crankshaft machining questions

[kotu100]

i've been calling a bunch of diff shops around my area gettin quotes and info on crank machining.
One place said it was impossible to grind a crank and use oversize bearings, said it HAD to be welded then ground, also said that to balance it he needed the crank pulley, dampner, rods, bearings, flywheel. And it was 200 to grind and polish and almost 400 to balance, so almost 600 total, seems waaaay too steep for me.

So the point of this thread is im wondering who got they're cranks machined? Did you get it balanced? What did they to to balance.

This is my 1st bout with machining a crank so any good info is appreciated.



Oh and this is the tech section, so no my buddy did this or i heard that, 1st hand experiences only plz.
Thanks!

 

[IJ.]

You can only go 10/10 before the crank needs to be rehardened and at that stage it gets very pricey.

If you haven't changed any of the reciprocating parts ie: rods/pistons you can get away with the factory balance, personally I always balance my engines no matter what.

 

[kotu100]

You can only go 10/10 before the crank needs to be rehardened and at that stage it gets very pricey.

If you haven't changed any of the reciprocating parts ie: rods/pistons you can get away with the factory balance, personally I always balance my engines no matter what.

thanks for the response Ian.
Any chance u could give me more info on the balancing aspect?

 

[IJ.]

The 7m is a 0 balance engine meaning you get the reciprocating assembly balanced as a unit ie: Crank, Rods, Pistons then the Flywheel, Clutch, Damper can be done on their own.

A lot of engines use an offset mass on the flywheel and Damper as part of the crank bobweighting.

 

[gaboonviper85]

The 7m is a 0 balance engine meaning you get the reciprocating assembly balanced as a unit ie: Crank, Rods, Pistons then the Flywheel, Clutch, Damper can be done on their own.

A lot of engines use an offset mass on the flywheel and Damper as part of the crank bobweighting.

For example: mustang 5.0 fox body's

 

[kotu100]

Out of curiosity, what would happen if someone were to upgrade their pistons/rods to a forged alternative, but didnt balance the crank with them?

 

[gaboonviper85]

You don't generally balance them together....you balance the pistons together.....balance the rods together....balance piston/rod combo....balance the crank

if you didn't balance then don't expect the motor to last 80,000miles.

 

[Van]

Hi,
The 7M I'm having built just finished having the rotating assembly balanced: pistons, rods, wrist pins, rod bearings, all zero balanced, then the crank spin balanced to 750 RPMs, with crank bolt, damper, flywheel and pressure plate. I'm building an enduro engine, and it is important to me to spend the time and $$$ to get that done. They spent hours getting the crank and pressure plate to spin balance at 750 RPM. I had some weight taken off the 6M crank, bull nosed the leading edges and smothed out the sides of the counter balance on the crank, in prep for the Ishi-J crank scraper and had the journals micropolished. You can see for yourself on my build thread, Race Ready 7M.
What are your plans for your build? Van

 

[kotu100]

well for this engine i just want to get it running, maybe 400 whp.
thats why i cant justify spending over $500 just to get the crank machined.

I'm still a little confused how the pistons/rods get zero balanced... do they just make sure everything weighs the same?
i've been doin google searches but the sites i find only talk about the principle behind balancing, not how its acutally done.

sorry if i seem stupid constantly asking the same question pretty much, i just really wanna make sure i know wat im doin before i start doing anything.

 

[IJ.]

Pistons,pins,rings all get made the same weights.

Rods are end weighted and made equal.

Once all of this is done they then calculate a bob weight and bolt 6 of these to the crank and spin balance it.

They will balance the damper, flywheel and clutch assembly on their own.

 

[kotu100]

thanks very much for the responses guys, i've learned quite a bit.

 

[suprarich]

IJ's last post explains the process just about to the letter.

It is about 200.00 to grind and polish the crank. A good high speed balance of the rotating assembly, to 1 gram or under in the 3 postionas measured on the crank, should be no more than 200-300. 400 is too much.

The machine shop will need the crank, damper, damper bolt, flywheel, and flywheel bolts to do a high speed balance. They should never take or add weight to the damper or flywheel, but they do need them to spin the crank on the machine. The damper and flywheel affects the rotation on the balancer. Also, it is much better to send the pistons, rods, and bearings along. They are not attached to the crank as it spins, but the machinist will need to weigh each item to set up the bob weights. If you cannot send the pistons, rods, and bearings,,, he will need to know the weight of each piston with its rings and pins, weight of each rod with its bearing installed.

Like IJ said, if you can send everything, then the pistons can all be weight matched, and the rods with bearings can all be weight matched as well.

You can call me at 330-541-1122 and I would be happy to explain the entire process in great length. I do all mine on a winona computer balancer and I have the NASCAR bob weight package which is the big $$ package.

 

[IJ.]

I got to do a bit of hands on with the last 2 M cranks I had done
(long time customer of the shop doing the work and was curious to get in there and play)

 

[kotu100]

thanks for the awsome info rich, i was actually gonna pm you to see how much it would cost to send my stuff out to you, but i found a useable crank in a extra 7m i had layin around.