B25SS Burning Oil

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B25SS Burning Oil

Post by Cocker Cox » Sat Aug 15, 2020 1:45 pm

Hi, All,

When I first got my SS in May, a puff of black smoke on start-up alerted me to a potential problem. A subsequent top-end strip down revealed a loose exhaust valve guide. End of problem, I thought. But no! I’ve now done over 300 trouble-free miles since sorting the problem (with a ‘new’ head); the bike runs like a bird, ticks over like a clock, is returning 70 mpg, and starts hot or cold with a couple of kicks. But the plug is oily, as is the tail-pipe, and a pal following behind me confirms that oil is being burnt; not a lot, but enough to be visible and odorous. I noted during the strip-down that the (standard) piston was a Starfire item not an SS Gold Star, but the cylinder bore seemed to be in excellent condition. So...

1. Would the consensus be to leave it as-is or investigate?
2. Might the problem be simply that the PO who changed the piston also used old rings not bedded-in to the cylinder?
3. What should I do to investigate and rectify the problem, apart from a re-bore and an over-sized piston?

Many thanks for any replies....

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Re: B25SS Burning Oil

Post by kommando » Sat Aug 15, 2020 2:52 pm

the (standard) piston was a Starfire item not an SS Gold Star
One and the same anyway so leave as is.

When is the bike smoking, on acceleration means its likely to be rings so hone and fit new rings, deceleration is guides and likely to be worn inlet guide.

Do not jump to a rebore unless the bore is damaged or oversized, the bore size in the manual is the freshly bored size, you have a further wear limit of 5 to 7 thou above the freshly bored top limit.

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Re: B25SS Burning Oil

Post by eebtr7 » Sat Aug 15, 2020 3:10 pm

Once apon a time, in a land far away and over the hill, this innocent young soul went out with cash in hand and brought home a brand new, bright & shiny, 1971 BSA 25SS from the local dealer. Being it was a four stroke engine, it was expected the spark plug would burn clean. No, instead it burned oily and carboned up like a two stroke.

Being the innocent he was, he went out to purchase a bunch of spark plugs to replace on a regular basis. Some months later after trading in the B25 for a brand new, right & shiny, 1971 B50SS, he learned the head and jug had been pulled where it was discovered the gaps on all the rings were lined up straight in a row. Thus allowing oil to gather inside the combustion chamber to pollute the spark plug.

Hindsight being what it is, for the cost of a cylinder base and rocker box gasket, along with a better quality paint job replacing the sun faded factory paint, our Hero would have never known the magical B50SS.

And so it was in that mystical land, over the hill, now far away.

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Re: B25SS Burning Oil

Post by Cocker Cox » Sat Aug 15, 2020 3:18 pm

So, what you’re saying is, whoever changed the piston may not have spaced out the piston rings gaps?!

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Re: B25SS Burning Oil

Post by Jeff K » Sat Aug 15, 2020 3:57 pm

Black Smoke? Black Smoke is normally carbon and indicates running way too rich, like a cold motor with too much choke. blue Smoke is burning oil, and if you can see the blue smoke, then will also be able to smell it. Lastly white smoke is water vapor. Rare in a air cooled motor until we were forced to use gas with 10% ethanol in it. Now more common when the motor is cold.
Pull the plug and get a compression reading after a good medium speed run and it has been all warmed up. Check the compression with a gauge and the throttle wide open while kicking. If it is low, add a little bit of oil in the plug hole and repeat. If the compression comes up, then it is rings. If it does not, then it is valves. If the compression is ok and it is smoking then it is probably your guides.
Also be aware that a lot of the newer oils are "thinner" then the old oils and will leak/burn easier. I spoke to a Oil company rep years ago and he explained that a say, 10w30 oil has to meet a certain spec at a certain cold temperature, the 10 in the 10w30 and then again at the high temperature, the 30 in the 10w30. With the new technology and chemistry, they can do this with thinner oils. Most new cars are running 0w20 or 5w20. But get 100,000 mile on them and they start to leaking and burning oil. He stated that is why you should start using 10w30 in high milage cars.
If your bike is running good, not smoking bad and the plugs are not fouling out, then I would ride it. Do the compression check to pin point the problem and get ready to fix it.
Jeff

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Re: B25SS Burning Oil

Post by Jeff K » Sat Aug 15, 2020 4:03 pm

Cocker Cox wrote:
Sat Aug 15, 2020 3:18 pm
So, what you’re saying is, whoever changed the piston may not have spaced out the piston rings gaps?!
Sadly, I have also seen this before when some one had just rebuilt their motor for the first time. A very good learning experience.
Jeff

