Henderson spark plugs

[exeric]

While perusing my 1919 Henderson owner's manual, (which I should have been studying for many years) I discovered that the spark plugs are required to have a 'reach' of 3/4" (19mm) from seat shoulder to points. That puts the spark well into the combustion area, which tells me why that thing could be so hard to start (duh). The problem I have is finding a 18mm threaded plug with 3/4" reach. I've looked on ebay, and parts store sites and no-go (long gone ancient stuff). I can get an adapter that will take a 12mm (3/4" reach) spark plug and go into a 18mm threaded Henderson spark plug hole. Does that sound okay and proper? I would love to find some nice period plugs of the right dimensions but I think that ship sailed a long time ago. Thanks.

[BigLakeBob]

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[BigLakeBob]

Ebay, Sixty Auto Parts has alot of vintage plugs. Maybe check them out?

[joe fxs fxr]

If you are concerned with judging, they let you use any plugs to prove the motor runs, but you need the correct ones for inspection.

[exeric]

Thanks for the chart, Bob. Unfortunately, those Champion plug #s are out of most inventories, and plugs on ebay are priced for collectors. Also, it is not impossible to get a new plug that is defective from the factory so there is that risk. However, I still appreciate the chart and will continue to look for those plug #s. Thanks, Joe, I have some 1/2" reach period proper plugs, but I really do want some easy to replace modern plugs of the correct 3/4" reach so I'll give the reducing bushing thing a try.

[T. Cotten]

If you are concerned with judging, they let you use any plugs to prove the motor runs, but you need the correct ones for inspection.

Please help me understand, Joe,...

Anything that fires is fine, but you need trophy plugs, too?

Thanks in advance,

....Cotten

[joe fxs fxr]

LOL yes, you know how the judging system works

[C.Flint]

@exeric…I was surprised to learn that your owner’s manual specified 3/4 inch reach. My recall was that the ‘19 Henderson motor was virtually identical to that used in the ‘17 Model G. I use Champion D16 plugs (1/2 inch reach) in my ‘17 and have put on over 6,000 miles (Cannonball and others). Easy starting - as is true with most low compression four cylinder motors.

[duffeycycles]

Won't an adapter lose some of that reach? D23 champion from Brillman is 5/8" reach..18mm Champion Spark Plug, 5/8" Reach - The Brillman Company

[exeric]

Well, first of all, I am a lousy mechanic which probably explains the troublesome starting. I also have a 1917 Hen and I have been using a 1/2" reach plug and don't recall any great difficulty with starting or sustained running. Both bikes have the worst starter jaw design I have seen on a motorcycle and that has been the Henderson curse for me. The poor engaging starter has (no-doubt) led to many other distracting, tail-chasing problems such as magneto, timing, and carburetor. Like I said, I was surprised to see that comment in the 1919 manual about plug reach and as you said, the cylinders are not much different from 1919 and back. Here is the verbatim quote from the 1919 manual:

"Don't use spark plugs with short points, as this may make the motor hard to start. The spark points should measure 3/4-inch from the gasket or shoulder of the plug, so as to place the spark well out in the gas in order to produce the best combustion. the points should not be longer than this, however, as they might be struck by the valves if they project too far."

I was pleased to hear you say the 1/2" reach plugs have been good to you. If worse comes to worse I'll just soldier on with the shorter plugs. Thanks for the reply.

[exeric]

Won't an adapter lose some of that reach? D23 champion from Brillman is 5/8" reach..18mm Champion Spark Plug, 5/8" Reach - The Brillman Company

You make a good argument against the adapter. I ordered a set of the 5/8" reach Brillman plugs. Thanks for helping me with that Roger, very much appreciated.

[T. Cotten]

LOL yes, you know how the judging system works

Actually, Joe, I do not..

(The goalposts keep moving, and I could never afford to pay attention).

So there's a swap-out loophole? Could the machine be started with a Linkert, but a dummy Schebler mocked up for "the show"?

What's the difference?

....Cotten

[joe fxs fxr]

So there's a swap-out loophole? Could the machine be started with a Linkert, but a dummy Schebler mocked up for "the show"?

....Cotten

I'm not a judge, and don't pretend to know how the rules were written. But I can guess that correct 100 year old plugs that actually work are getting hard to find. The loophole only applies to plugs as far as I know. I've heard that some people do a really good job rebuilding 100 year old carburetors, so swaps are not needed.

[T. Cotten]

I'm not a judge, and don't pretend to know how the rules were written. But I can guess that correct 100 year old plugs that actually work are getting hard to find. The loophole only applies to plugs as far as I know. I've heard that some people do a really good job rebuilding 100 year old carburetors, so swaps are not needed.

I know of nobody offering complete and comprehensive Schebler services at this time, Joe,

And nobody stepping up to the plate, (even for Linkerts, actually).

So,.. repop parts can be used to make it run,

And then fossils can be hitched up for the glory session?

.....Cotten

[joe fxs fxr]

I know of nobody offering complete and comprehensive Schebler services at this time, Joe,

.....Cotten

Oh, I though you were the one doing them. I can see where you are coming from, but can't offer any help.