Britbike forum Forums British Motorcycle specific Brands Bsa bulletin board horsepower from a A65

I don't know about the stock timing side plain bearing material, but for 660 bronze, Pmax is 4000 psi and vmax is 750 ft/min. This translates to 6000 lbs max load and 22900 max rpm for the 1.5" diameter bearing. So, load capacity and speed are not the problem. The two commonly cited problems with the plain bearings are: lack of an adequate clean oil supply and centrifugal forces preventing oil from reaching the rod bearings. The first can be cured by filtering the oil and making sure the oil pump and relief valve are in good working order. The second is a lot less clear than a lot of people make it out to be. The concern about loss of pressure is based on a very calculation that static pressure is supplied statically at the periphery of the main bearing and there is no flow or centrifugal pumping from the path to the rod bearings itself. Fluid models that I have run have actually shown the opposite to be true, though I must admit that the model needs to be refined more. Still, even high-rpm automotive race engines that run at even higher rpm with even larger main bearings still seem to work. I wonder how that is?

I think something else comes into it, most modern plain bearing engines tend to have narrow bearings of larger dia, with large oil delivery drillings. Plus they are split, made up of two shells that fit well into main caps. These shells are easy to achieve close and precise tolerances, with the long bush on the BSA its difficult to get and to measure. If you measure the crank up and down in the bush at say .001" it could be cocked and sitting on corners, possibly opening up a good oil path to hose oil out the sides. What it does at high RPM it would be interesting to see if the crank flexed a little and did the same thing. There is definitely something about it that can cause problems, usually from what I've seen its rod bearings failing.

I have been following this thread with interest. I think that Mark hit on the very problem with BSA bottom ends. Even if you precisely clearance the bearing to crank at .0015", if the bearing is even slightly cocked or the bearing is not perfectly Concentric, you will suffer premature wear and the oil will soon squish out the sides of the bearing and oil pressure drops quickly. Then the rods are in trouble. The only cure for this is to align bore the bush very accurately. The setup must be made with the two cases bolted together. SRM uses a special mandrell they have made that assures that the bores are aligned within a couple of tenths before they fit and clearance the bushing. This can also be accomplished on a jig boring machine or even on a good milling machine in the hands of a skilled machinist. BSA did not do this nor did they suggest this procedure in their manuals. The simply fit crank to bush with the proper clearances. Many engine builders simply grind the journal to fit the bush, but that will result in short life if the bores are not truely aligned.

Now if the crank flexs at high RPMS all bets are off. The only way to fix crank flexing, if indeed it occurs, is a center main bearing. I have no way to know if it flexes, but I suspect it does at high RPMS and others more knowledgeable than me think that it does flex. I just don't know for sure. The journal sizes on the A65 are adequate, and with clean oil, near perfect crank alignment and acurate clearances the motor will give satisfactory performance and life. The various roller conversions are also good solutions but the plain bearing done correctly will also work.

Mr Mike

"The only way to fix crank flexing, if indeed it occurs, is a center main bearing."

- Or a hardened or aftermarket crank. This was a basic requirement for a successful A65 flattracker "back in the day" (road racers too I'm sure).

I also recall that crank flex was not an issue for the Yammie twins, which do have a center bearing.

I once spoke with an A65 engine builder from New Zealand who said that sometimes the fore-aft play between the two halves of the crankcase is as much as ten thousanths, and that he sometimes had to precision-dowel the cases before fitting and reaming the main bush. BTW, this builder's name is John Cochran. (If I'm going to quote someone, I figure I should give credit.)

On the other hand, the guy who did both of my A65 lowers, was a "seat of the pants"-type mechanic, no exotic tools or equipment, and both engines (the lowers, anyway) have been trouble-free for many thousands of miles. But I don't rev to 7 or 8K, and I change oil about every thousand miles.

Hey Mark,
If in fact the cranks do flex, even a hardened crank will not prevent premature bushing wear if there is misalignment. A hardened crank is not a bad idea anyway. I think at reasonable rpms flexing is not likely an issue but I am sure no expert on this. I am a big proponent of doweling cases to maintain alignment and in fact I decribed a method my brother (a tool and die maker by trade) suggested in a previous post. It uses simple shoulder bolts and it easy to do on a milling machine or a vertical boring machine.

