Britbike forum Forums British Motorcycle specific Brands Bsa bulletin board A65 that actually 'blew up'

Passed a few semis today, glad mine didnt do that.

Gary, The picture I alluded is now sitting (on my computer) at 12 out of 32 pages so a bit less than halfway through. Posted by Petersen and about 20 or so posts before your Hornet. BTW thats for the info on the cut-out switch.

Cheers
Ray

Looking at the picture of the blown engine it kind of makes me wonder how the coils are wired. It could be that there is a center wire that I cannot see for the hot lead. If not it looks like it may be wired with two 6 volt coils in series using one set of points with a waste spark like Honda twins.

I seem to recall a certain moderator setting his pants on fire when his lightning motor blew.

now if I could remember his name..........

Craig, it has the telltale red terminal insulators of a Boyer kit.

Ray, I think this it.

'69 Lightning engine on the "Show BSA" thread about page 4.

when i bought my A65T it had a "rod knock." upon teardown, the primary side piston had marks up and down the skirt where it had been grabbing the wall, and the rebuild required .060" oversize, replacing third over pistons. the wear was progressivly worse in the lubrication path away from the timing side bush, indicating poor oil pressure. i thought the bush itself was the culprit, but i've had pressure problems since the rebuild, as some of you know from my thread about it. someone had obviously been in there before with the .030 pistons, so it could have been some mistake post-factory. dunno. didn't "blow up," but it did seem to have the infamous timing bush problem...

Q: Why do Brit bikes always blow up on the left side?

A: Because that's where the serial numbers are stamped.

This engine was just idling, when the right(!) hand side rod broke. No seizure.
http://home.online.no/~oseinbu/bsa2004/veivhusventilasjon.jpg

This 1971 crank just broke, and the bike was still running. It even started after the disaster, but made funny noises...
No damage to the cases or cylinder, but rh rod got some beating.








Best regards

Is it the Lightning/Spitfire motors that blowup the LH rod? Or do the T-bolt motors do it as well. (I have a theory...)

Ola - That crack through the RH Main bearing on that crank looks like metal fatigue/manfacturers defect? The journal on the RH side looks OK?

Don't know about that. I always thought it was all Tri/BSA twins and triples. The LH rod is last in line for oil.
I know every blown triple I've ever seen has a big hole in the left case.

From what I can see in the photos you had a beautiful example of a classic brittle fatigue failure.
Best seen in the 3rd photo.
From nearest to the crank case mouth you can clearly see the step marks.
I can count about 10 of them. This was the crack propogating with each stress cycle.
Then the lower section which looks really clean is the final fracture which happens really quickly as there was now insufficient material thickness to take the load.

There is no obvious distorted ( stretched or twisted ) material so I will hazard a guess that the crank has ended up being through hardened.

It looks like the crack originated in the cheek area so it could have been some bad machining and I would look really close at the alignment of the crank case which would have made things worse.

Being that it is a bike that is 40+ years old most likely to be problem attributed to previous owners .
Well done to catch it before it disintegrated the entire motor and a bit of good luck that you were not in high speed traffic when it happened.

Old cranks should always be treated as suspect until proved to be in good condition. It is worth wile to send them to professionals to be tested and heat treated before being put into service.

Interesting Trevor. Alignment is important. The journals were fine, so most likely factory defect. The present owner bought the bike this summer with "possible engine failure", so what actually happened is hard to say. Here are some better pictures of the breakage.








Best regards

Sure hard to tell from these pictures, but it looks like a regrind without leaving the proper fillet radius. If this is so it is not a factory defect.
HTH

I concur with Trev here. And yes, for possible causes I would look at the radius (since that's where it looks like the failure originated), surface finish of the radius and hardness. It's entirely possible that coolant loss or too high of a feed in the grinding process caused the steel to go brittle in the radius.

BTW, this is my fave:

Note the oil pressure gauge right next to the rod poking through the case (on the RHS, incidentally)

John sez:

"Sure hard to tell from these pictures, but it looks like a regrind without leaving the proper fillet radius. If this is so it is not a factory defect."

What are we going to do with you John. Don't you know that it may not be BSA's fault, but they are going to get the blame for it....

Manufacturing defects are usually (but not always) relatively low time failures. After 40 years....it was something that happened AFTER BSA made the crank.... There is even that pesky thing called fatigue failure.....

I can stare at photos of catastrophic failures for HOURS.

Am I wierd?

Sound like two completely separate topics! The answer to the second is an unconditional Yes ...

Lannis

That is almost funny Alex. As if the oil pressure gage would asuage the gods of bottom end failures! What I want to know was were these twin carb or single carb bikes? (I have a theory forming in my head...) Is one more prone to bottomend failure than the other?

If it is a failure due to the removal of the proper fillet by the crank grinder, which is an important engineering detail if one wishes to be successful at this hobby, it is certainly not fatigue, but the lack of information or in most cases laziness or economic considerations on the part of the operator.

Most American crankshafts have little, if any radius ground into the bearing surface. American engineers just used mass to gain strength. Nearly all British crankshafts, because weight was a big consideration when these motors were designed, use this strength increasing feature. Without the large fillet the cranks are very prone to breaking.

Because the crank grinder has to remove a lot of material to put the wheel back in the original condition after putting the radius on the grinding wheel to grind a British crankshaft they often cut corners. That is they fail to radius the grinding wheel enough to produce a proper filet. Shops that do a lot of British work often have a dedicated machine to do the work so they don't have to constantly redress the wheel.

For the lack of a better technical term for not following the manufacturers specification for regrinds we often call it stupidity, not manufacturing defect.

The only reason I can see for the single carb bikes being less likely to blow up is that, since they do not rev as freely under load, they are less likely to spend as much time at high RPM. Some failures don't show up until you've got her pegged at 7500 rpm for a while. Also, wild-eyed boy racers would have been more attracted to the sportier twin-carb models and, hence, I would guess they are more subject to abuse.

...of course that's all just conjecture. As they like to say on that hard-hitting news show on the Simpsons: "May not have actually happened".

I have to say that over the years, on many forums, I've heard of more catastrophic BSA failures than Nortons, and more Norton failures (90% Combat related) than Triumph.

Considering the fact that I've been far more involved in Triumph forums than Norton forums, and more Norton forums than BSA forums, that only adds to the disparity in catastrophic failure frequency between these three particular "popular" British marques.

What do ya'll think? Is this observation valid?

No.

In other words which engine has the highest percentage rate???? I've seen more Triumphs seize pistons and more BSAs throw rods, Triumphs suffer more often from ameteur mechanicing, and Triumph and Bsa suffer from oil pump problems more than Nortons.
Single carb BSAs are the most reliable of the twins, the a50s lower gearing and not having enough power to hurt themselves, should make them the most reliable. Rick

Are you sure about that Alex?