Spark plug re-thread and other issues

[Ozmadman]

Since I have had my bike I have been a bit dubious about the state of the spark plug thread in the head because the plug is loose /wobbly most of the way in and only "tightens" up just before seats on the head and also it doesn't really tighten properly so I have just been nipping it up. Well today whilst removing the tank and lowering the carb needle to weaken the mixture (black exhaust and black plug) I noticed some black gunk around the spark plug and presumed it was coming from a slight rocker box cover leak. Put it all back together and started it up (started 2nd kick as normal) and then I saw(when I revved the engine) a tiny leak from under the plug. I took the plug out once cold and it was black and slightly damp I then shined a light into the plug hole and with the engine at TDC I saw there was an oily deposit on the piston crown??? I put the plug back and this time it didn't tighten at all, the thread has finally gasped it's last!!! So why the oil? I took off the head and indeed there was some oil on the piston crown and both valve heads were black and slightly oily. I removed the head gasket and there was a large amount of oil under the gasket but not on top and there was carbon all round the ridge that sits into the head recess, the worst was just adjacent to the oil feed hole?.

The previous owner had done a top end overhaul just before I got it(rebore and new valves and guides) and the still almost shiny +40 thou piston seems this bear this out and no ridge at all at the top of the bore. But I am confused and maybe someone could throw some light on this. The bike starts virtually first/second time and runs great, ticks over evenly pulls fine and seems to have lots of compression on the kick start, it doesn't burn any oil apart from some smoke on start up if left standing for a couple of weeks( my son has followed me on a number of occasions and said there is no smoke , the exhaust is black but sooty and not oily (hence me trying to weaken the mixture ) the front pipe is dry also BUT I do loose some oil (more than from a few small drips here and there) the level in the tank drops about an inch after a run and then stays there.

So, is it possible that the oil is seeping out under the head gasket and running into the bore and if it is then maybe this could account for the black plug and valves and oil loss but surely it would smoke like hell?? There is no seepage of oil to the exterior at the head join as all the fins are dry. I can move the piston very slightly side to side in the bore, is this normal?( haven't removed the barrel as yet as it is outside (covered) and offers protection from the elements but will need to to replace the base gasket) And there are a few vertical score marks on the bore, very fine and barely detectable with a fingernail. Finally, back to the reason I took the head off in the first place..what is the best method of repairing the damaged spark plug thread? and can I get the valve springs off with out a special tool (might as well re-lap them in whilst it is apart)

Sorry for the essay, it just escalated like the issues with the bike

Many thanks

Paul

[robcurrie]

Paul, I can't help you with the plug thread, but I am sure other members will. Regarding the oil in the combustion chamber, the motor wet sumps from standing for a long while. This accounts for the level drop you see. If you start the engine without draining the sump (and refilling the tank), some of this excess oil might find it's way past the piston rings and cause a bit of smoke on starting and account for the oily black carbon on piston and valves. If the rings aren't sealing perfectly, you might have some oil consumption when the throttle is closed, but this might not produce much smoke. You might find the sparkplug oiling when the engine idles and idling might be inconsistent.

You will need to get the plug thread repaired before you try to adjust carb settings.

If you have very strong hands you might get the vave springs out, but you will battle to get them back, because the collets are very stubborn things and you will need three hands that fit in the space of three fingers!

Rob

[Rob Harknett]

Could be broken ring there. You can pull the springs off by hand but you won't get them back on without a tool as they need to be compressed. I prefer the lever and bolt type tool to the Half moon type for ease of use. you fit one spring almost in and lever the other with the tool, the springs compressing by leverage action. The spring first loosly fitted then only needs a thump. ( This type could be made with a bit of dexion and a bolt. ) I think I might consider getting other head. A repair would need the head off to stop filings getting in.

[Ozmadman]

Could be broken ring there.

Will check that out when I remove the barrel

You can pull the springs off by hand but you won't get them back on without a tool as they need to be compressed. I prefer the lever and bolt type tool to the Half moon type for ease of use. ( This type could be made with a bit of dexion and a bolt. )

Do you have a picture of such a device?

I think I might consider getting other head. A repair would need the head off to stop filings getting in.

The head is already off, there are lots of thread repair kits on ebay (some cheap) will they do the trick do you know

[SPRIDDLER]

The only mechanical failure I had in 4 years and 25 thou miles was when the plug shot out whilst waiting at traffic lights. I had the plug'ole Helicoiled and it's been fine.
I may have a pic of the spring fitting device. If not I'll photo mine and post it on here later. I think there's a pic in the original AMC Parts List under Special Tools'.

[Ozmadman]

The only mechanical failure I had in 4 years and 25 thou miles was when the plug shot out whilst waiting at traffic lights. I had the plug'ole Helicoiled and it's been fine.
I may have a pic of the spring fitting device. If not I'll photo mine and post it on here later. I think there's a pic in the original AMC Parts List under Special Tools'.

Thanks Neville.. Got the springs off (just a few yanks, not US yanks but good ol' tugs!!) and took out the valves. Well the inlet valve certainly wasn't replaced when he did the top end as it has definitely been round the bloc a few times, scored seat area and very thin on the edge and pitted face. The exhaust valve looks pretty new just choked up and it looks a though it has had a new seat insert in the head. But both guides look new and the valves are a perfect fit in them so no worries there. did you get your head done commercially or did you do it yourself?

Here is a picture of the head, looks like someone has used Wellseal or something on the gasket

Paul

[bob]

Hi paul
Recovering the plug thread with a helicoil is relatively easy , but go for a good quality insert , "Helicoil" or "recoil" brands use stainless steel threads ,
Don't forget you also need the correct size drill for the oversize and the correct taps for the insert , it will probably work out cheaper to take the head to a local engineering firm who already has the drill bit , taps and coils .

[56G80S]

There shouldn't be any sealikng compound on the copper head gasket.

I've had the oiling problem before (and today), For me I get some blowing past the front pipe / head join.

Doesn't happen when the Matchless is in regular use, I put it down to the pressures associated with wet sumping. IKBA

Johnny B

[SPRIDDLER]

....it will probably work out cheaper to take the head to a local engineering firm who already has the drill bit , taps and coils .

Exactly. I took mine to a local chap who is well known for old bike engineering who did it in 20 minutes over a brew. No hassle, professional job; just 25 quid (or approx one pound per thousand miles ridden since and still going strong).
I could have saved p'raps a tenner by hunting around obtaining the bits and spent a morning doing it in the shed if I could have been bothered. However I've always avoided the adventure of breaking down, regardless of how cheap it may be.

It doesn't matter if the inlet valve seat area is 'thin' or 'narrow' as long as it seals. In simple terms when it's open it's cooled by incoming fuel/air mix and when it's shut the combustion 'flame' doesn't touch the seat. When the valves have been refitted pour petrol into the zorst and inlet manifolds and look for fuel creeping into the head past the valve seats. There should not be any sign of leakage after, say, 2 or 3 mins.

[SPRIDDLER]

I may have a pic of the spring fitting device. If not I'll photo mine and post it on here later. I think there's a pic in the original AMC Parts List under Special Tools'.

Pics on page 26 and 27 of the half moon and lever type spring fitting tools here:

http://archives.jampot.dk/Book/Workshop ... dition.pdf