I hope I live long enough to run my A65 motor 50,000 miles or more to see if my efforts on the bottom end pay off, but I do have a preference for the singles and with two in the shed, the A65 gets ridden the least.

I would like to hear from Alex who has a daily runner but also a racer that he thrashes...both are plain bearing motors. Don't get me wrong, the bearing conversions are good way to go, but a plain bearing rebuild can be solid if you get that alignment right and have good oil pressure and clean oil.

I have done a test setup of an alternative oil pressure relief system with a gauge for early A65 models, but I have a few things to still work out before I report on the system.

Mr Mike

And no one has mentioned an oil pump this is adequate (barely) on the late motors. The early oil pumps, don't go there.....

For a street motor, the volume produced by the late pump is just adequate. For extreme use, it is probably marginal at best.

Everything flexes, just a question of how much. Hardening the crank does very very little to change the amount of flex for a given load, only the ultimate strength. The material would have to be changed to make any change and that would be small unless the material properties are significantly different. The biggest change would be larger journal diameters or moving the load closer to the bearings. Moving the counter balance to the outer webs would be an improvement. Dan Macias made a special crank for the Triumph twin with this modification and reduced the vibration.
BSA had a bush reamer that was piloted in the drive side bearing and reamed the timing side bush with the cases bolted together. Probably one of the best ways to get the bushing aligned.
Mark, was that 0.010"? That is a whole lot, equal to 0.00146" across the main bush width.
The XS650 twin has four crank bearings, one outside each web. The drive side bearing has approximately the same I.D. as the A65 but 10mm larger O.D.
I previously posted a graph of the main bearing oil flow requirement versus clearance and the pump theoretical output. I just ran through the Machinery Handbook calculations for the main bearing at SAE 30, 40 and 50 oil viscosity. As you would expect the thicker oil has more film thickness and uses more horsepower but it also causes higher oil temperature rise as it goes through the bearing, more than is recommended for the 0.0015" - 0.003" specified clearance. A lot of factors go into the calculations besides the bearing dimensions.
I need to go through the rod requirements and add them to the main bearing to see how close the pump flow to bearing requirements are.

DM,
I agree that a piloted reamer with the cases bolted together would be a very good way to do it. I doubt many shops had this tool. I think most shops reamed in-situ or ground the crank to fit. Neither will solve the problem if the bores are misaligned.

The calculations on required oil flow would be good to know. I think my 40 year old machinery manual takes one thru it. I would think that BSA engineers knew how to calculate this. And I think as long as the bearing clearances are withing limits, the pumps are likely to be adequate from a volume standpoint but they are in fact chintzy pumps, the early ones in particular. I think assembly at the factory was poor and that's where the problems started. It's a shame. Dealerships probably did not have the equipment and knowhow to fix the problems. So as a result, there are a lot of BSA twins around with bad motors and low mileage. You never see one with more than 10,000 miles, but that may be due to the Smith' speedo's.

Mr Mike

The problem with reaming is that it leaves a lousy surface finish and it's hard to control the diameter very well. I am now set up to align-hone these bushes and will probably be doing my first one in the next month or so.

Now, as for the crank flexing taking up the clearance of the bearing, I doubt it. Consider a bearing clearance of .001". This means that the journal has to tilt.002" over its length of approx. 1". To do this will take 1200 lbs of force at the center of the crank say, from centrifugal forces of the flywheel but, anyway it's worst-case scenario, it will take even more force at the rods. I got about the same answer from a simple beam model and FEA. Even considering the possibility of dynamic loads or crankshaft vibration, that is a LOT of force to generate. This equates to about 10,000 ft/lbs of torque at the crank. I don't think it's possible to generate those kinds of loads internally. Besides, in simulations, the crank itself started to yield at about a 10th of that torque...totally apart from components downstream.

