OIL FILTER TUNNEL LEAKS...Some thoughts.

[Les Howard]

This "Notorious" potential oil leaking area has caused so much grief to owners over the years. I just wanted to add some more thoughts and an idea to the numerous writings on this.1) The crankcases at the front of the engine and just below the oil filter tunnel are clamped together with a 1/4" BSFCY stud. This stud also acts as a fixing of the engine to the front engine plate. The engine plate however is in effect a thick "U" shaped arrangement that has to be initially loose enough for the crankcase to slide in between. It is in my opinion quite easy to tighten the stud thinking that full pressure is being applied to the crankcase but in effect you are tightening against the engine U plate only but leaving very little pressure applied to the very important crankcase area below the filter tunnel. After a while the immense oil pressure can leak out since the case halves are not really being clamped that hard together even though you think the stud is tight. I suggest this stud is in effect torqued up to its very maximum thus causing the front U shape engine bracket to flex inwards and transmit the "squashing" pressure to the crankcases instead of just feeling tight against the bracket but not really exerting any sealing pressure on the engine case. 2) One idea that struck me is the front oil seal cavity could be used simply as a container of a redesigned oil filter, but it would not see any direct oil pressure. If a new oil filter was constructed to seal at the rear (RHS case) where oil comes into the chamber, but the filter was in effect a sealed tube with the filter inside. The outer end also being designed to seal against the large end cap. I was wondering if this would be possible to achieve? The cross-case sealing would not then be required to withstand any pressure as it had now become just a sealed filter "housing". So, if anyone had a leaky oil filter tunnel, they would just use this new filter and the leak would stop...Thís would be a dream come true for quite some owners I reckon...Thoughts on the two things I mention?...All the best...PS: I originally placed this post on the wrong category hence I re-submitted it. Sorry.

[ChrisTheChippy]

Mine leaks from there. I just live with it but it bugs the hell out of me! If a definitive answer could be found I'm up for it.

[Les Howard]

Mine leaks from there. I just live with it but it bugs the hell out of me! If a definitive answer could be found I'm up for it.

Hi Chris...You're like me then....just one drop of oil oozing out from anywhere on an engine where it should be dry is mental torture, like I have failed badly. I'll take this opportunity to add that the thickness of the Factory recommended circular gasket around the tunnel joint is just 2 thou.....002". Not 3 thou and not 30 thou like some gaskets supplied by some spares shops. I have 3 historic write ups that say it is 2 thou. This makes sense. When crushed down it probably ends up as around 1 thou but is enough to seal...as long as the cases are not warped at all as when first manufactured. Historic road tests say the engine is utterly oil-leak proof no matter how hard it is thrashed. When my engine was first dismantled, the paper gasket was almost invisible so really didn't hold the cases apart by it wedging open as it will if thicker gaskets are used. I still think that a check should be made with the tightening torque on the stud I describe above...but once leaking it is not likely to completely stop if it is re-torqued but worth a try. By the way I found that modern kitchen grease proof paper is .002" thick and super tough too. You cannot get modern paper gaskets less than 6 thou (0.15mm) ....far, far too thick!

[Groily]

The late Alan Jacobs ("Biscuit") had a fair bit to say on sealing the cases a few years back on here.
I make a thin gasket to go all round between the cases with just a hint of Wellseal, others use a bit o' thread etc - and ditch the little gasket round the tunnel. I can't ever pretend not to have a bit of oil coming out where it shouldn't, but the crankcases don't leak (or not there, at least) even if the dynamo joints do with irritating frequency if engines are thrashed. That joint comes under my definition of 'notorious'!

Interesting thought though about a sealed cartridge of some sort - but would it need some sort of bypass in case of blockage or collapse maybe?

Ref the front engine plate preventing adequate tightening, there's maybe virtue in the separate unequal plates used on the earlier bikes - never thought about it until now. But I wouldn't want to wind the nuts up too tight. Think I'd consider a small inboard washer/spacer if I could wiggle one into a 'loose' fitting, rather than go beyond what feels 'normal' torque for the size.

[Les Howard]

Thanks Groily for sharing your info here. From your many contributions I have read on here, I have always considered you to do things the best way and this leaking tunnel problem is something you have posted on several times....The terrible problem is that that whatever way one chooses to attempt the fix, there is the immense amount of work involved in implementing it. In fact, to remove and rebuild the engine is more work than the factory needed to do to build the entire bike in the first place! After a total, very expensive and time-consuming restoration some years back I was literally in tears to find oil oozing out of the upper area of the filter tunnel joint...The reason was that I had used, unwittingly, the thick incorrectly supplied tunnel gasket leaving a "wedge" gap in the case joint. I left the bike for about a year not being able to face a complete pull down and rebuild. Eventually I decided to do the job but then spent weeks trying to decide what method was best. This is what I chose to do. Leave out the gasket and use Loctite 518. It is supposed to be designed for this sort of job with a sealing capacity of something like 1000 PSI...Cutting to the chase. Yippee it worked BUT after a few hundred miles I noticed a small leak from the bottom of the filter chamber that runs down the front and then underneath. Feeling absolutely gutted again I found that the fixing stud as discussed above was indeed too loose. Unfortunately tightening it has only slowed the leakage and a dribble still occurs. That is NOT good enough for me, so still gutted… Sometime, if I live long enough (I have other resto jobs on the go currently) I will try again. The brick wall on starting is one could spend a week of hard work redoing the engine and refitting and STILL have it leak! I toy with what way I will do it…maybe make a complete engine case gasket…maybe make sure and use a surface plate to ensure flatness…. Maybe use tried and tested WellseaL this time instead. Make a special tool to cut an O Ring groove …I just cannot make my mind up. The fear and dread of it leaking for the 3rd time makes me dizzy just thinking about it. If one could do the engine and be able to test it without fitting it into the frame would help greatly but that’s not going to happen. If only that first time rebuild had been done with the correct thin round tunnel gasket, I reckon all would have been OK.so very gutted!…Anyway I’ll let you know if or when I make the third attempt. All the best Groily

