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Dibnah Offline OP
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Greetings all,

My mongrel 650 has an annoying engine oil leak between barrel fins, I'm fairly certain it's from one of the through tapped holes for one of the outer head bolts. Doesn't leak all the time, stops completely on a long run and in the hottest (UK) weather. Reading through various threads (thanks) I can see that there may be an issue with a washer, might also need an "O" ring.

As a workaround, what is the downside of plugging the through tapped hole with silicone sealant? It appears to be accessible between the fins. At the moment, I'd rather be riding than spannering.

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Hi Dibnah,

When there is an oil seep somewhere in the top end, because that region is so hot and the oil so thin, that the oil can spread all over the place on its downward journey, encouraged by the airflow while travelling.
This makes it very difficult to be sure where the oil is actually escaping.

For the oil to be coming from BETWEEN barrel fins there would have to be a flaw or crack in the cylinder casting.
Also (unless there is a lot of oil in the combustion chamber leaking via the head gasket to the outer bolt hole, which I think is very unlikely) it is only the inner 4 bolts that can receive oil via a failed rockerbox gasket on an 8-stud head.

Your reference to some sort of washer problem I suspect is that you’ve read threads about the later (from some time in the early 70’s) head bolt system. This doesn’t apply to your 8-bolt head.

The places for you to look for the source of the oil leak are the rockerbox gasket, the upper pushrod tube seals, and the rocker oil feed manifold.

I suspect oil is escaping from 1 or more of those places, then flowing and being blown to confusing places. Is the leak on the right?

One way of identifying the source may be to thoroughly degrease with petrol, dry, then dust with talc. Go for a ride, stopping frequently to observe evidence of weepage.

Good luck!

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Dibnah Offline OP
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Thanks Koan58, the leak is on the left side (clutch side). Rocker box joints look OK, and if the oil can't be leaking via the through tapped holes for the outer head bolts then - logically - I'll need to look closely at the upper seals for the PRTs, probably the front as the rear looks dry. The top of the front PRT is caked in road debris but it does look damp.

The bike is running well, no sign of oil burning in the combustion chambers.

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Dibnah Offline OP
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I can update this thread.

The upper seal for the front PRT was seeping oil; I've sealed it using the time-honoured "iron-on seamstress tape" plus sealant. However, the oil leakage was still appearing between the barrel fins. A good light and clean eye glasses meant that I could see the engine oil oozing out through the bottom of the tapped hole for the cylinder head bolt. More oil flowed when the engine was idling than when the engine was revving.

Silicone didn't stop the oozing, but some epoxy resin has, although this obviously hasn't fixed the core issue, which I assume could include a cracked barrel. Bike still runs well, no sign of exhaust smoke although I really need someone following to check properly.

Road testing has been delayed by the need to re-spoke the rear wheel; does it ever end?

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Someone may well have run a tap down the bolt hole to "clean it up" and broken through, so the barrel is not faulty per se. You may be able to stop oil getting into the bolt hole to stop the problem happening, but I'd be happy enough to use a high temperature epoxy. If what you have works, I'd just ride off into the sunset, no big deal!

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Dibnah Offline OP
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Originally Posted by TinkererToo
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If what you have works, I'd just ride off into the sunset, no big deal!

Yes I will, thanks, although the unknown core issue will nag me, at least until the next problem. The fact that the seepage stops when the engine is heat soaked on a long run suggests that something is closing due to heat. Did I have the only Triumph twin that leaked less oil after a long run?

As a benchmark to remind me at a later date: currently, the only oil drips are from the crankcase breather.

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I can well imagine a scenario where the oil getting down the stud in the first place (head expansion?) could cut off the flow to the bottom of the stud hole. I know what you mean by the nagging, but count yourself lucky, or, of course, skilful, for no other oil leaks!

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Mick, how do you imagine the oil gets into an outer stud hole in the first place?

If it were an inner stud hole, it would be easier to understand.

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I agree, the inner ones are what I was thinking of, leaking out sort of sideways. Depending on the overbore, the barrel gets mighty thin next to all those bolts, not sure what the symptom would be if it broke through!

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We haven’t been told the oversize. I’ve had several 8-stud barrels at +60 with no issues. It would be helpful to know the oversize.
The casting is quite generous in the upper region to allow for the bolts (to the outside of the casting). It only becomes more slender below the bolt holes.

Dibnah, you say that you could actually see the oil seeping out between barrel fins, next to one of the left side outer bolt holes.
Can you be more precise, between which fins, and front or rear bolt?
Has this leak always existed with these barrels, or has it shown up after a considerable time of use?

Without there being a massive amount of oil in the combustion chamber, able to leak across a leaky head gasket to the outer bolt hole, I simply cannot imagine how the oil can get there. The engine would not run anything like well in this circumstance, and the smokescreen would be obvious.

