Connecting rod bearings, 1956 Johnson 30 HP

[olcah]

I have a 1956 Johnson 30HP RD18 and tested for clearance in the large and small con rod bearings by setting each piston on down stroke then pushing the piston with a dowel thru the spark plug holes. I got one small click on each piston . The top piston clicks at about 1/3 of the downward stroke. I estimate about 0.005 inch movement. The bottom piston clicks at about 90% of the downward stroke. Estimate 0.003 inch movement.

Two years ago I did the same test and got no clicking. Before then the motor had been out of service a long time. I put in a NOS crankshaft and replaced the crankshaft seals. The motor internals looked fine except for a damaged lower bearing when the carbon seal failed and the spring scraped the bearing seal surface.

I have run probably 120 gallons of gas through it over the past two years. The motor starts/runs/idles perfectly.
I expect to find that the two small end bearings in each rod have wear. I removed a piston from an RD18 parts motor to see how this is put together but darned if I can upload the photos. Each small end has two needle bearings. The piston pin is the inner surface race. If bearings are replaced shouldn’t the pin be replaced also?

Are the two small end bearings pressed into the con rod? If so how is pressing accomplished? It looks like the con rod surface may also be part of the bearing race? Can that surface be bad? Is the con rod and bearings available as an assembly? (Looks like they were in later models.) Are parts available? Should I smooth the piston bore at the end of the piston travel if I change the bearings?

Sure would appreciate if someone with experience can give me any tips before I go into this nice motor.

Thank you.

[frankr]

Its kind of hard to track down the history on those due to the frequent, usually mid-year, changes.

You probably have heard about the wrist pin bearing problem with the 30hp. The needles would get out and destroy the piston as they passed through. An improved rod was introduced the next year with bigger bearings. The new rod with w/pin bearings was part number 377272. W/p bearings (only) for he original rad was 304322. Yes, they are pressed in, one from each side. Always press needle bearings from the lettered end.

[olcah]

Frank,

Thanks so much for this info. If the wrist pin bearings are worn it seems the best and simplest way would be to find two # 377272. That would get me the larger bearings. Maybe I should also replace the wrist pins as the wrist pin is the inner bearing race.

Looks like next step is to take the motor apart and see. Again, Thank You for the help.

Charlie

[olcah]

Update- I opened the motor but did not feel up/down clearance in the rod bearings at the crankshaft end (checked by moving the pistons up and down). The rollers and bearing surfaces look excellent.

To check the wrist pin bearings I removed the crankshaft and pushed/pulled by hand on each rod. I did not feel vertical (push/pull) clearance in the wrist pin bearings. I did not remove the pistons to inspect the wrist pin bearings.

I had removed the exhaust cover and plate. And as I moved each piston I could see the rings move in their piston grooves. I could also feel the ring land against the top and bottom of its ring groove as the piston moved up and down. If the piston is moved quickly, the sound was the same as the click I heard when the motor was together. A 0.004 inch flat feeler gage fits in the space between the ring and the piston grooves. A 0.006 feeler does not.

I think I should just put the motor back together but would appreciate if anyone with more experience has any advice.
Thank you.

• This reply was modified 5 years, 4 months ago by olcah.

[frankr]

Personally, I don’t think there is a problem. Unfortunately, my crystal ball broke and I can’t predict what might happen tomorrow or next year. The didn’t ALL fail.

[olcah]

Frank, The discussion and info you provided are going into my notebook tonight. I have 3 of these engines and no crystal ball at all. Thanks again and Happy New Year. VR Charlie

[jerry-ahrens]

I think I’d go ahead and run it. I have a 1956 30 that has a million hrs. on it. When I had to replace the crankshaft (bad center main) I noticed some of the piston ring locating pins were missing. I could tell they had been gone for a long time, I left it alone and put it together that way. Some won’t agree with that, but it’s been running that way for yrs. In my experience, 50/1 ratio will wallow out the ring grooves on that motor. Saw it on a customers motor one time. I ended up changing the powerhead from all the wear on that one. Just my opinion..

[olcah]

Jerry, Based on your comment – Typically how much clearance might be expected between the piston ring and the piston grooves? I used a .004 inch feeler through the exhaust ports. That fitted but a .006 inch feeler did not go.
The motor started/ran/idled really well for the last couple of summers.

[frankr]

The 1956 service manual did not specify piston grove clearance, but I just looked at a 1971 40hp manual and the spec is .0015 min to .004 max. I’d say you are just fine. I’ve seen way far more pistons with stuck rings than too loose.

[olcah]

Great information! Thanks again.

(I run 24:1 mixture or even a bit richer in this motor.)

• This reply was modified 5 years, 4 months ago by olcah.

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