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Re: B25SS Burning Oil

Post by Cocker Cox » Sat Aug 15, 2020 4:48 pm

Cheers, Jeff. It’s oil burning not soot, so we’ll settle on blue smoke! I’ve just lapped the valves in, and they were ‘petrol tight’ when I inverted the head and filled the dome with fuel. I’m using Halfords Classic 20/50 mineral oil. I’m suspecting the piston rings and thinking either the gaps are in a line or old rings in a different barrel. I like your tests for compression though and will try it once I get a gauge (unless I can modify the scrap tyre pressure gauge I’ve got in my ‘inspiration drawer’!)....

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Re: B25SS Burning Oil

Post by Momus » Sat Aug 15, 2020 9:37 pm

Rings usually rotate so the chances of the gaps aligning is not that great.

I'd take that old beast out and give it an absolute caning for a few miles. Chances are the rings will seat.

Try with a different brand of oil; that has been known to help.

Really needs to be pushed for a couple of dozen rotations at a reasonable clip if anything like a reliable comp. reading is wanted.
What you do with the number thus obtained as part of any diagnosis is a good question.

A leak down test with a stethoscope would be a better approach.
If you love it, lube it.

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Re: B25SS Burning Oil

Post by eebtr7 » Sat Aug 15, 2020 10:03 pm

No, my friend, Cocker Cox. The ring gaps were set in a line by the boys on the engine line at BSA during the initial assembly. Had I taken it back to the dealer prior to the 6000 mile and/or 6 month period, repairs would have been for free under warranty. Or, so I like to fantasize.

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Re: B25SS Burning Oil

Post by Cocker Cox » Sat Aug 15, 2020 10:24 pm

Momus wrote:
Sat Aug 15, 2020 9:37 pm
A leak down test with a stethoscope would be a better approach.
Good advice. But it’s so easy to strip the top-end, which I’d have to do anyway, wouldn’t it be easier to do that and look at valve guides and valve seating with the head off, then, perhaps test the rings by filling the cylinder above the piston with engine oil and seeing if it holds (or testing how long it holds)?

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Re: B25SS Burning Oil

Post by Cocker Cox » Sun Aug 16, 2020 7:31 am

It’s just a thought, but if the the crankcase breather pipe was blocked, might this cause oil to be burnt? I hasten to add that the engine is oil tight!

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Re: B25SS Burning Oil

Post by Momus » Sun Aug 16, 2020 9:39 pm

Cocker Cox wrote:
Sun Aug 16, 2020 7:31 am
It’s just a thought, but if the the crankcase breather pipe was blocked, might this cause oil to be burnt? I hasten to add that the engine is oil tight!
No need to dismantle to perform a leakdown test.

You will see how long it holds compression for/rate of loss and where it is losing- ring seal, valves seats.

I'm not sure that using engine oil to test ring seal is going to achieve much.

Change the oil, give it a flogging for a few miles and see how that goes.
If you love it, lube it.

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Re: B25SS Burning Oil

Post by Cocker Cox » Sun Aug 16, 2020 10:54 pm

Cheers,

It’s not the breather... that works fine! Tomorrow, I’m going to perform a make-shift leak-down test. I’ve made a sealed plug-hole connection to my compressor and with the piston on the compression stroke listen for leaks through the inlet, exhaust and crankcase. But the fact is, the engine is definitely burning oil and I don’t think giving it a pasting will cure it. Since getting it road-worthy, I’ve put over 300 miles on it, mostly at 50 - 60 mph. I’m going to have to strip it down anyway, and the only thing that will prevent me from finding is if the head gasket is leaking, allowing rocker box oil drainage to seep into the cylinder, but that’s going to be obvious anyway. Weirdly, there’s engine oil around the base of the spark plug, and also traces on the threads. It’s not leaking into there externally as far as I can see; it’s as if the oil is coming up from the cylinder....

