Yanmar GM Series Injector -> Compression Adapter Conversion

[zephyr070]

Through the magic of craigslist, I was able to score a full set of used injectors for my Yanmar 3GM30 for the low low price of free. I had just had the injectors on my engine rebuilt, so I was looking to convert these other units to compression test adapters (did you know Yanmar doesn't sell compression test adapters for the GM series? go figure). At long last, I finally gutted the injectors, and now have a clear view from nozzle aperture to fuel return port on each. I intend to plug up the fuel injection ports, and use the fuel return ports to mount the compression test gauge fittings.

Absent of trying to get my thread pitch gauge in these ports (if I can even remember where I put it), I was hoping someone might happen to know the correct (NPTF?) thread dimensions so I can start sourcing the plugs and compressor fittings.

Cheers!

 

[dlochner]

Yanmar tends to use a Japanese standard thread which is the same as a British Standard thread. Look up the specifics and get your micrometer out to figure out which thread you need. McMaster may have adapters.

 

[zephyr070]

Look up the specifics

Exactly! I've been through the service manual cover to cover and haven't found those thread sizes. Guess I'll have to try to exhume my thread pitch gauge from the storage unit

 

[dlochner]

Exactly! I've been through the service manual cover to cover and haven't found those thread sizes. Guess I'll have to try to exhume my thread pitch gauge from the storage unit

it was for a different application, however I had to search for a plug that was a British Standard thread. Took a lot of time searching and measuring before I found it at McMaster. I now know more about threads and pitch than I ever cared to know.

 

[Ken13559]

I've converted a spare injector as adapter for Compression Tester. The plug on the side I use is standard 1/4"NPTM from Swagelok. The quick connect at top is from the Compression Tester.

Ken Y

 

[zephyr070]

I've converted a spare injector as adapter for Compression Tester. The plug on the side I use is standard 1/4"NPTM from Swagelok. The quick connect at top is from the Compression Tester.

Ken Y

Exactly what I was hoping for, Ken, thank you!! Off to buy some fittings.

 

[Ken13559]

Exactly what I was hoping for, Ken, thank you!! Off to buy some fittings.

Suggest you bring the injector along when buying the plug. 1/4"nptm could well be 1/8"nptm as I could be mistaken in the measurement.

 

[zephyr070]

Suggest you bring the injector along when buying the plug. 1/4"nptm could well be 1/8"nptm as I could be mistaken in the measurement.

The plug is indeed 1/4” NPT male, but the outlet port I was hoping to attach the compressor fitting to is something super weird. I may end up boring that out and retapping to something more standard, once I get my hands on a compression gauge with high enough range…that’s a separate issue.

 

[Ken13559]

The plug is indeed 1/4” NPT male, but the outlet port I was hoping to attach the compressor fitting to is something super weird. I may end up boring that out and retapping to something more standard, once I get my hands on a compression gauge with high enough range…that’s a separate issue.

Ok noted. Let me check mine to see what the thread size is for the top port where the Compression Tester connects to.

 

[RoyS]

I purchased an aluminum compression tester dummy injector from Hoye Tractor (I think). When I went to use it I found that it did not fit. The diameter was too large. I called the Tractor Company and they said, "Oh yeah, you have to have that machined to fit." Go figure. Anyway I had it turned down and it worked. Here is something to think about. Does the length of the hose between the dummy injector body and the gauge affect the compression reading?

 

[zephyr070]

I purchased an aluminum compression tester dummy injector from Hoye Tractor (I think). When I went to use it I found that it did not fit. The diameter was too large. I called the Tractor Company and they said, "Oh yeah, you have to have that machined to fit." Go figure. Anyway I had it turned down and it worked.

I have access to a lathe, so I may just end up doing that. But right now I have the injectors in hand, so that's the first solution to try!

Does the length of the hose between the dummy injector body and the gauge affect the compression reading?

Technically, you're effectively increasing your volume at TDC, so I would imagine the answer is yes. However, total displacement in this engine is 0.954 L, so each cylinder accounts for 0.318 L, and I'm not sure what the ID of the compression gauge hose is, but I know my gutted injector can't account for more than a couple mL of additional volume. Figure injector + hose is on the order of 20 mL, conservatively, that's a 6% increase in the volume, so your compression ratio would drop by the same factor. Roughly 15 psi at STP and a spec'd compression ratio of 23.0 -> 345 psi. With a 6% decease in compression ratio, now you're looking at 324 psi. Not terrible. And as many have stated, in many a forum regarding this issue, absolute compression numbers are just as important as the variance between cylinders.

So far as I can tell, such a test has never been done on this engine, so I'm *really* curious to know! I've just finished watching an engine maintenance series on Youtube by a cruising couple who'd experienced similar symptoms with their 3GM30F. Their problem was a bent con rod, and I'm hoping it isn't that...

 

[Ken13559]

The plug is indeed 1/4” NPT male, but the outlet port I was hoping to attach the compressor fitting to is something super weird. I may end up boring that out and retapping to something more standard, once I get my hands on a compression gauge with high enough range…that’s a separate issue.

The top quick connector thread is 1/8"nptm. Item 10 on the list. Photo of Compression Tester I use.

Ken Y

 

[zephyr070]

The top quick connector thread is 1/8"nptm. Item 10 on the list. Photo of Compression Tester I use.

Ken Y

Oh interesting! I took an injector to a fittings shop and a 1/8" NPT was way to big for the return port. On closer inspection of the first image you shared, it almost looks like the top nut that has the return port threading in it is of a different metal than the rest of the injector? If that is true, could it be that the piece is custom machined with a 1/8" NPT female port? That means one of two options for me: 1) ream out the threads, drill and tap 1/8" NPT, or 2) make a custom nut with the same. I can tell you which one would be less time-consuming Thanks for sharing!