The BSA A7-A10 Forum
Technical (Descriptive Topic Titles - Stay on Topic) => A7 & A10 Engine => Topic started by: terryk on 20.06. 2010 11:57
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edit: split from here (http://www.a7a10.net/forum/index.php/topic,2829.msg18879.html#msg18879), see last sentence.
Gday Brian, what has failed on the bottom end do you know yet?
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The timing side bush. I have made and fitted a new one, just waiting for some gaskets and stuff to turn up from the UK and I will put it back together.
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Please tell me pic is of the old one.............
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Where did you get that bearing, Brian?
White metal hasn't been used for years.
By the way, going back to our correspondence about bronze grades, I have found that the Fraser bushes that I fit into the original steel outer is LG2 which is the correct type.
Trev.
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As far as I know Trev that is the original bush. Its std and has been in the motor ever since I have owned the bike. I made a new one out of LG2. The crank is perfect.
There is absolutely no wear in the bush but as you can see it has started breaking up. The big end shells were similar, no wear but showing signs of fracturing, once again no wear on the journals.
The valves were also in need of work. A bit odd as well, no burning and no recession, the clearances havent altered since I last checked them about 10,000 miles ago (I know I should check them more regularly). But they were very badly pitted on both the valve face and seat, maybe the result of unleaded fuel. I have faced them and recut the seats but I have a spare head which I will get seat inserts put in and try and get some decent valves (maybe Black Diamond) and have that ready to go on when needed.
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The original bushes were White Metalled of course. Our man at the local Engineering Co, will re white metal, fit and ream the bush for about £65. He recons they are much better than Bronze bushes.
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Brian,
Are those wear marks on the face of the bush?
Has the crank been rubbing on it?
Do you think it tore the piece out of the face first before the bearing surface went west?
I've got 2 bushes out of A7s with pieces torn out of the faces.
cheers
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It does appear to have had pressure on the face. It has quite a sharp shoulder on the bush and even though the crank is radiused I think it may have been rubbing at this point which may have caused it to fail. Unfortunately I dont know how many miles this bush has done, it is std and has been in the bike since I have owned it so I would think it is the original bush. I know it has done at least 50,000 miles between the prevoius owner and myself but before that I have no idea.
When I pulled the motor apart the bush was complete, ie. no bits missing, but it was all fractured and as I poked it with a screwdriver all the bits fell out leaving it as you see it in the photo. I have replaced it with a solid bush made from LG2.
I guess this reignites the discussion as to just what type of bush to use. Obviously white metal ones can do this, sleeved ones can possibly (but not likely) turn in the steel outer which brings us back to solid or "one piece" bushes. At this stage I am going to use solid ones made from LG2 which after extensive research would seem to be the best material to make them from.
I have just assembled another plunger A10 motor with a solid LG2 bush and a ball bearing main so that end float is not an issue but I probably wont have it running for 12 months and then will need time to put some decent miles on it to see how it goes. This particular bike is going to be a sort of test bike for various ideas and modifications.
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Failures like this (cracking, disruption of the bearing surface) are very common with cast Babbit bearings, these failures are known since the 1930's, when these bearings were about the only ones used. With increased power output and lightweight cranks and cases (leading to larger deformation), bearings that worked perfectly well during many years in the 1920's, suddenly started to fail within short time.
White metal has a comparatively low fatigue endurance limit, so I think what you see here is simply an age-related failure, caused by many many miles and millions of load cycles, exactly where you would expect this to happen.
Usually, when exposed to high local pressure and temperature, the babbit melts locally and offers excellent failsafe running lubrication. So even if the crank had been rubbing on the shoulder, I suppose this would not hurt the bearing, as long as it is lubricated (and cooled) sufficiently.
The big advantage of white metal bearings is that they do not make high demands on the quality of the crank pin. The crank can be made from unhardened steel and still usually does not show any signs of wear when running in Babbit shells, even after considerable mileage. Therefore white metal still is used for bearings in large Diesel engines with their unhardened cranks (ship engines for instance).
OTOH due to the softness and the low allowable surface pressure, it is mainly suitable for moderate power levels. For an engine in standard tune you could definitely have new Babbit cast on the bearing shell and when machined properly, this will work perfectly well for the next centuries probably.