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Re: B25SS Burning Oil

Post by Jeff K » Sun Aug 16, 2020 11:28 pm

What type of oil did you use to break it in with? Some of the new oils will cause the rings to go a very long time before finally breaking in. I remember a old cycle mechanic pal of mine used to swear by using WD40 as a assembly lube for cast iron rings and cast iron bore. He firmly believe that any thing else took too long to break in.
If you are going to tear down the motor any way. Remove the Carburetor, exhaust pipe and valve covers. Put in 4th gear, block the tire and set at compression stoke. Take a old spark plug, bust out the inner ceramic part and solder/braze or JB weld a quick connect air line fitting to it. Now you need a thin fluid and a dye. Red permanent marker/felt pen comes to mind with some rubbing alcohol. Cut open the felt pen mix with alcohol and pour into spark plug hole. Screw in air fitting and pressurize, high pressure is not needed, just enough to get it to leak. Be careful because the engine will want to move. Keep pressure on for a few minutes, release pressure, be careful that it does not spray back out. Remove air fitting and pull apart. If it is rings, valve seats of head gasket, you will know by the red streaks. If it is guides then this will not work, but it will eliminate everything else. We used to use machinist dye, Dykem for stubborn cases like this.
Jeff

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Re: B25SS Burning Oil

Post by Cocker Cox » Mon Aug 17, 2020 7:47 am

Cheers, Jeff,

I haven’t done anything to the bike, engine-wise, other than sorted the top-end, so didn’t need to break it in. Having said that, the history of the bike is unknown to me, other than it stood for many years in Canada and a few more years when the last owner brought it back to Blighty. But I suppose there is the possibility that the piston was swapped by a PO, perhaps to make the bike sellable, and not used after that. It certainly had a rear chain fitted that was so tight, even with the swinging arm fully retracted, there was absolutely no up and down movement! There’s no way that bike was ever ridden with a chain as tight as that. I’ve come across a similar situation in the past where an owner was selling one of a few similar bikes and swapped parts, flogging the mongrel as a runner.

I’m going to pressurise the cylinder as you advise, sans dye, just as an opener to see if it reveals anything. But ultimately, I will have to strip the bike down anyway, even if I find the problem prior to strip-down, and all the culprits are detectable then: valve guides, piston rings, oil leaking into the cylinder through the gasket (I fitted a new copper head gasket when I sorted the head). My own suspicion is rings on the basis that the piston has been changed for a Starfire item. Also, the cylinder head was a 1/4” stud Starfire item married to a 5/16” stud SS rocker box. In my book, anyone capable of doing such a bodge has to be a suspect mechanic. I can’t imagine such a person would use new piston rings or hone the cylinder, having swapped the piston.

So, rather than spending time locating the problem, I’m just wondering whether it might be better to bite the bullet, and rebuild the engine anyway with the correct piston. The barrel is on a standard bore and in very good condition... no scoring, smooth as silk, not even a trace of a lip at the top. I’ve located a NOS standard piston in the US, but am unsure whether the factory L, M and H tolerance stampings apply to replacement pistons. My barrel and (Starfire) piston are both stamped M.

My other concern though is how much of the engine was changed when the piston was replaced. The SS con-rod was beefed up, requiring a modification to the crankshaft, so that’s different too. A Starfire piston will fit a onto an SS con-rod, so it’s more than likely that this is the only non-SS item fitted. As I say, I’ve done 300 miles on the bike, including sustained motorway blats at 60 mph, and the engine’s fine apart from burning oil. The spark plug is a (dark) biscuit-brown and remains clean, apart from being slightly oily. The exhaust tail-pipe is oily rather than sooty, but the bike starts hot and cold on the second or third kick. A lot of owners might run the bike like it is, but that’s not my way....

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Re: B25SS Burning Oil

Post by SteveS » Mon Aug 17, 2020 7:54 am

smooth as silk
I think you need to run a glazebuster down the barrel

My money is on Momus
Change the oil, give it a flogging for a few miles and see how that goes.
:thumb
I'd take that old beast out and give it an absolute caning for a few miles. Chances are the rings will seat.
:thumb
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Re: B25SS Burning Oil

Post by Cocker Cox » Mon Aug 17, 2020 9:24 am

Cheers, Steve,

I’ve put over 300 miles on the SS since getting it so I don’t think another thrash will sort it. Plan A is to fit new rings, get the cylinder honed (CCM Britain, a B25 specialist company is just a few miles away), and rebuild. Plan B is to rebuild the engine with a proper SS piston fitted....

