Longstroke valve timing

[Brian]

This is purely for information.

Some time ago there was a topic about the valve timing on the longstroke engine being incorrect due to the wrong timing gear being fitted. This is the gear that meshes with the crankshaft and the cam gears and has the dot and dash, also being the gear that carries the generator drive cog.

If you are building a motor and have a mix of parts it is very important that you have the correct gear for the longstroke engine. The gear for the longstroke engine is identical to the A10/A7 "shortstroke" except the dot and dash are four teeth different.

In the "longstroke" engine there are 16 teeth between the marks and on the later "shortstroke" engine there are 20 teeth between the marks.

In the pictures I have added the gear on the right is the "longstroke" gear. The "shortstroke" gear has 67-336 stamped in it and the "longstroke" gear has a P and what looks like a 2 stamped in. I dont know if these markings are consistent for all the gears so if you have any doubt count the teeth between the marks.

If you have the wrong gear in your "longstroke" engine the bike will start, run and idle perfectly but when you go for a ride about 55mph will be the maximum top speed.

[muskrat]

Good info Brian, thanks
I didn't think they did 55.
Cheers

[Brian]

I had a tail wind that day 

[trevinoz]

I have just built my Star Twin engine, the first long stroke for me.
There are a couple of traps for first time builders!
I thought that I would be smart and make sure that the generator strap was fitted before the inner timing cover. When I went to fit the auto timing gear, I found it wouldn't fit through the inner! Off with the inner and fit it.
I have just realised that the right hand front engine plate has to be fitted before the generator strap so the inner has to come off again.
I won't talk about the time taken to fit the rocker boxes, give me the later one piece with a comb any day.

Trev.

[Brian]

For a couple of years I had the Star Twin and a '51 A7 (shortstroke) so was able to directly compare them.

Both plunger models so I would think very similar weights and identical handling etc. So it came down purely to the engines.

The longstroke was a "nicer" bike to ride if you just wanted to go for a normal ride around the countryside. By "nicer" I mean it has a wider spread of power than the shortstroke and probably a bit smoother. So for a nice Sunday ride or on a rally with others it probably had the edge over the shortstroke.

However ! If you are a bit of a rev head and like to give it some welly then the shortstroke is significantly better than the longstroke. The old saying "ride it like you stole it" really does apply. They love to rev and respond really well if you give them a flogging. My bike would easily sort out a A10 up to about 50mph.

I dont own the '51 anymore and are in the process of selling the Star Twin but if a '51 or '52 (shortstroke) Star twin came up for sale I would buy it, they are a great bike.

The shortstroke motor did address some of the faults with the longstroke motor, the main one being camshaft wear. There is a good reason why longstroke cams are almost impossible to find.

They are both good bikes and I have enjoyed the miles I have done on mine.

[vinver]

Thanks for posting, I am building a 48 Long Stroke from parts, and it's good to know!
Vincent

[Stephen Foster]

Thanks for the "heads up" Brian !
Brian , what size rear sprocket did Your longstroke "Star twin" have please ?

Steve...

[Brian]

The std 46T.

I run std gearing on all my bikes.

[beezermacc]

If you go on the Cheshire BSAOC website, tech tips page, somebody has gone to the trouble of drawing pictures of the A7LS gears and A7 / A10 gears side by side.  www.cheshirebsa.co.uk