RB25DET Crank Balancing

[CLEM0]

G'day guys-

Might be a stupid question but i've never built an inline 6 cylinder engine, and I am looking at replacing the crankshaft only in my RB25DET.

I have tracked down a good condition crank which I would be getting the basic machine work cleanup (polished, crack tested, etc) and then installing myself- but since i'll be reusing the pistons, rods and most other parts besides bearings obviously- is there any balancing that needs to take place, i.e piston and rod weight checking etc.

Cheers all!

[GTSBoy]

If you're going to balance the rotating assembly, then you need to balance the whole rotating assembly.

In an ideal build, you balance the pistons so that they are all the same weight, you balance the rods so that they are all the same and the piston/rod combos are the same. Then you balance the crank with the assumption that the identical piston/rod combo is hanging off of each.

There is no way that you would go to the effort of balancing the crank to suit unmatched piston/rod combos.

By the same token, the are any number of engines that have been slung back together with completely unmatched/unbalanced parts. It's all a matter of what you want to achieve. Bazzilion revs? Balance it. Minimise losses? Balance it? Drive it to the shops below 4000rpm 99% of the time? Save some money (by not spending as much on balancing).

[Duncan]

I agree, I wouldn't (don't) bother balancing the rotating assembly except for race use. If you are going to do it you should include everything that turns from balancer to clutch

[CLEM0]

17 minutes ago, GTSBoy said:

If you're going to balance the rotating assembly, then you need to balance the whole rotating assembly.

In an ideal build, you balance the pistons so that they are all the same weight, you balance the rods so that they are all the same and the piston/rod combos are the same. Then you balance the crank with the assumption that the identical piston/rod combo is hanging off of each.

There is no way that you would go to the effort of balancing the crank to suit unmatched piston/rod combos.

By the same token, the are any number of engines that have been slung back together with completely unmatched/unbalanced parts. It's all a matter of what you want to achieve. Bazzilion revs? Balance it. Minimise losses? Balance it? Drive it to the shops below 4000rpm 99% of the time? Save some money (by not spending as much on balancing).

Thanks mate- it's only going to be at 450whp at most, and mainly a daily driver- maybe cop a limiter here and there but nothing insane. Thanks for the reply.

[CLEM0]

3 minutes ago, Duncan said:

I agree, I wouldn't (don't) bother balancing the rotating assembly except for race use. If you are going to do it you should include everything that turns from balancer to clutch

Certainly not a race car (yet, until it decides to fully blow itself) just a daily, around 450whp at the minute. Cheers.

[Duncan]

funnily enough (once metricised), that's more than my race car makes.....but anyway

[CLEM0]

2 minutes ago, Duncan said:

funnily enough (once metricised), that's more than my race car makes.....but anyway

Probably a substantially better Power:weight though  

[ChrisW434]

Inline 6s balance out primary and secondary rotation forces so are inherantly well-balanced anyway. Weight matching pistons and rods is a must do i think (many builders go to the extent of weight matching either end of the rod apparantly as well) but for the most part, the crank should be sweet i reckon, i asked all these same questions awhile back. Verifiy bearing clearances as well. Also, ask the machine shop to remove oil gallery plugs and flush the gallerys as well, replacing the oem plugs with screw-in plugs. Just some food for thought...