[Thomas]

When you dismantle the head be careful with the washers on the cylinder head bolts into which the rocker arm housing bolts are screwed. Later they need to be replaced onto the same bolt, respectively, because they level out the possible different bolt lengths and avoid breaking the rockerbox. Note: A chair on four legs can wobble, one on three cannot. 

[joebikerbloke@gmail.com]

Hi, just read your update, and yes that does appear to be the issue here... every day is a school day.

So I have attached some pictures which I think proves what you say, there is also a glimpse of a valve too. I tested this by adding a 1mm washer to the spark plug and hey presto... no more marks or bend ground electrodes. I've also realised why the L10 spark plug was more or less ok... the L86c electrode is far more bulky that the on the L10.

Whether the adding of a 1mm washer is a valid temporary fix, I'm not sure. I guess there is only one way to find out. If this is a bad idea, please scream at me accordingly.

So it appears, if this modification was made back whenever, it wasn't an issue due to the design of the spark plugs at the time. Roll forward to today, and the 'bigger electrode' spark plug is a problem. So when I last used the bike I guess I must have been lucky with the spark plug I used.

I'll have a proper read of the information supplied in that link and together with your comments, seems I'd better get some the spanners out. Before I do that, it sounds like some engineering will be required, so I will need someone to do the tapping for me. Last thing I want to do is break something.

If you know anyone local to Sheffield, please let me know.

Thanks again.

[wessex_man]

A thought I think your L86C plug is a 14mm, (just looked it up).Your 95L head like all Sunbeams used 18mm. I suspect an 18mm to 14mm adapter has been fitted and think this could be be the cause of your plug damage ie the 14mm hole is not tapped all the way through and the plug is longer than the adapter can take? So yes a head take off is required to investigate. To be safe remove the seat, petrol tank so you can gain easy access to the business end. Slacken off the tappets so they are loose. You don't wan't to crack the rocker box close to the fixings. Then remove the rocker box. Rocker box stud/ head bolts can then be undone so the head can be inspected and rectified. Once done reassemble in reverse. Take pics on the way down if you need to.

[Thomas]

Hi Joe, starting with page 9 this might help for removing the head: https://marston-sunbeam.org/1937-sunbeam-model-9-engine-overhaul/
Cheers, Thomas

[joebikerbloke@gmail.com]

Hi John, thanks for the reply.

I think you have hit the nail on the head... that is one thing I didn't think of. Its the action of screwing in the spark plug that is causing the issue. On closer inspection there is a restriction after the end of the thread where the spark plug screws into. Its this smaller diameter hole, after the thread, that is basically gauging the marks in the ground electrode.

On the basis that the spark plugs themselves are not at fault, I'm assuming that carbon build up over time has caused this hole to close up and hence is now fouling the ground electrode. The question is now how to fix this without removing the head. For obvious reasons if I use a file or similar the debris will of course fall on top of the piston... not good. I could try and use a hoover to suck the debris out as a gently file, but I'll have a think.

I will add a picture to better show the restriction / smaller hole but as its a bit late, so I'll do that tomorrow.

I could also add a washer to the spark plug so it clears the obstruction. Although this is just putting off the inevitable, i.e. removing the head.

Next question, are the any guides on how to do remove the head noting I have never done this before ?

Again, any advice gratefully received.

[singleminded]

Have you tried fitting and removing the plug without turning the engine over.
I know it seems like the piston is hitting the spark plug but If the head has not been off since it was last running I can't fathom how the piston would suddenly clout the plug.
John

[joebikerbloke@gmail.com]

So... My 95L has been resting for quite a while due to a number of reasons which I won't bore you with (one of them being the gearbox), but I decided this year I would get the bike going and attend the Annual Rally noting I hadn't attended for a few years and also see if the previous gearbox issue had miraculously fixed itself, and if not I could at least get it running and if the issue did persist, pick a few brains at the rally.

Roll forward a few weeks and having put some fresh oil in it, greased everything that needed greasing and generally giving everything a once over. Compression appearing to be as good as I remember too (the engine turns over without issue), having bought some new spark plugs (Champion L86C), I took the old one out and noticed marks across the side / ground electrode which I thought was a bit weird and the gap being quite small. Anyway I put the new one in after checking the gap, turned the engine over, took the spark plug out and found the marks appeared again and the gap was smaller again... Oh dear I thought, something appears to be hitting the spark plug.

Thing is, when I put the bike away after I last ran it, it was running fine, gearbox issue aside. I tried another spark plug and got the same result.

Assuming the L86C is the right spark plug, and the side / ground electrode is always in the same position when screwed into the head, otherwise surely the marks would not be the same, I'm not sure if its the piston or valves hitting the spark plug, and of course why ?

Unless there is that much build up of carbon on the piston / valves that is making them hit the spark plug, or the spark plug is sitting lower in the head, I've no idea what is happening or causing this to happen.

Any advice gratefully received.

Please see the attached pictures of 4, yes 4, different spark plugs I've tried, just in case (all are L86C).

Thanks.