Honda-SOHC

That type of plug cap was  also fitted to the Opel Rekord (D) Coupes back in 1973 I was drivng one down to Cornwall for our sumer holiday when the car dropped onto 3 cylinders in heavy traffic. Being an automatic it was it was almost undriveable.

One of those caps had failed plus the leads were the then quite new carbon fibre typ leads - I used a Nappy pin to replace the cap - it worked a treat.

There are plenty of other people fitting them to our bikes Ted for originality ... I bought 5 sets of each cheap and kept one of each set for myself and sold the rest on eBay. They didn't hang around for long  before being sold ! My 500K0 didn't have them as it is a 1st batch USA (squared off crankcase) bike and they were never fitted in the US. The 500 ones (fitted to UK and European bikes)  are the same as the 400F ones.

That type of plug cap was  also fitted to the Opel Rekord (D) Coupes back in 1973 I was drivng one down to Cornwall for our sumer holiday when the car dropped onto 3 cylinders in heavy traffic. Being an automatic it was it was almost undriveable.

One of those caps had failed plus the leads were the then quite new carbon fibre typ leads - I used a Nappy pin to replace the cap - it worked a treat.

550 and 500 same shape but later ones had a connector bit, the screwed in retainer does not have a seal, only the big tim pressed plate

So the seal fits into the sprocket cover plate Bryan not the retainer ring - that makes sense.
It confirms what seablower said as well.

I should have taken more photos!!

PXL_20240505_084731863 by Macabe Thiele, on Flickr

My UK 750K0, CB400F & 450K0 Bomber all came originally with metal plug caps.

Much maligned on here I know but for originality I am fitting them. I managed to get NOS ND ones for my 4 cylinder bikes  but the Bomber ones are hard to come by. also one of my NOS 4-cyl ones got badly damaged with acid. So this week I managed to de-crimp the corroded one and replated a grotty looking Bomber one. Pretty sure my CB250K0's and K2 would have had them but not found any of those yet.

I plan to do some tests on them to see why they tracked over the HT in the wet. It's not obvious looking at them why they would track. I suspect the rubber insert may not have sealed too well but a new part moulded in silicone may fix it. A lot of heathen dealers (like the one who supplied my 1st 400F ) just ripped the metal covers off and damaged the grey plastic body in the process.

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I'll fill it up with stuff in no time I'm sure, I'd have lived to have gone bigger but I'm doing it within permitted development rules (I think) and getting this far has cost a small fortune