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Bike Profile - Posted 7th October 2011

1953 AJS Model 18
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Classic motorcycle engines which pull a sidecar can get a bit hot and bothered. Neil Cairns suffers déjà vu and a seizure, all at once...

Luck seems to arrive in lots of three, so the old wives' tale goes. There appears to be some truth in this for me. Things also seem to repeat themselves during one's life, and I had a real déjà vu moment on recently as I came to a rather ignoble stop on my 1953 AJS combination.

Way back in 1978-ish I was out on my 1961 P&M Panther M120, pulling a sidecar with a camping trailer behind. We rode through the pretty scenery of North Wales with the wife and kids on board. The poor bike was working hard as we were en route to a Panther Owners' Club rally; these were always up on the top of freezing mountains in North Wales. About a mile from the campsite the bike expired, grinding to a halt sounding like a steam locomotive blowing off steam with big, flat, chuff-chuff-chuffs coming from the silencers (the M120 650cc sloper-single has two).

The exhaust valve has seized up and was stuck partially open. So we were towed (by another M120 also with a sidecar attached) up to the site. There, using my toolkit which was kept under the chair's seat, I removed the bike's cylinder head, then the valve, reamed the guide out (one chap had enough tools in his combination to rebuild a whole bike), bent the oil feed pipe to the rockers so it dripped onto the exhaust valve's spring (to get lubricant into the guide and then resulting in double the machines oil consumption figures); reassembled everything and had it running again in three hours.

'Using my toolkit which was kept under the chair's seat, I removed the bike's cylinder head, then the valve, reamed the guide out'... 1961 P&M Panther M120 - Running repair...

The photo is of me back then, balancing the cylinder on my knee as I remove the head... The engine is effectively the frame's front down tube. This photo made it to the front cover of the POC member's magazine! My fifteen minutes of fame, note the slim figure and full head of brown hair.

Now let us fast-forward to a Sunday in September 2011. I am en route to a Royal British Legion Riders Poppy Appeal show. I never got there. It is about 9.30 on a lovely morning as I approach the Black Cat roundabout by the A1 on my 1953 AJS 18S with its posh wicker sidecar. Then comes a familiar sound: pop... chuff-chuff-chuff.

The AMC 500cc single has lost its compression and has seized up its exhaust valve.

'I get taken home by Green Flag after half the customers in the Little Chef restaurant have taken photos of my poor bike being drawn up onto the recovery truck'... 1953 AJS Model 18 - Rolling recovery...

The TravelLodge reception girl let me use the telephone to contact recovery. Pushing an AJS combination is not good for OAPs like me, so I am pleased to leave it in their car park. I get taken home by Green Flag after half the customers in the Little Chef restaurant have taken photos of my poor bike being drawn up onto the recovery truck.

Once home I whip off the seat and tank, then remove the cylinder head. The exhaust valve is sitting about 2mm open, and solid. I drift it out, clean out the guide of hard carbon deposits and polish up the valve and its stem. When I try to grind the vale into its seating, I find it is only just touching it, the paste hardly doing anything. This is odd, I thought.

Not being done on the campsite this time... 1953 AJS Model 18 valve guide...

Investigating, I find that the bit of the valve's stem that opens out into the tulip-shaped head is binding on the end of the guide in the port. The guide protrudes too far into the port. It must have been like this for years and years. So I rebated (countersink) the guide by 2mm to clear the valve's stem, grind in the valve and re-assembled it all. I take the opportunity to use a new gasket under the rocker cover, as the old one weeped at the rear edge. By 2.30pm I have it all running again.

The oil feed to the exhaust valve's guide is from a little well in the head (see below). This I cleaned out of carbon, finding the hole from it into the guide blocked; the cause was a lack of lubrication? More often seizing exhaust valves are caused by excessive oil turning to hard carbon, though one chap on blamed leadfree petrol. (I use Redex additive to protect the exhaust's valve seat and have been for over seven years and 14,000 miles! The cast-iron seat was in excellent condition.)

Then I took the bike out to see how it all ran. I stopped at the Leighton Buzzard Railway for a cup of tea in their cafe, took a photo of the old toilets being dismantled, and dropped my camera. It broke! That was my No2 bad luck of the day.

Mind you don't drop any bikts in the long grass... 1958 AJS Model 18 handbook...

The illustration from the 1958 handbook shows the alloy cylinder head with the little oil well (No8). The text discusses seizing exhaust valves. Neither the workshop manual or Roy Bacon's Restoration Guide mention anything about seizures. Looking at the sketch, by 1958 the exhaust valve guide has a circlip fitted to stop it moving. Mine has no circlip and certainly had not moved and I was not going to start heating heads in oven, and hitting exhaust guides with big hammers. The countersinking took minutes and the bike was up and running again the same day. The forum brought forward many owners who had had exhaust valves seize up on these AMC heavyweight singles.

A bike pulling a chair works very hard and at slower speeds than if solo. Overheating is much more liable to occur and was something the old chariot riders / sidecarists / combination drivers were well aware of in olden times.

Due to clean modern fuels we no longer de-coke our bikes engines every 5000 to 6000 miles or so, so valve stems do not get cleaned any more. I've not had this head off for over seven years! Perhaps we need to do a de-coke every few years now?

Random AJSs on Right Now......

No3 bad luck was finding out that I had left the cylinder head steady loose, the one that clamps the head to the top tube. Sod it! Once home I had to remove the seat and tank again to get to it and tighten the nut and bolt!

Sadly the slim figure and brown hair all vanished many years ago. oh, the pleasures of real biking…

They don't make sidecars like that anymore 1958 AJS Model 18


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