spark plug thread depth

[will_curry]

Thanks for the photos. It's a round-barrel engine and they're like
no other Ariel single I know so the 1/2" plug thread may well be right.

[charvey]

Hi Will,You might well be right about VCH engines has somebody got a VCH out there and can confirm that they have a 1/2" plug thread depth just for the record,Thanks all, for your input,regards Clive.

[paul.jameson]

The VCH head seems to have been something of an experimental area for Ariel. The head I had featured cycle thread rocker box bolts, the same as for the iron heads. Naturally, in alloy, these stripped all too easily and the later VH heads used whitworth threads. It seems quite likely that the spark plug thread was a similar approach of using the same as the iron head. Later VH and NH heads use longer reach plugs as plug threads are standard (metric!).

The Mk I Square Four was almost contemporary with the VCH. I remember an article some years ago which described how the prototype Mk I was lent to Square Four enthusiast Arthur Bourne, also known as "Torrens" and Editor of "The Motor Cycle" for a trip to France. He decided to fit special spark plugs for the trip in order to test them and stripped the threads in the head as a result. Ariel had to machine up another head and get it round to him so that he could take the bike to France. To me, this suggests the first Mk I had short plugs, the same as the iron 4G.

[will_curry]

Waller recommends Champion L7 plugs for all Mk 1 fours and these are 1/2" plugs.
Both the Mk 1 heads I've had dealings with had bronze (?) inserts screwed into the head
at the plug holes with the insert being pegged to stop it moving. The job looked good
enough to have been 'factory' fitted.

I have several of the later VHA/VCH heads and these are all 3/4" plugs and whitworth
rockerbox bolts. One is also cracked right between the valve guides and with the exhaust
valve seat dropped in. Something of a mess.