LH250 Twin Spark

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andy.mcgregor
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LH250 Twin Spark

Postby andy.mcgregor » Wed May 23, 2012 11:05 am

I'm new to Ariels and have just bought a spAriel project - a pile of bits which I'm hoping to bring together to turn into a pre-war Red Hunter. I've started stripping the engine down but I'm a little confused by what I've found so far.

The engine is from a 1934 LH250, and the head looks like the pictures I've seen of that engine, with the remains of the clips for the valve spring covers.
But it is fitted with two 18mm spark plugs and from what I can tell that's not standard. Can anyone suggest why that would be? Race engine? Would it have been an either/or plug set up or would both have been firing at the same time?

A number of the fins have broken off and there are the remains of a stud tapped through into the top of the combustion chamber. I originally though this was just to fix on one of the broken fins, but it appears to have a hole right through the centre - some kind of sensor mounting?

Both head and barrel will need some work, but the bottom end seems to be in good condition. I think I'll probably start by rebuilding it as is, but I'm a bit concerned that the 250 will feel under-powered.
What would need to be changed to turn it into a 350 or a 500. Cylinder, piston and head but anything else?

Andy

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Postby john.whiting » Wed May 23, 2012 1:30 pm

People often experimented with old bikes in the days when they were worthless,its lucky to have survived.You could fit either a 350 or 500 motor, if you can find one thats period correct,ie 37 or earlier.The primary drive is a bit different ,nothing insurmountable.The frames have the same appearance,but the 250 probably had lighter tube.Your best bet would be a later 350 motor,as being easiest to find and fix.Regards John.

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brenton.roy
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Postby brenton.roy » Wed May 23, 2012 2:01 pm

Hi Andy, snap! - I have a '34LH. unfortunately, dad was only part way through the resto and much of it is in boxes.
However, I can answer some of your questions. The head would have been a single plug. I don't know where a second plug would go. Any chance of a photo? The clips mostly broke over time. The covers - and maybe cups under the springs (not sure about that) are available, but cost an arm and a leg.
There's about 3 different oiling strategies for the guides over the '30's. I think '34 has a breather from the rear of the crankcase to the inlet valve. It goes into the side of the head.
My head has serious porting and big valves. I had it club dated and apparently it was factory issued with a High comp piston. In Aus, a number of LH's were raced. I'm not sure why. This might be the same with yours?
I don't think it should be a plug in the centre of the head. About the only sensor used in '34 would be a thermometer...
You'd have to replace most of the motor to change capacity. The rod is unique to the model, possibly flywheels, certainly head and barrel.
I understand they are far quicker than you would expect.
Also, they had 20" rims, front and back and a pressed steel (not cast) front drum. I'm not sure the frame is the same as the larger bikes of '34. I'd need to measure it.
Mine has a pressed steel primary and the smaller brake pedal - late 20's style.
I'd also be interested in a photo of your front guard and stays.
I don't think your model is particularly common...
regards, brenton
'51,'56 Squares, '48 VH, '27 Model C, R67/2, Mk IV Le Mans, '06 Super Duke and Ariel projects.

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Re: LH250 Twin Spark

Postby andy.mcgregor » Thu May 24, 2012 9:07 am

Thanks for the info gents,

Here are some pics.
I've still got the cups under the valve springs, although there's some work there for the chromer.
When the engine is together, the extra plug comes out between the pushrod tubes and below the O/S rocker plate. It's well done and doesn't look out of place. The extra plug itself has a long shoulder so as not to catch on the head fins.
P1010630.jpg


I've no idea if the valves are standard size or not. You can just see the tapped hole, just to the left of the top plug in the photo. It would come out on the other side in the middle of a fin - if it weren't for the fact that there are a couple of fins broken off at this point.
P1010634.jpg


I'm afraid I can't help with other pics. This is a real mongrel bike, I've got an aluminium primary and the front mudguard came from an autojumble a week after the other parts.
But I did find some good 34LH pics here:
http://www.andybuysbikes.com/archivehtml/5116arl.html

Andy

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brenton.roy
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Postby brenton.roy » Thu May 24, 2012 11:06 am

Hi Andy,
The second plug was added later.
The tapped hole in the combustion chamber is another later addition. I'm not sure why it would be there..?
The spring cups are hard to get - even with bad chrome.
Is the Aluminium primary the earlier (round lump over cush drive) or later, (teardrop over cush drive) style? If the earlier, it's just as likely to be the original as a tin primary.
I'll take some pictures of my LH head over the next couple of days and post them. Your valves look to be as big as would fit.
This is a rare and very interesting bike. There are plenty of 350's and 500's of that era still around.
brenton
'51,'56 Squares, '48 VH, '27 Model C, R67/2, Mk IV Le Mans, '06 Super Duke and Ariel projects.

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Re: LH250 Twin Spark

Postby andy.mcgregor » Thu May 24, 2012 12:45 pm

Brenton,

It's a tear shaped primary. The rest of the 'bike' was put together by a dealer and I think it's just a pile of period parts that he came across in different places. I don't think any of the other parts relate directly to the engine, although the frame is of the same vintage. I'll maybe find out more when I get details from the factory records.

It was originally sold to me as a 350 - foolishly I didn't check the engine number before buying. But you're inspiring me to finish the bike with this engine. It was seized when I got it, but I was pleased to be able to get the piston moving pretty easily. Its is on standard size and the bore looks unworn. I haven't stripped the bottom end yet, but the oil looks clean - seems like the engine was rebuilt at some point and then hardly used.

Obviously the bike will run on just one plug, but i wonder if there is anyway to run two plugs from a Lucas magdyno?

Andy

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brenton.roy
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Postby brenton.roy » Sun May 27, 2012 3:32 am

Hi Andy, here are a couple of photo's of my LH head. The inlet port is a bit over 26mm.
Your valves look bigger (as the bishop said to the actress...), otherwise, single plug, no mystery threaded hole.
re the primaries, I have the early 30's cases. I'm not sure whether the later primary cover fits these? The mounts are different and there's no flange thingy.
If you get stuck, AJS or norton steel outer primaries are fairly close.
I'm happy to provide more photo's if you need to look for parts.
regards, Brenton
Attachments
LH Head bottom.jpg
LH Head top.jpg
'51,'56 Squares, '48 VH, '27 Model C, R67/2, Mk IV Le Mans, '06 Super Duke and Ariel projects.

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brenton.roy
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Postby brenton.roy » Sat Jun 09, 2012 9:24 am

Hi Andy,
this is the tin primary.
I think it attaches to the frame under the gearbox. I'm not sure about the top rear attachment.
The front two mount to the studs in the crankcase.
I don't know what all the holes are for. It has the short 20's style brake pedal, plus footrest, which accounts for two...
Brenton
Attachments
LhOuterprimarytop.jpg
LhOuterprimaryinside.jpg
'51,'56 Squares, '48 VH, '27 Model C, R67/2, Mk IV Le Mans, '06 Super Duke and Ariel projects.

patrick.robotham
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Postby patrick.robotham » Tue Jun 12, 2012 12:03 pm

What an interesting head. My guess is that it is for sprinting and that the extra drilling is for injecting some more exotic fuel element such as nitromethane or if it is a gas fitting even oxygen. Your valves are definitely bigger. Probably best not to try to repeat the experiment as it could be like sitting astride a bomb. Pat

andy.mcgregor
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Postby andy.mcgregor » Tue Jun 12, 2012 2:17 pm

You could be right there Pat.
But don't worry, bolting a nitro kit onto an 80 year old 250cc wouldn't be my first thought of a way of getting my speed kick.

Andy


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