Tech Tip: Fixing A Loose Fit Bearing On A Triumph Crankshaft
Fixing A Loose Fit Crankshaft Bearing
Finding a crankshaft in good usable shape that appears to have “life” can still be found today, although it is getting harder
In reality cranks are easy to come by, however most appear to share a common problem that is often overlooked
We understand big end journals and journal radius’s are all important factors, but what about undersized or worn bearing journals?
Today I wanted to take the time and explain to you what an undersized bearing journal is, how to measure your journal, and most importantly, how to fix your journal
For the rest of the tech tip post, I will be using a 1972 triumph 500 crankshaft but this post can be used in reference for most Triumph, BSA and Norton motorcycles and various other models
Refer to your workshop manual prior to performing any modifications or changes
Lets get right to it
What is an undersized crank bearing journal?
A crankshaft bearing journal on a single or twin cylinder motorcycle for example is a shaft on opposite ends of the crank that allows either a ball or roller bearing (or both) to be installed and fixed in place
It is vital that the journal is the correct size to allow the bearing used to properly seat on the journal
Journals that are undersized (below tolerance) will allow the bearing being used to “spin” on the crank journal which in return wcould cause a multitude of problems such as..
- Damaged / worm crank journal shaft
- Loss of RPM
- Bottom end rumble (sound)
- Bearing spinning on shaft
Each crank will be different and therefore may give different symptoms
Depending on how undersized a journal is will dictate some of the common symptoms above
What causes an undersized crank bearing journal?
I was once told that undersized crank journal shafts was caused by a bearing seizing and/or the manufacture then not being able to hold the proper tolerances needed
Cranks that suffer with a worn shaft will only get worse over time and should be fixed prior to an engine rebuild
Before we begin and cover how to fix you’r crankshaft, we first need proper and accurate tools
The single-most important step as we will be fixing our crank accordingly based off your measurements
Luckily today we will only require 2 simple but very accurate tools to begin the process
The tools covered today are..
- Bore gauge (1-2" range)
- Micrometer (1-2" range)
- Dial indicator
The bore gauge will be used to measure the inside diameter of our crank main bearings
The micrometer will be used to set the bore gauge and to also measure the outer diameter (OD) of both sides of the crank journal
Both tools below can be found on eBay for pretty cheap
I am using a Sterrett dial indicator on my Fowler bore gauge that measures in .0001" (tenths) as well as my digital micrometer
Above is a snippet taken out a late Triumph workshop manual (500 models) that shows the correct specifications for both bearings and bearing journals
The drive side journal which shows a roller bearing (which happens to be the same specs on ball bearing models 1957-1968 - 350 /500) has a shaft diameter of 1.1805" - 1.1808"
1.1805" - 1.1808" is the "low" and "high" end of the spectrum which is .0003” (three-tenths)
Ideally, our journals need to measure in the acceptable tolerance range to allow a good bearing fit on the crank end shaft
The timing side bearing as used from 1969-1974 shows the shaft measuring in @ 1.3774" - 1.3777"
Again, 1.3774" would be the "low" end while the 1.3777" figure would be the "high" end which is again (like the drive side) .0003” (three-tenths)
The book shows 2 basic measurements for each of the bearings in metric
- Roller bearing (Drive Side) (also same as the early ball bearings) - 72MM X 30MM X 90MM
- Ball Bearing (Timing Side) 72MM X 35MM X 17MM
We will measure the journals and bearings next to figure out where our crankshaft falls in the specs
Measuring the crank & bearings
Here is a 1972 Triumph 500 crankshaft that will be used for my race bike
We will be taking measurements in inches using this crankshaft
Using my micrometer I will measure both sides of the crank journals first
Drive side shaft
The drive side measures in at exactly 1.1800"
According to the Triumph workshop manual it should measure anywhere from 1.1805" - 1.1808"
On the low side, it appears that I am .0005" under - and in the high side I am .0008" under
This would be the reason why my roller bearing "slips” over the shaft
The drive side is in need of repair, and will be fixed
Timing side specs
The timing side measures in at 1.3771"
That appears to be approx .0003" - .0006" undersized according to Triumphs tolerance range
This side will also need to be repaired prior to use
Drive side roller bearing
Using my bore gauge I will measure the inside diameter of the roller bearing
I used my micrometer (not shown) to set the gauge to find the "true" I.D. of the inner race
Trying different figures I have found that the 30MM I.D. measures @ 1.1800" (inches)
The reason why we are measuring both inside dimensions of the bearings is to find the true diameter to calculate the fit on each shaft
In this case, the bearing happens to be the same size as the shaft..
Timing side ball bearing
The timing side ball bearing has an inner diameter of 1.3775" while crank journal measure in @ 1.3771"
This inside diameter of the ball bearing is larger by .0004" which is contributed to a worn shaft..
How to fix the crankshaft
There are a few different ways or methods to fix this type of problem
Some would "peen" the shafts, use bearing loctite, shim stock or perhaps leave it
Upon further investigation I found that the most logical and economical method (which is also permanent) in my situation would to have the both journals spray welded then ground back down to the proper size
I have had a few cranks "spray welded" before with excellent results
The company that will be performing this work is called "Marine Crankshaft", they are located right here in SoCal
I will box up this crank up and send it out to Marine Crankshaft
Luckily here in SoCal Marine will have it the next business day
Crankshaft back from repair
After a week away the Triumph 500 crank is back from Marine Crankshaft and from what I see, they have done a fantastic job!
The finish on both journals is chrome-like (far better than when it left the factory back in 1972)
I checked both journals and both come up to spec as per the book and the information I have provided to Marine Crankshaft
The process consists of welding each bearing journal shaft then re-grinding to the size as shown in the Triumph workshop manual and to the documents I supplied to Marine Crankshaft
I am very happy with the results, having both shafts at the proper diameter it will ensure a positive fit on both the roller bearing inner race as well as the ball bearing
My 2 cents...
When searching for a crankshaft you will start to see that most cranks have issues that will need to be addressed
However, one must not frown upon a good crank that can be salvaged
Each crank will be different and some will require more work than others
When repairing your crank use a company that can perform all the work necessary at one go
People to know
I have used a few different shops for crankshaft grinding, balancing and welding and these days quality work is getting harder to come across, not to mention working with someone that is "dedicated" to vintage Triumph, BSA and on Norton motorcycles
AJ Richter @ Richter Machining does a wonderful job on British machines for crank grinding and balancing on the east coast - you can send AJ cranks by dropping them off or sending them in via mail
(He also offers engine rebuilding services too - reach out to him for questions)
Marine Crankshaft also does great work too here on the west coast (California) - they too accept “mail in”cranks for work
Both Marine and Richter Machining have my trust when it comes down to any crank work
Thanks for reading
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