I'll use this new thread to document the installation of a Honda GX690 into my 1976 Sears ST12 tractor. After looking at all the major manufacturers for a new V-Twin engine, I settled on a Honda GX690. The specific engine model is the GX690 TXA2. All I really needed was the GX690 TDW, but the TXA2 model includes an oil level indicator and a factory mounted control box that includes a key switch and hour meter. The oil level indicator circuit will shut the engine down in the event of a low oil quantity condition. Since the ignition wiring is different between the HH120 and the GX690, I plan to remove the control box key switch and wiring and install it in the tractor dash. The hour meter will be an added bonus for tracking when maintenance is due.
I will be using a Honda GX690 muffler, P/N VHRM6 that has the exhaust outlet facing towards the front of the tractor. I looked at several other mufflers and this one seemed to be the most well constructed (and hopefully the quietest).
My initial measurements suggest that I will need to cut the hood and both upper and lower grill sections to allow clearance for the new engine and muffler. I was initially against cutting these items, but I changed my mind since this is likely the last engine that this tractor will ever get and I have no plans to go backwards to the original Tecumseh engine.
Once the parts arrive, I will remove the front bodywork, existing engine and wiring. Once the new engine is installed, wired up and running, I will clearance and reinstall the bodywork.
I have concerns that the existing clutch pulley installation will hit the new engine, but have a few ideas to gain clearance if necessary. I also expect to modify the firewall and battery box to allow access to the rear sparkplug.
Here are a few pictures of the tractor as it currently sits with the Tecumseh HH120 engine.
2017-07-04 11.17.04 HDR by rewilfert, on Flickr
2017-07-04 11.16.47 HDR by rewilfert, on Flickr
2017-07-04 11.15.33 by rewilfert, on Flickr
2017-07-04 11.15.48 by rewilfert, on Flickr
2017-07-04 11.16.03 by rewilfert, on Flickr