These rocker caps replace Ariel part# 857-38.

These are currently out of stock.

They fit all OHV big singles from 1938 to 1959.

This includes Red Hunter and De-Luxe models NH, NG, W/NG, VH, VG, VHA, VCH, HS, HT, HT3, HT5 and the pre-war 250cc LH Colt

The original components are cast from some soft gray unpolishable alloy (Mazac? pot-metal?) and I find have a tendency to round off as the cast hexes are a little tapered and don't seem to fit any wrench in my toolbox. I thought making some from a lump of Fortal bar stock would be a good exercise.

There are some differences from the originals as follows.

• These are made from Fortal high strength aluminum alloy. I got this stock on eBay, just search for it and you can get all the specs - it is harder than the original cast caps, that's for sure. This polishes to a mirror finish. I have run them through the buffing wheel a little and got most of the machining marks off. I figure anyone who wants them really, really shiny probably has better polishing gear and more patience than me!
• The outside diameter of the cap is a bit more than the original at two inches. I ran them a bit wider thinking I would drill for locking wire. I found that it is easier on the fingers to grip the knurled edge with it wider as there is some clearance from the rocker box.
• The outside diameter is knurled rather than scalloped. I don't have a rotary table or rotary indexer for my mill, so this was the most practical solution.
• The hex size is an accurate 7/8". I think the original is about 3/4". Bigger hex and accurately cut. The sides are vertical too - from being milled rather than cast. The hexes are a very nice fit in my 7/8" ring spanner.
• Threads are longer. These have 2-3 more theads than the originals. They should stay on better, particularly if 60 years of routine maintenance have taken a toll on those first three threads in your rocker box. I have test fitted these caps on my 56 NH and there appears to be no clearance issues with the rocker or tappet. I advise that you make sure those previously unused threads in your boxes are clean before screwing these in - scrape them with a 20 tpi whitworth thread guage.

The thread is cut manually with a 55 degree single point tool to give the correct 1-7/16" x 20 tpi Whitworth thread form. Each cap has been screwed into my 56 NH's rocker box to ensure fit and clearance.

There's a lot of steps involved in making these. See this page for details. I am also going to make some filler caps for Burman GB gearboxes. Let me know if you might want one.

Here are a couple of pictures of my prototype caps fitted to my machine. Note that these prototypes are not the actual ones for sale (see main picture). They differ only in that they are unpolished.

I now have a rotary table so the next batch will have the correct outside diameter and scalloping.