[Groily]

I agree Les - it's such a lot of work and no guarantee sometimes that the effort will be rewarded!
To be honest, I just do what seems sensible (to me anyway!) at the time, having read marked and inwardly digested what Biscuit said on many subjects, and ITMA Ken too when he was posting. It helps to have run these beasts for nigh-on 50 years continuously of course: no parts are strangers and many are all too familiar. On the oil filter chamber thing, I'd guess the O ring option + annular groove would be best, but haven't ever gone there personally.
None of my engines come even close to passing oil tight or shininess tests but I would claim that a) they run well and b) for a long time between serious rebuilds. They have some annoying foibles and glitches like the one under discussion, as we both know, but they are extremely reliable as long as one is prepared to accept that perfection can be the enemy of the good if using them as daily riders.

Last week I was in Spain (Los Picos and Santander rallies) with my M31 along with hundreds of other people. There were 4 Twins in evidence (mine from France and three members from Hampshire) and they all went very nicely for close to 1000 miles (in the case of a very fine CSR rather more) in arduous conditions and some pretty foul weather as well. Three on dynamos, the CSR an alternator model.
Given that the commonest machine present was the Norton Commando; that the majority of machines were half a generation newer (from the '70s) than AMC twins; and that these events tend to run at pace where conditions allow and under heavy load ALL the time in the mountains, I reckon the twingles did darn well. Mine ran with four old friends on a Trident, Commando, R90 Beemer and XS650 Yamaha and stayed with them OK despite the horrendous braking deficit plus lack of a 5th gear or the sheer torque of the 4-speed Norton. It was ridden pretty hard.
I used a quart of oil and broke a clutch cable. That was it. It never faltered and always went 1st prod. The oil use was 50% due to the bung on the end of the oil filter chamber backing off a flat or two and needing a tweak about last Tuesday. A Good Result, I thought anyway.

I'm not quite sure how to keep machines spotless and beautiful though. I raise my hat to you and all those who do - but I'd be bothered about riding them on grotty days. I'm very bad, terrible in fact, at washing and polishing, but do try to keep further cosmetic deterioration at bay. The objective being that when I fall off my perch, the beasts will be good restoration projects with (largely) correct bits for those with the spotless gene, or can continue as well-proven daily riders for those who like 'em that way..

[Les Howard]

A fabulous reply Groily...enjoyed reading it so much. You should write a book on all your motorcycling life.. I am plagued, yes plagued by being a born worrier that sometimes makes me think I am a perfectionist. The two are hard to separate in my opinion, seeing it from the inside so to speak. I've always been like that even as a small child. You see old vintage machines smothered in oil and I think they look fine, well maybe my inner self doesn't, so I think truthfully, if I had one, I would immediately try to stop every leak it had whatever it took. Back to the twins, I reckon I could stop it next time around. Surely the O ring method is THE only way? But then again, maybe a full gasket is better. I say this as the AMC gearbox kickstart has an O ring and it leaks like a dripping tap!. maybe I should just TIG weld the front of the engine together...It will last me out before it needs any rebuilding. Thanks again Groily....keep on posting and writing

[ajscomboman]

FYI too all. The original paper gasket was 2 thou, this material thickness is no longer available and the replacement in the gasket sets is too thick. The suppliers have been made aware of this issue and can only offer the nearest metric thickness available. They did come up with a polymer based material but couldn't give any indication as to how it would work on hot sweaty twin engines. I have used full gaskets cut by hand and sealed with Threebond 1184. I now use the same compound and the thinnest paper gasket material available and it's been just as good as the full gasket. All engines have remained bone dry on the case joint. I tried wellseal once and found it to be hopeless on the twin motor cases but fine on singles.
The ultimate has to be the o-ring route but I haven't got the skill or machinery to carry it out.

As for O-rings on the gearbox, bin them and use X rings they seal so much better.

[shaunstaples]

One solution I was thinking about is reaming both halves out and making a hollow dowel that's a tight(ish) fit as long as the filter goes through it.

If there was sufficient room between the filter and housing, then the dowel could be thick enough to get grooves in and an o ring for each crankcase?
Don't know the diameter of filter and chamber to work with for it to be possible. Should be nothing holding crankcase faces apart

Filter chamber sealing.JPG

[Groily]

[quote=ajscomboman post_id=252696 time=1695754783 user_id=95324
As for O-rings on the gearbox, bin them and use X rings they seal so much better.
[/quote]

Big YES to that. Transformative with worn shafts (which are a quite expensive 'replace').

Les, your bike is a joy to behold. (At least with the orternator you have a chance of staying dry on that side of the crankcase . . .!)
Here's my M31 that's just had an oily rag wiped over it plus a few basic checks, on the ramp today after a tough week or two, and is all set for another good thrashing this weekend. Unless I take my Notrun instead . . choices choices . . .
Can't help but love 'em all.

Oily rag.jpg