Norton model 18 1946 Piston

Hi Martyn, 

It may pay to check your frame number as 1946 Model 18's were fitted with a rigid rear end. Frame number for 1946 should start with A (for 1946) 3 (Model 18) then a 4 digit number.

An ES2, which had plungers fitted, has a model 4 designation. e.g C4 for a 1948 ES 2.

.. why you would want to lock the plungers? They can be tricky to dismantle if seized (I had a real odyssey with mine) but once sorted out I find both comfort and handling far better than with a rigid frame.

I had a new spindle made for mine by a helpful ex-toolmaker for about £20 or so. Unless the alloy sliders are fractured they should be retrievable by boring and fitting oversized spindles - or maybe even bushing.

Locking them would be very tough on the frame. And the plunger frames are allegedly not over strong to begin with. The rigid frames have a simple rear triangle.

Hi Martyn,

   By your description it sounds like your frame has had an after-market plunger conversion which were popular in the late 1940s and early '50s. If this is so you would need to have bespoke parts made to restore it to working condition. The other option would be to remove the rear end of the frame and get it re-tubed and fit the rigid frame lugs as original if you can find donor parts. The worst scenario would be to lock up the plungers as you will be carrying the extra 15 pounds of weight for no benefit and this would be less stable than a rigid frame. If you can post a photo of your bike someone might be able to identify it. As regards your piston, the pre 1948 flywheels were slightly larger diameter and thinner than the 1948 type, but piston weight complete with rings and pin should be around 1 lb for both. If you can lose an ounce off the piston it will be better at high revs.

To me that looks like an original Norton plunger setup. According to "Norton Singles" by Roy Bacon plunger suspension was available as an option on ES2 from 1939, becoming standard equipment from 1947 along with Roadholder forks instead of girders. My understanding, at least for UK models, is that Model 18 always had a rigid frame. Could it be that model specifications for export markets differed?

Ian McD

Hi Martyn,

   Yes as suspected this is a post production fitment. The frame, forks and engine all look correct for a 1946 Model 18 rigid frame. The main plunger support lugs look to be modelled closely on Norton's own castings but have slight differences. This is also the case with the bronze sliders. The frame still retains some of the features used on the rigid version and has the original rear chain guard.  To achieve this fitment the 4 rear frame tubes have had to be bent and re-aligned.

   If I was in your position I would try and restore the plungers, perhaps by getting new vertical spindles made and sleeve the slider units. If you do this make sure you have the correct plunger rear wheel with the one piece spindle. You could fix the plungers rigid by putting substantial aluminium tubes in place of the springs to the exact length but you would have still have the wear issue side-ways.

   BTW,   I might have over estimated the extra weight, it's probably only 14 lbs 15 oz.

My apologies, Richard has picked up on details that I missed.

Ian McD

.. look very like Norton ones apart from the colour. Are you sure they're not just painted? It's hard to imagine a small manufacturer going to that level of detail.

BTW your primary chain could do with adjustment!

The plunger assembly looks very like the very first BSA 500 plunger set-up in 1946. It had a unit engine, of which I have a few parts somewhere. Yours has a very large Battery, is it electric powered as it has no petrol tank visible. In which case it is a Green Norton, so setting a trend followed today.

Hi Ian McD,

   This had me scratching my head for a while, so no worries. Just remember, it's a minefield out there !!