Good afternoon - I have my pony block and crankshaft at the machine shop being refurbished. The crank was welded and ground back to standard. I ordered a new set of standard rod bearings (3B3014). On the spec sheet of the manual (page 7), the specifications are listed as:

Crankshaft
1) Connecting rod, journal diameter - 1.7495-1.7500

Connecting Rod
1) connecting rod clearance (babbitt) .002 - .0035
2) maximum permissible clearance .009.

On page 243, under the heading Connecting Rod Bearings, the first paragraph reads "Precision babbitt-lined bearings, machined to provide proper clearance, are used. These bearings do not require fitting, scraping, filing, or reaming, and can be put into the engine without removing the piston assembly or connecting rod." On page 244 it says "the proper clearance is .005" - .008" with a maximum permissible clearance of .013".

Do these two sections of the manual contradict themselves in regards to rod bearing clearance? The machinist tells me that the rod journals are ground to just below 1.7500 and there is .003" clearance with the rod bearing and he's concerned it may not be enough space for oil to reach the bearings, since there is no mechanical way to get oil into there. The range on page 244 indicates this may not be enough clearance. Any advice?

Since what you posted shows .002 thou as the minimum clearance, I would think that .003 is within limits and will always open up a bit with running. Whatever the manual says for breakin is probably important and figuring out what the best oil is for a ten HP motor that will only be run for short periods of time would be important also. I am going to have to do a rebuild before one of my ponys throws a rod or leaks most of its oil out if starting the big engine takes too long. I suspect that the number of hours that a pony lasts is short because they are only run for short periods of time. I am not a professional mechanic so take my advice with a grain of salt. Grant.

Re-reading your post made me dig out my D2 book and I see the discrepancy in the listed clearances. I wonder if the manual was re-written during a time of oil improvements that allowed for tighter tolerances and the proof readers didnt pick it up? I also looked in a modern engine manual for a V6 gas engine and it gave rod clearances of .0008 to .0019. Service limit is .0024 which is when they say to replace. Their spec for worn out is tighter than the minimum clearance given for a pony rod bearing. Again , I suspect that modern oils have changed things but checking with an oil company tech and see what they recommend. Of course a clunky old 2 cylinder is much different than a modern high revving V6 but I think relative improvements would make a difference. Good Luck, Grant.

Hi Team,
Cat may have addressed this with the release of an Engine Specs chart in July 1951. They should print out clearer I am lead to believe.
Hope this helps.
Cheers,
Eddie B.

Thanks Eddie. The specs on the documents you shared are the same as what my manual says, which is .002-.0035 with a maximum clearance of .009. What's confusing is later in the manual it gives a clearance of .005-.008 with a maximum clearance of .013. Does .003, which is the clearance the machinist has now, sound right? Are the replacement rod bearings that I have considered babbitt lined?

Hi,
the Cat rod bearings for the big ends are babbitt lined, the main bearings from Cat are a bearing grade aluminium alloy.
Cheers,
Eddie B.

"Cat wrote:

Thanks Eddie. The specs on the documents you shared are the same as what my manual says, which is .002-.0035 with a maximum clearance of .009. What's confusing is later in the manual it gives a clearance of .005-.008 with a maximum clearance of .013. Does .003, which is the clearance the machinist has now, sound right? Are the replacement rod bearings that I have considered babbitt lined?

-common automotive practice....

-rule of thumb....001" bearing clearance per inch of journal diameter...that's pretty average...being on the loose side doesn't hurt..
-maximum clearance of .009" i can understand...but not more..? not .013" ?

Thanks everyone. As far as the method that the rod bearings and journal are lubricated, it is my understanding that this occurs when that portion of the crank immerses into the oil and the oil seeps into the clearance, since there is no oil pump to pump oil into the bearings. Am I correct on this? The concern of the machinist is that he was wondering if .003 was enough clearance to lubricate in this manner, and what I'm hearing is that it is.

No there isn't a oil pump exactly, and no dipper. The big end is moving at some good velocity when it dashes through the pool of oil which isn't exactly seeping in, more like shutting a door against a hurricane. Would be interesting to know what the Briggs & Stratton clearance is, something in the small engine range would be a better comparison than an average auto engine.