So if you’ve eliminated all of the usual sources (rocker boxes and pushrod tubes) then you’re not left with many options.
I would suspect a flaw or crack in the barrel casting, especially as you have observed the oil’s egress from between barrel fins.

Barrel castings can have flaws (like any other casting they can have porosity, holes or worse, which would only be detected by X-ray). My 8-stud barrel was rebored to+40 in 1998. The honed surface appeared perfect.
When I had cause to examine it again 10 or so years later (so just a few thou wear) several cavities had been exposed in the bore surface.

So what I’m saying is that hopefully your problem is caused by such a flaw that has been exposed by bore wear, allowing a path for crankcase oil to reach the outside.
I’ve not known a Triumph barrel to leak to the outside, but?

The worst case scenario would be that it is a crack forming, probably just below or at the lower threads of the head bolt end. I hope not.

Not wishing to be a harbinger, but if the usual suspects have been eliminated, must consider the unpleasant suspects. Cheers Dave

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Hi, I’ve observed on several bikes that leak at head gasket. The combustion gasses seep by head gasket. Condense into oil, or what looks like oil.

To be clear this is not a blown gasket. Just tiny seep. It may never grown to blown gasket.

Putting straight edge on heads they can have small dips here. & there as well as cylinder top. I’m not talking about bent head. Less than .001”.

You can flatten head & cly top on surface plate using 120 emery paper.

I flatten until the straight edge rocks on a .0005” feeler blade.

Also head gasket must have a little wiggle room around head bolts so it can expand as it flattens during torquing.

Also the head gasket compresses. Very little. If you reuse head gasket it may not flatten all the way in its new position.

Personally if your epoxy is holding, I’d leave it.
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Hi; this thread is interesting for me due to I still think that I had some issue with the pre unit head and may be did not blown the copper rings gaskets.
-In which way did you used the epoxy? Is not clearly.
-About the 0.0005 gauge! where did you find one? Mine has 0.005 or is a typo?

Thanks

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Dibnah Offline OP
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I think it's demonic possession.

Thanks for the detailed replies.

I've no idea re: piston size, but if there is a developing crack in the barrel then I might find out sooner rather than later. I attempted to take a photo of the oozing oil, but my feeble camera skills were as feeble as usual, the camera focusing on the fins rather than the oozing oil.

Any thoughts on the significance of the oozing at idle rpm but no oozing at higher rpm?

It's now obvious that the oozing has probably been occuring for a few years, the symptoms being no obvious leakage observed when riding, but park the bike on the side-stand and there is a drip-drip of oil from the barrel onto the primary chain casing.

Can oil seep between the liner and the barrel? i.e. if the liner isn't cracked then it's not so potentially catastrophic.

Edit: "> Can you be more precise, between which fins, and front or rear bolt? <"

Primary chain side, outer front bolt hole

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There’s no liner (or shouldn’t be) in a pre-unit 650, I must admit that I’m puzzled!

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Between which fins?

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Hi Reverb, Not a typo. The .0005 feeler blades are sold by various industrial supply houses & ebay. Not sold in auto parts stores. The one I have is Starrett brand, I got on eBay when I got my Starrett straight edge. But was from industrial supply tool seller that sells to machine shops etc.

I got .0005" & a .001" 12" long. Both are sold in rolls as well.... They are very flimsy & fragile. Especially the .0005". In USA many feeler gauge sets have a .0015" included in the set. It's rather fragile as well. Often this thin one gets ruined & discarded, or breaks off.
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It was pointed out to me some years ago, that standard* domestic aluminium cooking foil here in the UK is 0.0005" thick. I got out my micrometer and checked it out and you know what, it was correct!

So yes, a ready supply of more half-thou material in a kitchen near you!

Terry.

*Note! There is also a heavier (thicker) grade foil used in commercial catering, often called Oven Foil.

[Linked Image from i.ibb.co]


Bike History: Jawa 50 1956, Bridgestone 50/90 Sport 1967, Triumph T120 Bonneville 1970, Yamaha 125 DT125 Scrambler 1974, Kawasaki 125 KE125 Scrambler 1978, AJS model 18 500 Single 1964. Current bike Triumph T100R Daytona 1972.
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Dibnah Offline OP
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@ Koan58, between 3 and 4 where 1 is the top fin.

Screwdriver inserted into the identical hole on the other side of the barrel (gearbox side)

[Linked Image from i.postimg.cc]

@ reverb, I applied a "plug" of epoxy resin to the hole using small screwdrivers and some dowelling (wooden rod). This is not related to the head gasket.

@ TinkererToo, my mongrel bike probably spans three decades. Physically, the engine is unit construction, although the engine number starts T110 and includes 6T. As Ginge previously pointed out, the 6T was sold with unit engines in the 1960s. I speculate that perhaps the crankcases were a replacement set, with the engine number perhaps being stamped after the cases left the factory

[Linked Image from i.postimg.cc]

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Hi Dibnah,
Thanks for the extra info.

