TheSamba.com :: View topic - 40hp 83mm: must I split the case to balance?

Thu Apr 29, 2021 7:11 pm

Recently-finished 65 bug restoration with stock rebuilt 40hp motor (from a guy in Detroit) had about 2,000 miles on it until last week, when pulling off the freeway, the motor loudly clatter-clatter-clattered and lost power. Last Saturday my son and I pulled the motor and diagnosed a broken spring on the #1 cylinder exhaust valve.

I had already been thinking about upgrading to the 83mm kit, and took this as my cue to go ahead with that idea. Parts are on order, including a big bore kit from SS. Today I spent about 3 hours reading up on tips and tricks for the installation and have an unanswered question about balancing.

Namely: do I have to split the case to balance the setup? One poster said there was no way around it, but several other builds described balancing only the pistons and wrist pins with no mention of the rods. Since the engine was recently rebuilt, is it reasonable to assume they are close-enough in weight to allow matching only the pistons + pins? Or alternatively, can I weigh one end of the rods while they are still attached to the crank to confirm they're the same weight?

Or should I bite the bullet, split the case, and get the entire assembly weighed and balanced at a machine shop?

Would appreciate any thoughts, thanks.

Here's a shot of the bug last summer, after picking it up from the pinstripers:

And here's the offending spring:

Thu Apr 29, 2021 7:38 pm

You can pull the rods without splitting the case.
If you don’t know if everything was balanced then I’d tear it down and have everything balanced. It makes a huge difference.

Thu Apr 29, 2021 8:35 pm

If you pull the rods without splitting the case, how in the world do you think you're going to get them all back together and torqued properly (checking that they don't hang up anywhere while setting the torque)??

Short answer, yes, you'll have to split the case halves and balance everything properly.

Fri Apr 30, 2021 10:19 am

Well, heck.

I was hoping for some magic idea to bubble up, but splitting the case it is. That'll be a first for me (motorcycles, inline 6s, and small blocks are more familiar ground), but I expect another evening of reading up will point me in the right direction.

Thanks for the feedback.

Fri Apr 30, 2021 5:46 pm

even what the factory manual calls "within tolerance" has a lot of room for improvement. Balance the crank and flywheel together, the rods end to end as a set, and the pistons with their pins. It really makes a difference.

Fri Apr 30, 2021 6:17 pm

motofly196 wrote: If you pull the rods without splitting the case, how in the world do you think you're going to get them all back together and torqued properly (checking that they don't hang up anywhere while setting the torque)??

Short answer, yes, you'll have to split the case halves and balance everything properly.

Rotate the crank.
But yeah, it’s more efficient to just tear it down.

Sat May 01, 2021 8:22 am

Braukuche wrote: If you don’t know if everything was balanced ..

This statement reminded me that the guy who built the motor races Formula Vee, so I reached out and got ahold of him. Turns out that the engine was balanced before I bought it, so balancing the new pistons by themselves should do the job.

Very happy to have learned that! Thanks again.