Leaded bronze otoh will withstand much higher loads, so definitely is the first choice for a tuned engine, but you have to be aware that with the use of harder bearing material the wear on the crank pin might be increased (depending ont he crank surface hardness). To counter this, I always have my cranks nitrogen hardened. As this hardening process does not cause any distortion, it can be done after regrinding. In case you have the chance to get you crank (as well as cams and followers) re-hardened, it definitely is worth doing so, it's actually rather cheap.
Something to think about. *smile*
Cheers, Markus
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There is no doubt about the wear ability of the white metal. There is no wear in the bush or on the crank and I have no doubt this bush would have done a considerable mileage had it not broken up. Incidently the big end shells were also starting to break up.
I have heard of people fitting phosphor bronze bushes and wearing out cranks in record time but havent experienced this myself (and hopefully I wont !)
The stress on this particular motor I would rate as probably slightly above average. It is running a 356 cam with 7.25-1 compression and gets ridden at around 65mph, none of which are particularly excessive but it does get subject to fairly high temperatures. I regularly go on rides of 3 to 4 hundred k's in our summer with the temperature in the 40's C. I dont know how hot the engine gets in actual degrees but some days I reckon you could fry chips in the oil !
I will run the solid LG2 bushes from now on and see how they go. Trouble is now I dont have any high mileage motors so will have to wait a couple of years to get some decent mileage on one of them.
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To counter this, I always have my cranks nitrogen hardened.
Unless you are talking about cryogenic hardening in liquid nitrogen, nitrogen dose not significantly increase the hardness of steels.
The post machining process that introduces nitrogen into steels is called nitriding ( fairly obvious ).
The nitrogen sits in interstitial cavities in the lattice and prevents disslocations ( atoms that should not be there or holes where an atom should be ) from moving under stress. If these disslocations can not move then dissolved hydrogen gas can not form hydrogen molecules and the material will not crack ( well not crack as readily to be acccurate).
As such the steel is significantly TOUGHER and in particular it has an increased FRACTURE TOUGHNESS but the wear resistance will not be increased in any way shape or form.
OTOH if you had your crank treated to with both Carbon & Nitrogen (as would be the case with an ammonia furnace enviroment ) then the additional carbon will cause additional case hardening and thus wear resistance.
This process is termed "carbo-nitriding" and goes under trade names such as "tough-ride" ,"tough-proof" , "carbo tough" , "nitro tough" etc, etc, etc.
I strongly recommend every one do one or the other whenever their cranks come out of their bike.
But you must be careful which process you use as adding carbon to high strength forging grades of steel can have drastic & unpredictable effects on the crystal structures particularly if it is a second hand crank of unknown history.
Once you have introduces extra carbon into the steel you really can not get it back out again and if the previous owner had carbon treated the crank to make it harder, then you add even more you run the risk of ending up with either way too deep casing or a way too hard wearing face.
Sorry to appear to be a bit pedantic but very few owners ( and even fewer mechanics ) have a sound understanding on heat treatment and while a little more hardness can be a good thing, too much promotes brittle fracture which aint funny, particularly if it happens some where real busy and it is your bum on the saddle.
Heat treaters have a bad habit of doing exactly what they are told to do even if it is the wrong thing to do.
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i done a job for the company who does nitriding near me , i got to know one of the managers and he offered to do a crank if i wanted to , never bothered as it would mean stripping down the engine. also i was worried if the crank suffered damage other than normal wear eg. seizure could the journals be ground undersize again.
also i believe that nitriding actually softens some grades of steel if they have been heat treated by conventional methods , this happened with a batch of camshafts for triumph twins and they wound up soft as putty .
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Nitriding dose not create any problems with machining at a latter date.
As it involves heating the part for a considerable length of time, if you nitride a part that has been case hardned it will have the same effect on the carbon as prolonged tempering.
When ever you heat a metal all of the disslocations will try to evenly distribute themselves throughout the material.
The speed of this will depend upon the relative concentrations and the over all temperature.
When you case harden typically you immerse the steel in a arsnic salt.
the salt has a very high concentration of carbon so the carbon will try to evenly distribute itself between the salt and the steel so the rate of transfer into the steel will be high.
If you kept it in the salt for long enough the entire thickness would be hardened ( this is called through hardening ).
If you pull it out after a short while there will be high concentration gradient of carbon between the surface and the center and in this state it is called "Case Hardened"
However the outer edge will usually be too hard so you reheat it to allow the thin band of high carbon to dissolve deeper into the steel and reduce the concentration gradient this condition is termed "tempered".
