Go BottomJohn Deere 850 rear crankshaft seal
 | carkivey3
Join Date: May 2025 Posts: 1 fayetteville,nc 
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2025-05-20 201887
I recently rebuilt a 86 jd850 with yanmar 3 cy diesel. Among other things the crankshaft was turned down .030". the rear oil seal also rides on same journal as the bearing. Did not think of it at the time.
Tractor back together and running great. however, I do have an oil leak out of bellhousing and I am sure its because now the oil seal is .030" too big.
I would like to have a new seal to install before seperating tractor. there is only one rear seal listed as parts. i would like the manufacturer info to determine outer dimension. Service manual has crankshaft at 3.542". minus the .030" =3.242" for seal id. John Deere just refers me to part numbers. Yanmar was very helpful but basically said against policy to give info about where they get parts. anyone have any info that would be helpful
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| Tractorpoint
Join Date: Oct 2006 Posts: 20 NJ
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2025-08-01 201905
You're right to suspect that the oil leak is due to the crank being undersized relative to the standard seal. Here's what may help:
1. Original Rear Main Seal Info
For the John Deere 850 (Yanmar 3T80 engine), the original rear main seal is typically referenced as:
John Deere part number: CH15699
Superseded by: CH12387
Commonly supplied as a double-lip rubber seal with a steel outer casing.
Unfortunately, Deere and Yanmar won’t release the full dimensional specs, but field replacements from brands like National (Federal-Mogul), SKF, or Timken sometimes list them with dimensions.
2. Typical Dimensions of OEM Seal
While not officially published, several rebuilders and parts suppliers report:
Outer Diameter (OD): 4.724" (120 mm)
Inner Diameter (ID): 3.543" (90 mm) — matching the original crank journal
Width: ~0.472" (12 mm)
With your crank now at 3.513" (89.2 mm), the standard seal is too large by about 0.030", confirming your suspicion.
3. Options to Address the Undersize
A. Custom Undersize Seal
You can try to find a seal with:
OD = 120 mm
ID = 89.2 mm (your turned crank)
Width ≈ 12 mm
Search using SKF, Timken, or National catalog sites. For example, SKF or Chicago Rawhide may carry metric ID/OD combinations or be able to custom-make one.
B. Speedi-Sleeve Alternative
Another approach: use a Speedi-Sleeve to return the crank to original diameter. That way you can reuse the stock seal. SKF makes these wear sleeves and you’d just need one with a 90 mm OD and the right width.
4. Next Steps
Search SKF or Timken sites using the crank new OD = 89.2 mm and OD = 120 mm.
Consider industrial suppliers like Motion Industries, Applied Industrial Technologies, or McMaster-Carr — they sometimes carry off-standard seals.
As a backup, order the OEM seal and consider installing it slightly offset (some seals allow 2–3 mm depth change) to ride on an unpolished section if available. ....
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