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Balance shaft with aftermarket rods and pistons

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Hi All,

First off, my first engine build was a success, thanks to HP Academy.

I have been hunting down some engine noise on my car and have the following question:

Are there known issues with using OEM balance shafts with aftermarket (lighter) rods and pistons?

The balance shaft on my car (Mazdaspeed, same as for Duratec) contains a gear that meshes with a gear on the crankshaft.

During assembly, there is a requirement to shim the clamping point to prevent gear binding when components reached temperature. My balance shaft or crankshaft gears were not machined, so I did not change the shims.

After assembling the components, parts would spin freely, but there was quite a bit of lash.

The engine idles super smooth on warm idle, but I can hear the balance shaft rattling around under the car especially when varying throttle input, and it leads to some concerns.

Any experience sharing is appreciated,


Lash in the gears is an issue with any gear drive system. Is one of the gears a two piece design that's spring loaded to eliminate lash? This is common with Toyota scissor drive cam system. If you've got that and the haven't installed it correctly then you'll have a problem with lash. Personally I remove balance shafts on the engines I build. You'll normally gain a small amount of power and often the balance shafts can be a reliability issue.

Andre, Thanks for the reply.

I deleted the balance shaft and the suspicious noise is gone. It just sounded like there was a loose bolt in the oil pan.

NVH is slightly worse, but at least I know the engine is running now, thank you!

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