ATF Pump Socket Replacement

From Paul Beesley

The Moto Guzzi Convert's torque converter system is arguably the defining feature of the bike. The most common problem with this system is the failure of the ATF pump that circulates transmission fluid through the converter and dumps waste heat into an oil cooling radiator mounted at the front of the bike.

The trochoidal ATF pump sits inside the timing chest cover at the very front of the engine, driven off the camshaft via a 6mm hex shaft and a corresponding socket that protrudes from the pump body. It's this socket that seems prone to accelerated wear, stripping out after just a few thousand miles and leaving the hex shaft spinning freely inside. Once it strips out, the transmission fluid is no longer being cooled and it quickly overheats and thins out, preventing drive. You can find further details in Guzziology and at Gregory Bender's site

My Convert came with a ruined pump socket and a newer, decent condition, deeper socket. I considered having the newer one hardened but decided to have a completely fresh one made up. Having created a design by reverse-engineering the newer socket, I wanted to share it here so that others can also make new parts to keep these bikes on the road.

CAD File Downloads

This is the version that I had produced via CNC milling and then hardened. I've tested the fit on my Convert engine using the original Guzzi hex shaft: the socket turns smoothly without binding and the timing chest cover buttoned up nicely. I have no data on how many miles this new socket will last, only the expectation that it should be much longer lived than the original due to its much higher hardness.

Any decent manufacturing firm should accept this STEP file for providing a quote and performing the production.


The designs made available here are provided "as-is", in the hope that they prove useful but without any warranty (express or implied), including warranty of fitness for a particular purpose. Differences in production processes and raw materials may mean that the resulting part(s) do not function as designed. You install any part(s) produced from this design at your own risk and you assume all costs of repairs, servicing, etc resulting from the use of these parts.

Design Notes

  • Ask your fabricator to ensure an absolute minimum of 6.05mm for the flat-to-flat distance of the hex and an absolute maximum of 14.0mm for the diameter of the socket. The design has been adjusted to stay within these limits when produced with a tolerance of +/- 0.05mm.
  • Hardening was performed, followed by tempering, to a final hardness of 58Rc (Rockwell C).
  • The original Guzzi hex shaft protrudes slightly further from the end of this new socket when compared to the template pieces I have here (the later, 11mm depth version of the socket). The difference is approximately 1-1.5mm. At least on my engine, this didn't cause any problem when refitting the timing chest cover.
  • The original socket lets the shaft have more wiggle (lateral play). I wanted to reduce this on my design so the hex socket flat-to-flat distance is smaller than the original - I don't have exact numbers for the orignal as there was no realistic way to measure it. There is still some play to allow for misalignment of the pump and camshaft.
  • This socket fits without binding in the pump bore, certainly slightly smoother than my original piece. There is some discussion that variation in the diameter of the original sockets might cause binding and early failure.