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Re: B25SS Burning Oil

Post by JB » Mon Aug 17, 2020 10:16 am

What oil are you using for breaking in?
Your better off using a straight 40 grade for running in a cast iron ring & liner set up and then switch to multigrade of your choice after 4-500 miles

The piston grading was a factory method for mass assembly allowing piston selection from a stock of graded parts: all cylinder rebores are done on a 1 to 1 basis with the cylinder honed to give the recommended clearance to match the supplied piston.
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Re: B25SS Burning Oil

Post by Jeff K » Mon Aug 17, 2020 2:39 pm

"A Starfire piston will fit a onto an SS con-rod, so it’s more than likely that this is the only non-SS item fitted. "
The crankshafts and rods are different between the two models and the factory used a different ratio to balance them.
Heavy oil use due to rings normally equals clouds of blue smoke, low compression and hard starting. Remove the exhaust pipe and shine a light up in the exhaust port. Compare what you see to the spark plug. How many miles are on the bike? I would suspect the valve guides.
If you do replace the guides, be careful the valve stems on the new valves can be rough and will wear out a new bronze guide in no time at all. I use a drill press or a lathe and start with 600 grit sand paper and work my way up to 1200 or 2000 automotive body work sand paper. Just a light bit of pressure, you are just polishing them. Then I will buff them on the buffing wheel. I find the Bronze guides last longer this way. I have posted this before. You will then need to check the valve seats. I use a black felt pen and a dab of lapping compound. If they are ok, then lap them in. If the are off then they will need to be recut or ground.
I have been very happy with the Kibble White Black Diamond coated valves and their hard bronze guides. But they need to be honed to fit, not a job for the home mechanic.
When you go to remove the guides, use a propane torch to heat the head, evenly heat it and put the flame in the ports, rocker boxes and combustion chamber until the aluminum starts to sweat, it will actually get moisture on it. Keep heating until the moisture is gone and then using a valve guide punch gently drive them out. Reheat if you took a long time to get the guides out. I like to use a small dab of nickel anti-seize on the nose of the guide and using the factory tool pull them into the head. Let cool and then check the valve to valve seat contact.
ED V. has some good advice on the 250 crank on his web page.
https://www.shopevengineering.com/techtips
Jeff

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Re: B25SS Burning Oil

Post by Cocker Cox » Mon Aug 17, 2020 4:58 pm

Thanks, Jeff... lots of useful info. there. I’ve now tracked down a set of NOS rings and an NOS SS piston, so all bases are covered in that sense. While I’ve got the head off, I’ll take it to Mark Cook at CCM Britain (just down the road) for inspection of the guides. I’ll leave any work to him. I’ll let Mark run his expert eye over the barrel as well. It’ll need honing but I’m sure it’s also in good condition... no scratches or damage and no lip at all at the top....

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Re: B25SS Burning Oil

Post by Jeff K » Mon Aug 17, 2020 5:54 pm

Excellent choice using Mark!
Jeff
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Re: B25SS Burning Oil

Post by Cocker Cox » Thu Aug 20, 2020 3:53 pm

Excuse the hairy piston! But notwithstanding the differing opinions about rings rotating, the gaps are all within 60 degrees of each other. I’m not saying this is the cause of the oil burning, but I’d be interested in any comments. Not sure why it’s upside down!
Also, the copper gasket was covered in oil. There’s no evidence it was blowing, so I don’t think oil was being sucked into the cylinder, but, again, all comments valued. How can I stop oil leaking out of the drain channels running from rocker box to crankcase?
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Re: B25SS Burning Oil

Post by Jeff K » Thu Aug 20, 2020 6:34 pm

Just a odd problem and normally you would see oil leaking on the outside due to it. How is the fit between the head and the cylinder liner. B40's had a issue with aftermarket copper head gaskets being too thin and the liner would hit the head before the head could seal the copper head gasket. You could have the save problem if a previous owner machined a bit off the head. Put the head on the cylinder and use feeler gauges to get the gap thickness and then compare that to the head gasket thickness. You have carbon build up on the top of the piston, but I am not seeing evidence of blow by on the side of the piston. You would have blow by if the ring gap was in a line or they were worn or not sealing. Next on your list should be the simple check the head/cylinder gap, then the valve seating and then the guides. I am betting on the guides being worn.
Did you notice the stud in the fore ground is wasp waisted just bellow the top thread ? That would normally would indicate being over tightened or worn from rubbing against something. I still use the OEM style rings with the one piece oil ring. But I have also been using the three piece oil rings like the USA Hasting ones more and more. I think that they do seal better.
Jeff

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Re: B25SS Burning Oil

Post by Cocker Cox » Thu Aug 20, 2020 6:47 pm

Cheers, Jeff. I’m taking the head and barrel to a specialist so the issues you raised can be checked by them. I’ve just looked at the guides and I don’t think there’s any doubt that they’re worn! The exhaust valve stem is worn by 2 thou. and wobbles by about 1/16” side to side in the guide. The inlet valve stem is worn by about 1 thou. but has a similar wobble. By comparison, the valves on my scrap Starfire head are worn by less than half the amount, and there is much less wobble....

