175GT Engine Teardown

175GT Engine Teardown

The 1957 Moto Morini 175GT started easily and ran strong right off the truck, but what follows is a classic case of one thing leading to another until the entire engine was scattered around my shop. At the bottom of this page you’ll find links to pages with greater detail about the disassembly process. In my defense I intend to actually ride this motorcycle and I needed to have confidence in its mechanical integrity. Here is a synopsis of the adventure so far:

  • I drained the oil on general principles and discovered that there is no mesh filter in the sump. At first I feared that the pickup tube for the oil pump was missing too, which could mean trouble in the oil pump and elsewhere. Michael Harper-Smith had an old mesh filter laying around and kindly sent it to me. It cleaned up pretty well.
  • Since I was concerned that the bike may have run with the oil pickup tube missing, I removed the left engine cover to gain access to the oil pump. The oil pump turns out to be in fine shape (pump body, pump gears). Somewhere along about here I also determined that the oil pickup tube was actually in place so fears about poor oil circulation were banished. The tube just doesn’t protrude as far down as I expected.
  • After a year of working on other projects I came back to the GT and reassembled the primary side. When I got to step of installing the kickstart lever I discovered the engine would not turn over.
  • There’s nothing to do but disassemble the primary side again. This time I removed the idler gear between crankshaft and clutch basket, which necessitated pulling the rocker arms and pulling the pushrods. The clutch basket turns freely so it seems the problem must be at the crankshaft or in the cylinder. Could there be corrosion in the cylinder from sitting?
  • I pulled the cylinder head and cylinder. There is some corrosion in the bore where the engine must have sat for a long time. It will require boring. The piston is already an oversize (60.6 mm vs. 60 mm stock) and it is quite fresh, almost no carbon on the face but the combustion chamber has plenty of carbon build-up. Rings are all free. Someone bored the cylinder but not quite enough to get it all.
  • The rod has a worrisome amount of rocking play. I made a video of the play after I’d finished splitting the cases and removed the crankshaft. The crankshaft still doesn’t turn freely in the case.
  • After seeing this video my machinist, Steve Choquette, www.studioenginewerx.com, wanted to see it in person. He diddled it in various ways and concluded that there is more side play than he’s used to seeing but it may well be that that was a standard construction practice when this was built, especially since it relies on oil splash for lubrication. All other indications are that it’s a perfectly good crank-rod assembly. Therefore I’m going to use it as is.
  • I didn’t really think it could be a problem but I went ahead and pulled the dynamo off the right side of the crankshaft; no drag there. Can it be a bad main bearing?
  • The only thing left on the primary side is the oil pump, but the pinion gear has observable play against the worm gear on the crankshaft. Nevertheless when I removed the oil pump again the crank freed up. Eureka! When re-installing the pump everything is fine until the first snugging down of one of the screws, then it locks up. The problem may be that I left the original gasket in place. Maybe it absorbed enough oil while sitting open to swell and throw off the alignment of the pump drive. I will need to be extremely careful on final re-assembly.
  • At this point I decided to take the final step and split the cases, check everything inside, replace seals, maybe bearings, and clean out 65 years of oil dregs, etc. After all I still had concerns about the connecting rod big end bearings so the crankshaft needed to come out to properly investigate that. I’m glad I did too, because I found a bad bearing on the countershaft that would probably not have lasted long in service.

I Own This

I want to be as clear as possible I do not in any way hold Michael Harper-Smith responsible for any of this. If I had simply put the oil drain plate back on without a mesh filter and refilled with good oil the GT would probably have run fine for quite a while, or at least until the countershaft bearing failed. The rings probably would not have sealed either and there might have been issues with blow-by. I am sorry that my excessive curiosity/paranoia is going to keep me from riding the GT for a while longer, but at the same time I’m glad to have the chance to understand how this engine works and to know that everything inside is in good order. I had no assurances from Michael that he had gone through the engine carefully, only that the bike ran decently and was in excellent cosmetic condition.