engine balance (updated)

[Jim Comet]

Last summer my 416 dropped a valve. So I am having 2 sleeves installed and getting 2 new pistons. Here is my question? I ordered 2 new Mahle pistons of the exact same part number as what is/was in the motor. Do I need to rebalance the motor? This motor was going through the traps at 7400rmps. I am going to limit it to 7000 now with the new rebuild. I can weight match 1 of the pistons, but due to the other having a hole in it, I cannot weight match that one. Thanks for your thought. Jim

[1968galaxie]

You just need to have the piston and pin assembly weigh the same as the previous ones.
No need to re-balance the entire rotating assembly again.
Have the new pistons weight matched to the one that you can measure.
Also make sure you weigh pins as well.

[Jim Comet]

The new Mahle pistons did not come with pins. Is there a reason I could not reuse the old pin?

[jayb]

You might have to have the pins fit to the new pistons, your shop should at least check that.  No reason you can't reuse them though.  All your piston/pin combinations should weigh very close to the same, so if you are +/- 10 grams with the old ones you don't need to rebalance.  Probably a good idea to mag the rod where you dropped the valve also...

[JimNolan]

On my Bob weight Card it just has one entry for Piston & Pins. I wouldn't think the same piston from the same manufacture would be that much different. Jay Brown probably said it best.

[blykins]

On my Bob weight Card it just has one entry for Piston & Pins. I wouldn't think the same piston from the same manufacture would be that much different. Jay Brown probably said it best.

You'd be surprised......I've ordered replacement pistons of the same job number and they were 10g different than the previous ones.  I think it just comes down to the particular batch of aluminum that they received in.   

[1968galaxie]

As you have a piston from the previously balanced engine - it is easy to measure any difference in weight.
You should have a previous engine balance card (bobweight card) and can go from there. Easy peasy.

[Jim Comet]

Thanks for the responses, R&R in Minneapolis is doing the work. I'll check with them.

[pbf777]

     Have the rods involved checked for straitness!  Also the pins if reusing! 

     Scott.

[Diogenes]

As Brent alluded to above, you should never assume anything. I've seen a defective piston within a set, where piston/bore clearance was way out compared to the rest of the set. It is probably pretty uncommon these days, particularly among the "premium" brands, but like Brent stated, many variations can occur throughout the manufacturing process.

[Jim Comet]

The shop already has the rods to mag. I am dropping of the 3 pins to go with the new pistons for inspection. I am sending 2 original pistons to compare weights to the 3 new ones. Thank you for the tips/info. If all goes right, I will be running the 416 on the dyno some time this spring. The plan is to do initial start/break in on VP110 and then switch to E85 and see how it compares. I have a new E85 spec custom carb for the test.

[Posi67]

Several years back I needed 2 pistons for my 427/454. The replacements from Arias were way heavy like 60 grams or something. My balance guy (who was very good btw) got about 1/2 that weight out but didn't want to cut any more. His advise was to put them in the middle of the engine and on opposite sides. Thing was smooth as silk but your results may vary.

[Jim Comet]

I took the pins to the machinist today and he measured the pins and new pistons and will work fine with no special fitting. He then weighed the 3 new pistons and 2 of the old ones and they were all within a gram of each other. I should have stopped and bought a lottery ticket. Things are slowly coming together. Jim

[machoneman]

Jim, buy that lottery ticket: winning numbers are 416416. LOL

[Jim Comet]

I brought my 2 new Molnar rods to the machinist and he called back and said the small ends weighed the same as the old rods but, the big ends of the new rods were 4 and 7 grams lighter than the old rods. He recommended running them but would rebalance the assembly if I wanted him to. What are your thoughts? Molnar rods running on a 3.98 stroke steel RPM crank with Mahle pistons spinning up to 7000rpms. Rebalance or run it? Thanks, Jim