Tech Tip: Fixing A Loose Fit Bearing On A Triumph Crankshaft

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
  • Friction
  • Heat
  • 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 



Dial Indicator & Micrometer

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 




Crank Journal Specifications - Triumph


Above is a snippet taken out a late Triumph workshop manual (500 models) that shows the correct specifications for both bearings and bearing journals 


Drive side

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


Timing side

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

 Crank Journal Specs - Drive Side


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

Timing Side Journal - 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

 Measuring Drive Side Roller Bearing With Bore Gauge

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 


Measuring Timing Side Ball Bearing With Bore Gauge


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


Triumph 500 Crank Prior To Welding


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

Crank Finished


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 


Crank Journals Welded & Polished


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


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


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

Thanks for reading today's Tech Tip post 

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Want to join in on the conversation? - leave us a comment in the section below

Ride safe!


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  • Classic British Spares

    @ Boyd – thank you Boyd for clearing that up, makes sense to me

    We appreciate you

  • Classic British Spares

    @ Ray – thanks for commenting and reading today

    Yikes! At least your OK, glad you are still using the machine today

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