The tempered surface is a little softer ( which is the aim of the exercise ).
Nitriding is done at roughly the same temperatures as tempering so if you nitride a tempered steel you will "over temper" the steel in effect drastically softening the skin that you originally hardened.
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Trevor,
You are absolutely right, the process is called nitriding. That was simply a translation mistake, sorry for causing confusion. In German there are various names for the process, obviously "nitrogen hardening" is not one of them used in English?
I also agree to the diffusion of nitrogen into the ferritic metal matrix, where it prevents dislocations and creates residual compressive stress, thus improving the fatigue strength and the "toughness" in terms of crack deflection.
But: This does apply for the diffusion zone only, while on the surface of the part a very thin link layer of (epsilon and gamma-) iron nitrides, iron-carbon-nitrides, nitrides of the alloying elements and cementite is formed (10-30 micrometers thick). This is what gives the part a very high surface hardness and what increases the wear resistance as well as the tribological properties (the iron nitrides form a more or less porous surface at the very outside which can be used to carry lubricants).
With alloyed QT or nitriding steels hardness values of 65HRC and above are reached.
For the reason you mentioned I always use the nitriding process and not carbo-nitriding. Why you mentioned the ammonia furnace treatment in that context is not clear to me. Where would the carbon come from in ammonia atmosphere (=NH3)?
Compared to the nitriding process, carbo-nitriding is not true to size, so can not be done without subsequent grinding. The nitrided journals will only need a bit of polishing and are ready to fit.
bonny,
If the process parameters (especially the temperature) are set correctly, there is no danger of softening pre-hardened parts. It is common practice in automobile industry to have parts like camshafts hardened (martensitic hardening here), then ground, and finally nitrided. The nitriding temperature has to be lower than the tempering temperature of the steel. Plasma nitriding is done between 380-570°C, while the QT steel 42CrMo4 (common steel for cranks) has a tempering temp. of 540-680°C, for example. Usually the nitriding temp. is chosen 30-50°C below the tempering temp.
Regrinding is not a problem, a case hardened crank will have about the same or even higher surface hardness, while the hardened layer is even thicker. It just will need to be nitrided again after a regrind.
The problem however is that we usually don't know what heat treatment or hardening had been done before or originally. In case of the BSA cranks I performed hardness tests on them (micro indetation), showing values of 35-38HRC, which could be significantly improved by the nitriding process. I assume the cranks had been quenched and tempered (probably directly after forging), and that was about it.
Many repro parts (cams, followers) I have seen for the A's seem to be nitrided as well, I recently got a set of followers for my Matchy that definitely had been.
Cheers, Markus
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I work in engineering MG but have never had much to do with the heat treatment of parts and nitriding only once, in a works where i used to work we were having problems with spools that connected two screw augers in sewerage machinery, sewerage is highly abrasive , the spools were supported in polyamide bushes and no matter what we did they wore away quickly, the poly bushes acting as a lap , we first tryed simple case hardening and that didn't work , so we then sent them out to have them nitrided in ucd (university college dublin) i can't honestly say how that turned out as i simply can't remember . but i seem to recall that it was recommended that if nitriding was going to be done as part of the process , then a steel with a high chromium content was suggested .
to be honest , i am sorry that a brilliant company like bsa with massive engineering resources and expertise who made such a vast range of products ever perservered with the bush set up. maybe the end-fed conversion is the road to take.
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Generally speaking, the higher alloyed the steel, the harder the surface will come out after nitriding, while the depth of the diffusion zone will decrease.
Considering the mileage Brian's engine has covered with the (presumably) very first bush, I'd say the needle roller conversion is not really necessary, especially as it is a rather costly operation. The most important thing is correct and accurate machining of the bush, then it should last at least until the next rebore and pistons, assumed that oil is kept clean and changed regularly. I think the setup is not as bad as it's reputation.
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another thing about the bush which i'd like to know is the endfloat issue , what are the symptoms of the clearance being too much and too little ? knocking and banging i presume ?
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I've just got the A7s bottom end back from the machine shop.
They turned up a new main bush from the small end of a large perkins or isuzu diesel.
The cam and crank bushes have been aligned and reamed so hopefully all will be good.
