Crankshaft main journals

[Tom in Iowa City]

Here's a question for someone in the know on crankshafts:

I am doing a rebuild on a 52 N. All torn down now.
I miked the three main bearing journals on the crankshaft per Fig. FO45 on page 35 of the FO-4 Manual..
The specs call for making four measurements on each journal to determine 1)wear, 2)out-of-round and 3)taper.
All 12 of my measurements are VERY close to 2.2285.
Standard journal diameter is 2.248-2.249.
Therefore, my journals have been turned by somebody, and are .0195 undersize.
Are these sizes such that I can use .020 undersize bearings (technically, the bearings are oversized with undersized journals) without having to turn the journals? It seems to me you need some clearance and that the 5 ten-thousandths would give this clearance.
The journals seem to be very smooth everywhere, with no grooves.
Thanks, Tom

 

[oldhousehugger25]

Take it to a good crank shop, preferably one that rebuilds industrial engines. They will put it on an engine lathe, mike it for round and undersize and also check endplay surfaces for wear. I had a crank out of a perkins 3 cylinder deisel that could have been ground .04 undersize but the guys there told me that the thrust lands were way under spec and they had no way of fixing that because even if they built it up with weld the temper would come out of it. The new crank was not cheap but when it went together everything was where it should have been. Thats a good feeling.
John

 

[TheOldHokie]

(quoted from post at 14:19:02 09/12/08) Here's a question for someone in the know on crankshafts:

I am doing a rebuild on a 52 N. All torn down now.
I miked the three main bearing journals on the crankshaft per Fig. FO45 on page 35 of the FO-4 Manual..
The specs call for making four measurements on each journal to determine 1)wear, 2)out-of-round and 3)taper.
All 12 of my measurements are VERY close to 2.2285.
Standard journal diameter is 2.248-2.249.
Therefore, my journals have been turned by somebody, and are .0195 undersize.
Are these sizes such that I can use .020 undersize bearings (technically, the bearings are oversized with undersized journals) without having to turn the journals? It seems to me you need some clearance and that the 5 ten-thousandths would give this clearance.
The journals seem to be very smooth everywhere, with no grooves.
Thanks, Tom

• 
  [*:6bab9f7710]Taking absolute (as opposed to comparative) measurements to tenths (.0001) with a micrometer requires a well maintained, recently calibrated, high quality mic and someone experienced in it's operation. Although I've been dabbling in machine work for a couple years and spent a lot of money on tools I don't trust my micrometers or my skills to be accurate or repeatable to tenths. So let's limit the discussion to thousandths.
  [*:6bab9f7710]To understand the nomenclature you have to think like a machinist. "Undersize" shell bearings are fitted when the journal has been turned or ground undersize - they have a smaller (undersize) ID to compensate for the material that has been removed from the journal. "Oversize" shell bearings are fitted when the housing bore has been bored or ground oversize - they have a larger (oversize) OD to compensate for the material that has been removed from the housing.
  [*:6bab9f7710] Automotive shell bearings are manufactured with the required oil clearances built in - you don't machine the part +/- to produce it. Rule of thumb for crankshaft bearing oil clearance is .001 for every inch of journal diameter. So a .020 undersize set of mains is manufactured to give the required .001-.002 oil clearance when installed on a 2" journal that has been machined .020 undersize - i.e. 1.980. If your original cranksahft journal diameter is 2.248 the proper diameter to mate with new .020 under bearings is 2.228. Simply subtract the undersize from the original.
  [*:6bab9f7710]The most common calculation errors made by budding machinists involve diameter calculations. Your crank @.0195 under doesn't give you .0005 running clearance on .020 under bearings - it gives you a .0005 interference fit - it's oversize by .0005 :shock: In reality that interference is offset by the .002 oil clearance allowance built into the bearings so you wind up with .0015 running clearance. Dang - I said we were going to avoid talking tenths....
  [*:6bab9f7710]To actually answer your question - if the journals are all round, without taper, and within .001 of the nominal undersize you are good to go.
  [/list:6bab9f7710]
  
  TOH
  
  

 

[Tom in Iowa City]

OldHokie,
I enjoyed reading your response! Thank you. Very informative. Bottom line: I'm going to take things to the engine shop (I think I found a well-respected one)to measure everything up. I was just having fun trying to be an amateur miker and checking things out. Thanks, again. Tom

 

[TheOldHokie]

(quoted from post at 15:53:58 09/12/08) OldHokie,
I enjoyed reading your response! Thank you. Very informative. Bottom line: I'm going to take things to the engine shop (I think I found a well-respected one)to measure everything up. I was just having fun trying to be an amateur miker and checking things out. Thanks, again. Tom

Probably a wise decision. They will check more than the crankpins and mains.

TOH

 

[Dan (NYC)]

Tom, Can you post back with what the shop finds. I would love to see what difference they come up with. I am learning a lot at this point, have a micrometer can "use" it but wonder how much difference a pro will come up with etc. I DO NOT want to suggest you may be less than right on just curious... Thanks
Dan- nyc

 

[trucker40]

Its just like anything else,it all has a level of experience to do it right.There is a real good chance you are close,but the machinist with the high dollar calibrated mic will likely have a little different reading.Thats the way it is for me anyway.Its good to get a general idea of what you have,but to get down into whether or not a crank needs to be turned or how much,its a good idea to not give your measurements all that much credit.Its what machinists do,and they are very good at it most of the time.They have the tools and experience to get the job done.Depending on how many beers you drink first,you might take your micrometer and your crank to the bench grinder and try to perfect your readings(I have thought of doing that before on a lawnmower crank)but I dont care how fancy of a riggin you come up with,it wont work,or at least not as accurate as what a machinist can do in a lot less time with a lot less ruined cranks.