Crank Pulley Cracked on the Quantum Two

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Crank Pulley Cracked on the Quantum Two

Back at the Mount Equinox Hillclimb in 2017, on the first run of the weekend, I melted the #2 piston in the Quantum Two. Now I am finally getting around to rebuilding the engine with a new piston. As part of that, I am cleaning and inspecting all the parts that I remove from the car.

Recently, SAAB guru Tom Donney of the SAAB Heritage Museum USA and Tom Donney Motors posted a video about some crank pulley issues he’s coming across on the SAAB two-stroke. As a consequence of this video, I took a much closer look at my pulley than I otherwise would have.


This pulley is going on the “Rafter of Disaster” with all the other broken racecar parts.

Honestly, I was a bit surprised to find a crack in the pulley as I didn’t think it had many miles on it, but really I don’t know for sure.

So I went digging in my parts collection looking for another pulley. While I have a few on the shelf, none are for a 750. The diameter of the sealing surface (where the crack is) is smaller on the 750 than on the 850. It is a simple task to replace the current 750 front cover (the thing that keeps all the grease in the distributor drive area) with one from an 850 so I can use one of the pulleys I have.

The 850 pulley should be stronger too, because it has more material around the center hole. So that’s a good thing.

Now it is time to take a look at the pulley I want to use, i.e. the cleanest one on the shelf. It is a brand new one (aftermarket) that came with a rebuilt crank we had done a while back.

Thanks to Mr. Donney’s video, I knew to look for a couple of issues with this brand new pulley.

  1. The casting didn’t have enough relief (wasn’t deep enough) in the center to allow the tensioning cup to install fully and properly tighten down the harmonic balancer weight.
  2. The sealing surface where it goes in the front cover is unfinished (with a rough surface) and will wear out the oil/grease seal in short order.

So, onto the lathe it went!

This shows the modification I made to the pulley to gain clearance in the center.
A polished finish will help the seal do its job and last a long time.

Here is Tom’s video that helped me so much in identifying the problems I had and the solutions I needed to put into effect. Thank you Tom!

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