Like Brians this will get a ball bearing. I wanted to use a NTN TMB206C3 or TAB206C3 with seals but have not been able to obtain one.
These bearings have special heat treatment (nitrided balls??) and are designed for transmissions.
The husaberg boys retro fit TMB206C3's to their scramblers.
If anyone knows where I can get one of these bearings (with a seal) please let me know. The local agents just go glassy eyed.
Does anyone know if the distance between the crank and the timing bush matters if the crank is fixed?
Will it effect oil pressure if the gap is large ?
cheers
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G'day Olev,
just did a google on that bearing and got nothing, I found the size 30,62,16.
Even with a ball I'd get the end float as close as possible, say 2-4 thou. Less stress on the bearing and wont affect the oil pressure.
Cheers
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The bearing is a 6206, metric. 62mm OD, 30mm ID, 16mm wide. I used a SKF explorer bearing in the one I put the ball bearing in and left the outer seal in place. If you have 53' or later cases they will have a seal so you can remove both seals from the bearing. The roller bearing is a NF206.
We could probably discuss for years the pro's and con's of a ball or roller bearing so I wont go into that except to say just about all modern motorcycles run on ball bearing mains, not rollers. The reason I chose to try a engine with a ball was to eliminate end float, the thing is no matter what you set up the end float to it changes with heat. I had the primary cover off my 61' when the motor was hot a few months ago and hot it had about .010'' end float (only a estimate as I didnt measure it accurately) as the motor cooled down the end float dissapeared back to the zero I had put it together with. Just how much end float affects the wear etc I dont know but it cant help having the crank being able to float around.
All this started because it wouldnt go up a big hill as easily as the A7 !!!!!!!
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For the reason you mentioned I always use the nitriding process and not carbo-nitriding. Why you mentioned the ammonia furnace treatment in that context is not clear to me. Where would the carbon come from in ammonia atmosphere (=NH3)?
Compared to the nitriding process, carbo-nitriding is not true to size, so can not be done without subsequent grinding. The nitrided journals will only need a bit of polishing and are ready to fit.
Comes from not checking my post properly before I send then and changing my train of thought on the fly ( somthing about playing on the net when I should really be in bed )
I am nearing the limit of my Ferrous metallurgical knowledge as I was principally in tertiary metallurgy and non ferrous foundry to boot so did not do much with steel after I left uni.
As for the formation of nitrides & complex nitrogen phases at the surface , this is something that I was totally unaware of and would have thought both the concentrations of nitrogen and temperatures were too low for the formation of such phases in sufficient quantaties to alter the bulk properties if at all.
I no longer have access to TTT curves so could not verify how near the tempering temperatures nitriding would be done and what affect if any this heating would have on the microstructure of the particular alloy. I was always led to believe that gas nitriding was done very close to the tempering temperature to reduce the heating time ( & costs )
A couple more things that I have learned for which I thank you.
The principal object of the post was to distinguish between the two processes which just sound like different names for the same thing and as we have established are totally different and clarify which you were talking about.
As stated I always recommend nitriding and have had a couple of locals looking to shed my blood after they Carbo ritrided their cranks which failed very quickly.
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I've been co-working on a university research project on that stuff (tribological behaviour and wear properties of nitrided surfaces), so gained a little knowledge about it, but in fact I'm a mechanical engineer and no metallurgist, so mainly have to believe what these tell me. *smile*
Since plasma nitriding has become common, the process temperatures could be reduced significantly, compared to gas and salt bath nitriding. I was told that if necessary they could go down to 350°C (with an increase in nitriding time though).
Thank you however for the clarification of the process names. It is not easy to keep things apart, especially with all those creative trade names around.
Cheers, Markus
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Muskrat,
Here is a couple of links that headed me in this direction.
http://www.ntnamerica.com/pdf/Other/tmbspec.pdf
http://www.husaberg.org/forum/viewtopic.php?f=5&t=6195&start=270
The second link gets into it about half way down. They talk about SKF TMB bearings but the SKF distributers don't know anything about it and its not on their web site. I'm pretty sure its an NTN.
NSK make an interesting variation called TM206 sealed clean where they fit seals that lets the oil in but keeps the lumps out.
My engine is a 52 so like Brian says it would be good to have a seal on the drive side.
cheers
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Engine rebuilt and all back together and running again. Now, what else can I pull apart............................