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Re: B25SS Burning Oil

Post by Jeff K » Thu Aug 20, 2020 7:16 pm

And we have a Winner! Bad Guides! :ban
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Re: B25SS Burning Oil

Post by Jeff K » Thu Aug 20, 2020 7:26 pm

I have been using the Kibblewhite Black Diamond valve and Bronze guides for a while. The bronze guides are so hard that you can not use a reamer on them and they have to be honed to fit. Here is their recommended clearances.

Aluminum cylinder head recommended valve stem-to-guide clearance is as follows:
• Intake: .0008" to .0012"
• Exhaust: .0012" to .0015"

You need very expensive equipment to (Sunnen Hone) to hone these to size and some very expensive pin gauges to check the ID on them. Prior to using these I had very good luck with the Alfa valves and Bronze guides-- ONLY after going through many steps to polish the valve stems. You are being smart in having Mark cook do them for you.
Jeff

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Re: B25SS Burning Oil

Post by Cocker Cox » Fri Aug 21, 2020 8:11 am

Wow! According to the manual, the valve-guide clearances on a standard set-up are twice that... inlet: 0.002” - 0.003”; exhaust: 0.003” - 0.004”. BTW, CCM have declined to take the work on as they’re so busy... I’m taking it to RTS. It’s a bit of a lick from me but Roger Taylor knows BSAs inside out. You were right about the SS crank and con-rod still being in place, so that's a real positive. I’ll never know why the piston and cylinder-head were changed, but anyone capable of putting an SS 5/16” stud rocker box on top of a 1/4” Starfire head isn’t going to worry too much about putting a used Starfire piston and rings into a glazed bore I wouldn’t think....

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Re: B25SS Burning Oil

Post by JB » Fri Aug 21, 2020 9:19 am

Taking into account BSA's original 0.0005" tolerance on the valve stem diameters then the original factory stem clearance figures are:
• Intake: .00020" to .0035"
• Exhaust: .0025" to .0040"
Which is huge by any modern standard.

I made my own B25T's guides from Colsibro and as an experiment I went all the way down to 0.01mm (0.0004") on the inlet, with a stem seal and 0.020mm (0.0008") on the exhaust. My bike is used purely for trail riding and only has just over 2K miles on it and runs really well, pulls strongly, it has a very clean exhaust with no signs of any burning whatsoever.
I admit my clearances are right on the low end as I wanted to aim for this. My thinking behind this is that obviously the inlet valve isn't subject to the heat like the exhaust always is and in addition to that it's actually being cooled by the inlet charge. I then ran some numbers on the temp range that the relatively small .3125" stem would have to be ABOVE the already hot exhaust guide and concluded that there was next to no chance of that difference occurring, so I was willing to go for 0.020mm (0.0008") on the exhaust.

I've yet to pull the head to inspect it so can't conclude anything certain other than it runs well like this.

HTH
John

I run my own toolmaking business so these exact clearances weren't difficult to hit and are loose by our normal stds.
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Re: B25SS Burning Oil

Post by Jeff K » Fri Aug 21, 2020 3:02 pm

The Kibblewhite Valves are specially coated SS and their guides are a very hard copper based C63000 nickel-aluminum bronze. The valve will not fit into the guide when you get them. The using pin bore gauges that are in the ten thousands range you have to hone them to fit. Then once the head is heated , guide pulled in and it has cooled down, you will find the valve no longer fits. The clearance is that tight. You then have to hone the guide again to the final fit. And be very careful on final assembly. I for decades used a mix of STP and motor oil as a assembly lube. I had sent a 850 Norton head out a few year back to one of the very best Norton Machinist, James Comstock. He put in threaded Bronze exhaust ports, and the Kibblewhite valve guides. I used the STP mix and seized a valve on start up. He told me that STP does not play nice with the very tight clearances. He redid the head free of charge and now includes that warning with his work.
links to Kibblewhite:
https://shop.kpmivalvetrain.com/c/briti ... 630-bronze
https://shop.kpmivalvetrain.com/c/briti ... ck-diamond
Jeff

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