Between the third and fourth fins must be close to where the bolt holes end.

As best I can see from your pic, the head bolts and rocker box bolts are a mixture of types, ie at least some of them probably aren’t original to an 8-stud 650.

If a longer head bolt were used, it is possible that it could go too far into the barrel hole, it could bottom out or reach to where the threads taper out, or bottom against a plug of muck at the bottom of the hole.

Any of these scenarios would exert a damaging force on the barrel casting, possibly leading to a crack between the inner cylinder and the outside, where you see the oil emerging.

I can’t see anywhere else the oil can be coming from, other than from inside the cylinder itself.

As for why it’s worse at tickover, I can only try guesswork.
Perhaps the timed engine breather works to greater effect at moderate running rpm, creating more crankcase vacuum.
Or maybe the slower movement of the rings at low rpm simply allows more time for oil to enter the crack.

Just ideas mate.

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Man alive, you really do have a mongrel, but if it works, who cares? Not a great picture, but looks to be oil in frame, so '71 on. The crankcases have the TDC hole so '65 on, a pic of the front right of the cases would show if the tappet oil feed was there, which would be '66 on. Chaincase is pre '68. As it's an 8 stud top end, it has to be pre '63. By the way, I can't see any left hand rear engine plates?
None of this explains the oil leak, koan58 is as likely to be correct as any!

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Dibnah Offline OP
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@ TinkererToo, a photo from the gearbox side. The engine number is mid-1950s, but again I speculate that these were probably replacement crankcases.

[Linked Image from i.postimg.cc]

@ Koan58, thanks for the suggestions. The PC side outer front and outer rear head bolts do look different on the photo, but my 5/16" Whitworth socket is a snug fit on both. I can't get a depth gauge under the tank to check the distance from the top of each bolt head to the washer, but each bolt head sits at approximately the same depth within the ring of a 5/16" Whitworth ring spanner when measured with my eyeball.

Edit to add: this useful thread refers to differing headbolt lengths https://www.britbike.com/forums/ubbthreads.php?ubb=showflat&Number=683539

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Hi Dibnah,

Thanks for the extra photo. Though the resolution makes it difficult to be certain, both of the timing side outer bolts appear to have domed tops, as does the driveside rear bolt.
However the driveside front outer bolt appears to have a flat top.
This is what I was meaning when I suggested that the 4 outer bolts were not a matched set for an 8-stud 650, ie the front driveside outer bolt is a stranger.
It may just be coincidence that this appears to be where the problem is, but it may mean have relevance.

AFAIK all B-range headbolts had the same size head till about 1970, so that doesn’t inform about the length.
However, I’m doubtful that any later B-range (including 750) used longer outer headbolts than in the pre-unit 650.

So I retract my thoughts about it being too long a bolt causing your issue.
But there must be a reason for that odd bolt. It could be that the PO simply lost one.
Or that a problem was encountered with the original bolt in that hole (maybe the bottom breaking off and being left in there?).
Pure guesswork until such time as you remove it and examine the hole.

I’m not suggesting that you take any invasive action at this point, as its running well and you’ve stemmed the leak anyway. Wait until something more obvious crops up.

My curiosity would tempt me to pull that bolt and probe the hole, but letting sleeping dogs lie is probably the better approach!

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I'm pretty sure that this is an oil in frame setup, so 50's crankcase and gearbox would not fit at all, so not a replacementas such, just a bit of creative stamping. However, any of the unit engines will fit the frame. Again, you have cropped your picture so that the rear outer engine plates are not visible!

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Dibnah Offline OP
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Thanks again for the replies.

@ TinkererToo, the frame is OIF, registered 1972. No cropping of earlier photos by me, this one shows the gearbox side rear engine mounting plate

[Linked Image from i.postimg.cc]

The engine number is a mystery, although later 6Ts (up to 1966) were apparantly sold with unit engines. Could an earlier 1950s 6T frame have been fitted with a unit engine to replace the pre-unit engine?

@ Koan58. The earlier photo is deceiving, the outer front PC side head bolt is domed

[Linked Image from i.postimg.cc]

I would be loathe to remove one head bolt without loosening the others, which could create new problems. The quantity of oil that was being leaked was negligible but annoying.

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The unit engine is much shorter than the pre-unit, so really not interchangeable. However, any of the B range unit engines will fit the oil in frame, and I can now see that the engine is fitted as standard. Why someone got "creative" with stamping an engine number, who knows. I recently bought a '63 6T Thunderbird (always wanted one of the bikini 6T's) that has what I think is a 68/9 engine fitted, but partly re-stamped with the frame number. I have a set of the correct cases, and will build the bike with them, so I'll be selling the later case. I will be honest about the number when selling, but down the line there will be yet another "mystery" engine number, I've seen so many!


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