Tech Tip: Crankshaft Bushing & Journals

There are many vital components inside a vintage Triumph engine that must be fully inspected and addressed before an engine is put back together.

Unfortunately a lot of home builders and professionals alike need to understand that it is absolutely important and necessary that every key component is checked and replaced when worn.

Today I would like to go over some "tips" in regards to crankshaft bushings that where fitted on unit triumph 350 and 500 from 1957-1967 and all BSA A50 / A65 models.

This same method can also be applied to A7 and A10 models...

Performing this method or "tip" I will cover will give you more service “life” from your crankshaft rather then having it reground -.010", -.020", etc every time the journal is worn or an out of round condition.

To be clear, I am referring to the plain bushing journal on the crank, NOT the rod journals..

Let’s dig in...

 

There are few handy measuring tools that are needed before we start this project.

If you do not have access to these tools then you can take your crankshaft and bushing to your machinist or crank grinder and they should be able to assist you.
 

• Micrometer or caliper

• Telescopic Gauge or bore gauge

• Mill or lathe

• Factory workshop manual

 

If you do not have a least a micrometer or a caliper I highly suggest purchasing one. You should be able to find them locally or on ebay for just a few penny's on the dollar.

Measuring the crank journal

 

Measuring the O.D. of a Triumph crank journal

With your crankshaft on the bench you first need to mic your bushing journal with a caliper or micrometer to figure out what the true outer diameter is.

Take multiple readings in 4 different locations and compare your results... Refer to a manual to decipher if your crank needs attention.

Here are some key points that you must look out for when measuring:

• Out of round condition

• Pitting or any inferior markings

If your crankshaft is out of round, journal is undersized beyond what the factory workshop manual recommends or has any markings then your crankshaft journal should be turned down to the absolute minimal.

I state “absolute minimal” because you want your crank journal turned down to make it usable which consists of a perfectly round and polished journal.

This is no need to turn a .002” worn std journal crank down to -.010” When all it will need is about -.005” to make it round with a perfect finish.

 

What about the crank bushing?

Suppliers (including myself) sell crank bushings in sizes that follow:

• -.010”
• -.020”
• -.030"

So you might ask, if my crank is -.005” undersized, shouldn't I require a custom bushing? (Most bushings are in -.010" increments)

The short answer is no..

In a nut shell, you are trying to remove the minimum amount material from the crank journal as possible and use an undersized bushing (then bored) to match the crank.

Bushings are disposable, use them and save your crankshaft.

 

Boring the bushing

 

Crank bushing chucked in the lathe ready for boring

When boring the crankshaft bushing it’s best to remove the material using a lathe or mill. I used my mini lathe for this project and set the chuck with the bushing installed to .0005” run out to ensure a concentric bushing before I start boring..

With your crank ready and back from the crank grinder you must again measure the final outer diameter of your journal.

Use your new measurements and measure the i.d. of your crank bushing that you will be using. You will then see how much difference or material you will need to remove from the inside of the bushing to match the crank.

The goal is to remove material from the inside bushing to the point that the bushing will barley slide over the crank journal but keeping the bushing concentric.

When boring the bushing you must be patient. Measure twice and cut once.

If all is ready and correct, then you will need to fit the bushing into the case.

 

Installation

 

When installing the crank bushing in the case you have to keep in mind that the bushing will “crush” because of the press being applied once installed in a cool or room temperature case.

If done properly, once the bushing is installed in the case the crankshaft SHOULD NOT slide into the bushing.

It should be a tight fit to where it will bind or lock-up or not fit at all.

If all works well using the steps above then your job is done

 

Tips / next steps

Once the bushing is installed you will have to hone it with a rigid honning machine such as a Sunnen to accept the crank leaving a .001” - .0015” clearance for the journal.

You can also have the bushing line bored or set up in a mill using the drive side bearing as a guide to keep everything aligned.

Once the proper clearance has been achieved you should have a free rotating crank and also a crankshaft that you can obtain a longer service life out of.

Cranks with bushings have always been more time consuming to get set up properly. Once assembly, you should have a strong running engine that will last you many years and miles.

 

Recap

To summarize, we recommend using the crankshaft bushing to "match" the crank instead of removing material from the crank journal to fit the bushing.

We have been using this method for a long time with great results.

You can use method as discussed above on original Alpha style bushings or on Kibblewhite / USA made solid bronze bushings we stock.

 

Thanks for reading today

I hope you have found today's tech tip post informative. 

Should you have any questions please feel free to comment in the section below.

You can also click here for more blogs and technical information like this one.

Ride safe!