux, shot peening, cyro-treating, etc. that the number start adding up faster. jack

Yeah, everything crack-checked, crank polished, pistons, rods + crank balanced to .25g, rods peened and bores cleaned. No cryo or final assy.

Did you have to hone the block? Im asking because I was told you had too in order to get a 2.9 from a 2.5 thats why I haulted my project until I could find a willing machine shop locally.

Jack wrote: Yup, it's when you start adding magnaflux, shot peening, cyro-treating, etc. that the number start adding up faster. jack

and line boring

or is that align boring?

I paid $75 to have my crank balanced on top of the other machine work. I assume that included balancing the rods and pistons.

Balancing in a lathe or on rollers is not good for a crank shaft as it is only "single plane" balancing a crank is long and has to deal with "couple imbalance" it needs to be done in a machine and spun. Bring everything that ataches to the crank! Fly wheel or flex plate, Timiming gear, lower pulley etc. This way the guy can balnce the crank to "zero" ad on thing at a time and corect on it then add the next section this way if you F'up one peice down the line they'll no how much residual was in it and where.

Try www.LINDSKOG.COM

44Dwarf

So the boring was only basically to clean the cylinder walls using the same stock m50 pistons?

You typically hone the bores just to clean them up before a re-ring. Engines are overbored when the bores are no longer straight (they are tapered) or they have scored walls. When they are overbored, they will usually hone the cylinders the final few thousanths with progessively finer stones.

So can you use m50 stock pistons with the s50 crank and rods in a m50 block to get a 2.9?

44Dwarf wrote: Balancing in a lathe or on rollers is not good for a crank shaft as it is only "single plane" balancing a crank is long and has to deal with "couple imbalance" it needs to be done in a machine and spun. Bring everything that ataches to the crank! Fly wheel or flex plate, Timiming gear, lower pulley etc. This way the guy can balnce the crank to "zero" ad on thing at a time and corect on it then add the next section this way if you F'up one peice down the line they'll no how much residual was in it and where. Try www.LINDSKOG.COM 44Dwarf

Correct me if I'm wrong, but I beleive four bangers can be effectively balanced statically. They do not need to be spun up.

As the Dwarf said, typically, you need to bring one of everything (piston, with rings, pin, etc) for them to fully balance and yes, bring everything that spins, from the front pulley to the clutch pressure plate and disk.

It was wicked cool when my machinist told me that he had to do essentially nothing to balance my crank. The factory had balanced it incredibly well, considering that it came out of a 1964 TR4. he drilled two 1/4 inch holes about 1/8 deep and added a tiny bit of weight elsewhere.

Jack

I real shop will spin them to corect for couple. Just like your car tires it might need weight removed or added on both sides to get it mint, you can static balance a tire and it feels bad but spin it up you might find it out to lunch.

44