Its a GN cycle car engine that use the same two lobes for both cylinders.

Another good reference for that era might be Phil Irving’s book Tuning for Speed. He goes into cam design for bike engines of the mid-20th century era but not in great depth. Some of Harry Ricardo’s books from that early 20th C era might be more specific but I don’t have any of them so cant say for sure.

I do have a reprint of Cam Design and Manufacture by Frederic Burnham Jacobs, 1921, but it goes more into manufacturing than specific cam design. Like Irving, he covers the basics but that is about all.

Interesting history. Thanks.

If the engine is one of the inlet-over-exhaust models, the equivalent Harley would be the J Model. If you Google Harley J Model you will find there are numerous forums and a few companies making aftermarket parts, including cams, who may be able to provide cam specs that you could copy. The IOE is pretty specialised as OHV and SV cams usually have different characteristics and this has one of each!

The more you look at it, the less important Lobe Separation Angle would seem to be. All that really matters is valve events: valve opening, valve closing and lift. All the rest is secondary. The LSA will fall wherever it ends up depending on valve opening and closing specs. Keep the valves open longer, the LSA widens. And so on.

Of the secondary considerations, probably the most important would be the flank angles. Steep angles open/close the valves very quickly, holding the valve fully open for as long as possible. This puts great strain on pushrods and rocker arms etc. Gentler flank angles open and close valves gently, running quieter, with less stress, but giving less performance due to valve being not fully open for most of the duration. Looking at J Model cams here https://www.competitiondist.com/products/cam-001-harley-j-jd-cam-gear-1917-1929-61-74-inch  it looks like they open fast and close slow, probably to save the soft valve seats of the day before lleaded petrol?

This page here http://www.geocities.ws/pravgeusau/engine.htm gives some cam timing specs for the J Model IOE engine, unfortunately in inches before and after TDC etc. But it is a 3-1/2 inch stroke and there are online calculators that let you convert that into degrees.

“The exhaust valve should start to open about 9/16″ before bottom centre (BBDC) and close 1/32″ after top (ATDC), after 1917,  3/32″ (ATDC). The inlet valves start to open 1/32″ before top (BTDC) and close 1/8″ past bottom (ABDC), after 1917 5/32″ (BTDC) and 3/8” (ABDC).
It was found that although the 1917 and later models have different cams shapes for the front and rear cylinders, the timing is the same for both cylinders. Actually measured values are; exhaust opens 14 mm BBDC and closes 1 mm ATDC. The inlet valve opens 5 mm BTDC and closes 22mm ABDC.”

It looks like there is not much overlap at TDC, indicative of a wide LSA. But a lot of degrees translate to not much linear movement close to TDC so would need to be calculated out to be sure.

Have fun!

I have a feeling the lever type cam followers come into play when working out what is going on as if the tappet were just radial to the cam axis it would not work as you can’t get the cam to actuate both valves the same relative to TDC.

Have you asked on something like the GN Cyclecar FB group from which I sniiped this

On 27 July 2024 at 10:10 JasonB Said:

I have a feeling the lever type cam followers come into play when working out what is going on as if the tappet were just radial to the cam axis it would not work as you can’t get the cam to actuate both valves the same relative to TDC.

Have you asked on something like the GN Cyclecar FB group from which I sniiped this

Elegant in its simplicity. But also got me scratching my head. Those cam followers appear to contact the cam above the centreline, so looks like there is 90 degrees or camshaft rotation between the two followers? So one cylinder would fire 180 crankshaft degrees after the other one? Then a looong 540 degrees until the first cylinder fires again? Kind of a “big bang” timing, as used by some racing bikes in recent years to get unevenly spaced power impulses to allow rear tyres time to cool down and regain shape between cycles. Harley tried it on their XR750 dirt track racer but abandoned it. Seems to work better on the 4 cylinder bikes.

None of which has anything to do with Lobe Separation Angle between the intake and exhaust lobes of course, but very interesting nonetheless.

https://www.facebook.com/share/r/PCXwivvNJDT4yC1h/

Still scratching my head. I know the answer must be simple — but I must be simpler!

Looking at other photos on the FB page, the OHV engine runs the pushrods parallel, not crossed. The rocker arms are just offset to match the pushrod location.

Now, looking at that video I posted a link to, it looks like the followers are spaced apart perhaps at 135 degrees, not 90 as it looked in the other photo? So 135 degrees of cam rotation gives the requisite 270 degrees of crankshaft rotation equal to one revolution of 360 minus the 90 degrees the cylinder is offset. The next firing interval is the long one, 450 degrees, ie 360 plus the extra 90 degrees around to the next cylinder.

270/450 is the firing interval of the new modernday Triumph Bonneville twins, so it is quite do-able. Triumph though keeps the cylinders parallel and uses two crankpins with a 90 degree offset, aka 270 degree offset if you measure it that way. Gives a beautiful V-twin loping exhaust note out of a parallel twin. And like the 90 degree V twin, has virtually perfect primary balance so runs very smoothly.

A very clever arrangement in its simplicity by the gentlemen at GN if that is the case.