Shortstroke 500 Scrambler, Slow Build

[johnmain]

Well after a lot of soul searching, I have decided to pull the plug on this project. There were more and more issues coming up from searching for more power, costs were spiraling, and the didn't seem to be a reliable end in sight. Thak you all for all the fantastic help and advice I have recieved here.
Just going to build her up as a trials engine and sell her on (
She's going to have a Jawa motor in her now, as it just works out more cost effective and fit for purpose. I do though intend to revisit this out of sheer bloody mindidness
Thanks
John

[david.anderson]

John
It would be hard to argue with your choice of a Jawa motor. Good luck with the venture.

The stumbling block for John has proved to be the short stroke bottom end. The 350 crankpin is not up to the job for a performance 500. A new set of flywheels with a stronger 38mm crankpin is available in Australia but the costs are prohibitive. And John received advice from a performance engineering shop in the UK that the crankcases were not up to the job and would break. While I had not previously heard of the VH breaking crankcases John also checked with Australian Ariel tuner Clive Harrop. Clive advised that he had been breaking one or 2 sets of crankcases per season so he made some heavier crankcases that have proved satisfactory. Those crankcases are available here in Australia at $2000 per set.
What I can say is that Clive Harrop must be putting out more horsepower than the successful Ariel tuners of days gone by to break crankcases. I am envious.
So the shortstroke VH was going to cost in excess of $4000 for a reliable bottom end and there is still the rest of the engine to build. Clive Harrop had also been breaking cam followers and now sells stronger sets, at $500 a pair. Postage costs add considerably to the outlay.
A strong purpose built Jawa is clearly a much cheaper option for a performance single for MX.
Again good luck with your venture John.
David

[Simon.Gardiner]

David - thanks for the update, very interesting and slightly depressing!

SG

[johnmain]

Thanks for all your help David, and an excellent post thee. Which I should have posted lol

[Simon.Gardiner]

I found an article in Jan 2010 Cheval, in turn a reprint of an article in Motorcycle Sport, Jan 1964.

It's about a sidecar scrambler successfully using an Ariel single modified to 600cc. I thought it might be appropriate to add a summary of the engine mods to this thread.

-pre-war Red Hunter steel flywheels with 15/16 crankpin, bored (and re-faced) to take a wider International Norton bearing with pin of shank dia 1.125" at a radius of 50mm, for a stroke of 100mm, with an International rod ('or get Alpha to make up special crankpin to take the Ariel rod');
-(crank) case halves free from cracks ('which often appear in the region of the front mounting lugs') and with slots machined to clear the Norton rod;
-cam compartment oil level altered ('...block the existing drain hole and re-drill one on a level with the cam spindle centre...it is impossible to over-stress the importance of this modification') (no mention of draining the mag chain compartment but there's also a comment about needing to use vegetable oil - eg Castrol R - if 'the camshaft and followers are to last');
-pre-1954 iron roadster barrel bored to take an 85mm Gold Star piston (eg Hepolite 13052, 'about 9:1');
-through-studs between head and crankcase (there's a detailed description of this but essentially it uses the method with the adapter screwed into the crankcase with an offset hole to put the through stud in line with the cyl head holes to take the head sleeve nuts);
-'saw the corners off the barrel flange to get it to fit';
-barrel raised with a 3/32 spacer and relieved (at the edges) hemi in the head to clear the piston (this will be with the Norton rod which might be shorter than an Ariel rod);
-alloy head (seems to be using standard valves);
-modified push rod tubes;
-cast-iron Mk 1 HS cam (with HS followers - this is considered the most effective cam/follower combination );
-racing magneto, methanol, 1 3/16 bore Amal type 29 carburettor.

'while....not as quick as an oversize Norton or very special 650cc Triumph or BSA it is normally quicker than most ordinary six-fifties and affords infinitely more wheelgrip for a lightning quick getaway.'

I thought the reference to breaking crank-cases was interesting, in the light of Clive Harrop's problem. (And looking again at the picture of the new Harrop crankcases, they seem to be for a 4-stud barrel, I think he must be through-bolting to the head so I wonder if the crankcase stud holes are moved to help with that.)

SG

[nevhunter]

The "normal" place for crankcase cracking is the top front lug where the stud goes through the engine plate. I've not come across other places of much weakness, but I imagine the timing case area is much weaker than the drive side due making allowance for camshaft bearings etc. The ARIEL was always regarded as the "poor man's JAP" and a good clubman bike for the fiddler and tuner. Probably the exhaust valve is bigger than it needs to be on the earlier ones, and if you followed JAP speedway practice the head would have had the valves at a smaller angle leaving the head a bit shallower with a less domed piston needed. for higher comps with alcohol.
Through bolting is not made easy by the bolt centres being quite close to the cylinder, at the head face especially if you want to seriously over bore. It's surprising how good some of them go on the track, even pulling a chair. Nev