Saturday, 10 June 2023

Velocette KTTs 55 and 305: Wheels – Part 2 – Cup-and-Cone Bearings

All the front wheels use cup-and-cone bearings and unfortunately, I was sadly lacking sufficient of these in good condition; I had one good cup and one damaged cup. The pictures below show pitting on the cup and typical brinelling damage on the cone.

The cone is from a different Velocette hub. There are a number of possible reasons as to the causes of such damage (interestingly, there is little agreement in the open literature - see eg here in the context of angular contact bearings) but, in this instance, it is probably the result of poor adjustment and/or inadequate lubrication.

The earlier hubs (part # W1/2), 2 of which are shown below with the bearing cups and the bolt-on brake drum removed,

use a really strange size of bearing. The hubs, where the bearing cup is located, is bored to 1.735” - that is 0.015” under 1 ¾” inches? Why did Velocette choose that size?

Now, the simplest solution would have been to bore the hub ID to 1.749” and fit off-the-shelf taper roller bearings. However, these bearings are not cheap; the best price I could find for the bearing that I would have liked to have fitted - Timken 05068/05175 TS was 182 GBP each – which adds up to 728 GBP for the 4 bearings required for 2 wheels! Whilst I don’t mind spending money on my restoration projects, I consider that a bit too much to spend on 4 wheel bearings.

Deep groove ball bearings are in plentiful supply and much cheaper but are not an optimum technical solution because they are designed to take radial loads but only minimal axial loads (they also need a spacer between the bearing inners to avoid the clamping force putting a large axial load on the bearings which would otherwise destroy them after a short period of time). Wheel bearings experience axial loads when cornering and huge axial loads if a sidecar is fitted.

On the other hand, angular contact bearings will take axial and radial loads, however, I could not find bearings of the size that I wanted and one that could possibly have been used was, again, horrendously expensive.

I therefore decided to make my own replacement cup-and-cone bearings.

The parts manual very usefully lists that these hubs use 9 balls of 3/8” diameter each side (part # W15).

I measured the radius of the cup using a radius gauge

and this was 0.25”.

Based on the dimensions that I had and not having an original cone to reverse engineer, I sketched out what I thought would be the optimum layout, shown below.


Here, I have used a 450 contact angle for the design (see picture below for the definition of this, shown for angular contact bearings) and the radii of the contact surfaces of the cup and cone are 0.25” and 0.315” respectively.

Picture courtesy of SKF

The overall plan would be:

1)    Use O1 tool steel to make both cups and cones

2)    Make blanks of the cups in the lathe and then use a ½” diameter carbide ball-nose cutter in the milling machine to machine the 0.25” radius bearing surface

3)    Use a 16mm diameter carbide cutter on the lathe to machine the cone bearing  surface profile. This cutter gives a radius of 0.315”

4)    Harden and temper all parts to ~ 60 HRC



The first step in making the cups was to machine 4 blanks to size, leaving sufficient material for the ¼” radius in the corner.

 Each of these was then mounted on the milling machine in the dividing head chuck

and, after a lot of handle-turning, there are 4 nearly completed cups. A damaged original is shown at the bottom of the picture for comparison.

The final operation was to chamfer the corners. The hubs have a well-defined radius of 3/32” at the corner where the bearing seats

and it is important that the bearing cup does not foul this corner. Another radius tool was therefore ground from a piece of square HSS bar to machine the corner of the cups.


The bearing surface profile was machined using a 16mm round carbide insert, as shown below. This has to be machined carefully because the tool is in contact over the whole length of the profile.

After tapping a ½” 26 TPI BSCY thread through the centre and milling spanner flats on 2 of the cones the machining of these is complete.

Before heat treating the cups and cones a few final checks were made on the assembly

and to ensure the balls are not in contact with each other when the cone is inserted.

All was good here so time to proceed to heat treatment.


Heat Treatment

Heat treatment – hardening and tempering, was carried out in exactly the same way as for gears described in a previous project (see here).

The first step is to make each component into a little package using stainless steel locking wire so that they can be easily removed from the furnace when hot and to coat them in Kasenit anti-scale compound.

They are then put into the high temperature furnace and heated to 815 0C

before quenching in oil (vegetable oil from the supermarket). The parts are then put immediately into another furnace for tempering at 200 0C and soaked for a couple of hours before removing the Kasenit coating with a wire brush and put in a tumbler overnight.

Although I can check the hardness approximately using a fine file it is much better to have a quantitative measurement if possible. Luckily, a local company has a hardness testing instrument and a couple of parts were selected for measurement. This came out exactly as I had hoped for, namely 60 HRC.


Last, but by no means least, new wheel spindles (EN16T steel), nuts and dust caps were needed.

All of the components for these 2 front wheels are now completed.

One Final Job

I have 3 sets of strutted KTT forks – 2 of these obviously belong to KTTs 55 and 305 but I plan to use the 3rd set on the KTP-framed special. However, the brake plate used on strutted forks differs from that on the standard forks by virtue of the brake cam boss being longer to avoid the brake lever and forks interfering. I only had 2 correct brake plates and a standard brake plate was therefore modified by making an extension piece that was held in place with a substantial phosphor-bronze bush.

The picture below shows the extension piece and bush before assembly.

After assembly, the bush was reamed to size, the extension piece was fettled to blend in with the brake plate casting and a new brake cam made to fit.

The work on the front hubs is now pretty well complete.

Thanks to Peter at Jayess Tools for grinding the carbide ball-end cutter and the HSS radius tool used to make the cups. 

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