Where/how to weight rods, pistons, etc?

[cstreit]

Gang,

I need to weigh my rods, pistons, wrist pins, etc.. to balance them for assembly. A few questions?

1. To what detail do you need to reasonably measure? (GRams, 0.1 grams, .00000001 grams?)

2. Any suggestions about where to find such a scale?

[BK911]

I'd like to follow up with an additional question..

When balancing an S rod, what happens to the nitrating?

Thanks!
BK

[gregk1]

When balancing rods you weigh one end at a time hang one end and let the other end lay on the scale, Then the other end, A rod balancing scale has the hanger and all bilt in to the opparation, I tried to do it on a triple beam, I have at home, Didn't work to well Unless you had a box full, And not worried about screwing a few up while trying to get it right.I wouldn't advise it, BK isn't nitrating A form of case hardning?? If you ground the area off I would think you will have to at least re do that area. Just my 02

[cstreit]

Gang,

I'm not talking about end-to-end balancing, that's already been done. I'm talking about weighing the parts to make sure equal weight parts are opposed to each other on the crank...

[911pcars]

As Gregk1 suggests, nitriding is a form of surface hardening. It's mainly used to strengthen and create harder friction surfaces in crankshaft main and rod bearing journals. When material is removed from a connecting rod for balancing, I suggest shot-peening them to increase their strength.

Chris,
Although one can purchase a con rod jig to weigh big and small ends (e.g. Jegs has a con rod balancing fixture for about $145.00), balancing engine parts is not a DIY operation without some special machinery, jigs and a lot of experience. It doesn't cost very much to have a specialty shop balance all the reciprocating parts (crank, rods, pistons, pulley, flywheel, pressure plate, etc.). While the factory weight tolerances are pretty good, balancing can get all the pieces a little closer to perfection. On a race-based engine where you may need to replace an individual component (for whatever reason), having records of component weights will make replacement that much easier to accomplish.

Sherwood Lee
http://members.rennlist.org/911pcars

[Bill Douglas]

I have heard you put the rod, piston etc, all the same parts that will be together with te same piston on the scales and take the weight out from inside the piston. I don't know how similar you make them though. Lots of offices and labs have scales that wiegh to .01 of a gram so ask around your friends.

[HawgRyder]

In the past I have used 2 scales to weigh rods.
I find that if you make a mark (grease pencil) on one of the scales ans use this as a fall line for one end of the rod...then weigh all rods using this line..it will give you a good idea of total weight (add the 2 scales) end weights (large and small)...and with this knowledge you can set up the engine.
With horizontally opposed engines (911 type) the difference in weights is not as critical...but should be as close as possible.
Also...if you pair up the rod weights you can even out the balance per rod pair.
Bob

[TimT]

Chris, somewhere along the line I came to posess a triple beam balance. I use it to weigh the parts. The procedure I use to wiegh the pistons, and rods seperately, and mark the weights on them with magic marker.

Then juggle around components until you have 3 sets of evenly matched pairs of piston/rod assmblies. If your rods have already been end balanced, you may only have to do adjust at the piston weights. If you need to lighten a piston to get a suitable match you will want to remove material from under the piston dome.

[911pcars]

If you're going to button up the engine and never touch it again until the next rebuild, then weighing complete subassemblies is probably okay. But this is a race engine isn't it? If so, you may want to place more emphasis on the reciprocating parts. What if you have to replace a single piston or a single connecting rod and you don't have the details of its weight? IMO, adjusting the weight of each individual component is better in the long run.

While pairing the equal weight pairs on opposite sides of the crank is permitted by the factory, I would attempt to get all six rod/piston assembly weights as close as possible. I believe Porsche's spec is 9 grams max. difference between rods (with no weight spec ? between big and small end). By pairing the heaviest and lightest rod/pistons, it's possible to have opposing cylinder pairs that are as much as 18 grams heavier than the lightest set and still be within spec. Not much, but I think the engine will live longer and be smoother at 8000 rpm the closer you can get.

Sherwood

[TimT]

If you're going to button up the engine and never touch it again until the next rebuild, then weighing complete subassemblies is probably okay

The way I do it is get the components weighed, then isolate into pairing of similarly weighted pairs. I know the POrsche tolerance is 9g....


I like to see delta= zero on the six assemblies...

On my engine I spent along time, and got weights within 1g.

What to you do about the friggin clutch disk?

[ChrisBennet]

A friend brought his 3.2 rods to balancer. The spec is 9grams but his stock rods where within .2 grams. 4 were identical to a tenth of a gram and 2 were .2 grams heavier.
Measured as the difference from the lightest rod; 0.0, 0.0, 0.0, 0.0, 0.2, 0.2
If you think about it, it would cost nothing for Porsche to do this. All they'd have to do is weigh the rods as they recieved them from the rod maker and put them in bins sorted by weight.
-Chris

[911pcars]

[b]"What to you do about the friggin clutch disk?"[b/]

Let's hope the stuff spinning inside the engine lasts longer than the friggin' clutch disk. However, since the disk is considered a "consumable" but also rotates (albeit only when in gear), you could balance this independently - same with the pressure plate. The clutch manufacturer probably does this too before it goes out the door, but do you think they have a <9 gram tolerance factor?

Sherwood

[cstreit]

Guys,

I've already weighed all the individual components (Rods, wrist pins, pistons separately) down to a gram. They were all within 1 gram of each other... THe guy that built my motor did a lot of balancing work for me...

So I figured the tolerances for a race motor were going to be balance of less than a gram across the crank.

What I'm looking for is ideas where I could get the components weighed again to about 1/10th of a gram or less to get that perfect side to side balance.

Any idea where a guy could find a scale that has at least a kilo range and tolerances of 1/10th gram or less (besides the local crack house).

Thanks,

[ChrisBennet]

The place I buy my metric fasteners from has a pretty accurate scale. When you buy 100 8mm washers they don't count them, they weigh them. I used their scale to weigh my rods.
-Chris

[Quicksilver]

When weighing to balance the piston/rod/pin sets you should total all of the weight of the rods big end and half the weight of the piston, pin and the rods small end. The reciprocating weight is calculated at %50.

With the weight tolerances that you listed, you won't find anything more in power, durability or noticeably smoothness, by matching the parts. The only gain will be that you can sit back when you are through and know that you put extra TLC into it.


Wayne

[Neilk]

Originally posted by cstreit

What I'm looking for is ideas where I could get the components weighed again to about 1/10th of a gram or less to get that perfect side to side balance.

Chris,

I think 1/10th of a gram would be overkill and an excercise in frustration as you try to shave off milligrams of metal and never get all your parts to the .1 gram spec you desire.

It would be interesting to find out what tolerances are used by various race engine manufacturers. How balanced is a 917 motor, a NASCAR V8, or a BMW F1 engine that revs to 19k?