Again, I have no reason to believe that the "problem" with these motors is anything other than clean, reliable oil flow and properly machined bearings/journals.

Quote: I have no reason to believe that the "problem" with these motors is anything other than clean, reliable oil flow and properly machined bearings/journals"

I agree with those two 100%. I think the early pumps are chintzy and problematic although the original in my 66 makes good oil pressure. I just don't like the fact you put finger tight torque on the bolts to keep from binding the pumps. A nice billet pump would be the ticket. I'll have a oil pressure gauge on mine before long so i can keep an eye on things. First time it hiccups I'm gonna break the bank and find the best opump available.

Mr Mike

The crank does not have to tilt. Force on the piston will push it down into the bearing. Indicator diagrams for a Cooperative Fuel Research engine show the cylinder pressure reaching 500 PSI at 1200 RPM, full throttle 40 degree spark advance. That translates to 3333 lb force on a 74mm piston. Peak pressure is at 16 degrees ATDC so the downward force on the crank is 3204 lb.
Going through the Machinery Handbook calculations for journal bearings I plotted the following for the oil film thickness and flow. The lines with symbols are 500 PSI the other are 900 PSI peak cylinder pressure. SAE 50 oil at 180F and 50 PSI.

The minimum recommended film thickness is 0.0001". You can see for 500 PSI cylinder pressure at 0.0015" clearance diameter it is at the limit. More pressure (horsepower) pushes the crank below the 0.0001" limit. The rod does not have the centre groove along with the larger diameter so it has more capacity. If the main bearing were full width without the groove and end fed for the rods it would have 22% thicker film at 0.0015" clearance.
You can also see why clean oil is imperative. 1/10,000" is not a lot of clearance for dirt. The rod bearings have the advantage of the centrifugal sludge filter.
The oil flow appears to be more of a problem. With the crank and both rods at 0.00225" clearance the flow requirement is at the theoretical limit of the pump.

wow it makes you wonder how there are any a65s left !!!! all i can say is iv had a65s for 30 years and had the pleasure???? of rebuilding 75+ engines mainly ones from the jordon army in the 80's and very few of them had bush issues, use a genuine bush etc and you wont go far wrong.

Thanks DM,
You must have just finished a course in fluid mechanics! I dropped that course about 45 years ago. Were you calculations based in the earlier lower volume pumps or the newer ones? Your work clearly supports proper clearances, the best pumps available and a good full flow filter.

Mr Mike

"Your work clearly supports proper clearances, the best pumps available and a good full flow filter."

Duh......seems that has been said for years......but it falls on deaf ears....

DM, yes, that's mostly correct...as a worst-case scenario. You're talking instantaneous load, which is spread out by the inertia in the rod, piston and crank and damping in the film. That stuff can be difficult to model accurately because you have to make a lot of assumptions and approximations. The conclusion, however, remains the same, particularly if you're going to ask more of the motor than it's original design intended: Clean oil in adequate quantities and proper bearing setup.

Well this thread is about horsepower so I'll put some graphs on, they would need some sort of correction factor to match with normal rear wheel dynos but they do compare to themselves, and show what different things do.
These are datalogger graphs, A65 with norton crank at 818cc started testing with 48HP and proceeded to fiddle with tuning to finaly get a best of around 57BHP(though its not this graph) then added big bore cyls 883cc graph in blue, the dates are on the bottom so was a while ago.



This second graph has some mods to push power up the RPM, offset rocker buttons for more lift and C/molly pushrods. This was with first alloy cyl with liners.


Second alloy cyl with Nicasil bores and 'D'd inlet ports is noticably better but I don't have a decent graph because the computer lost the couple I had. This is a speed against time graph from one, and shows 1st and a bit of 2nd gear. 1st gear isn't the best gear because the motor doesn't spin up fast, its probably a bit heavy in the crank, but the higher gears have some really nice zoom in them when its going a bit, and uphills are a good place for it.

The load is not quite instantaneous. The cylinder pressure rises from ignition and then decreases as the cylinder volume increases on the down stroke until the exhaust valve opens. Each bang has to impart enough force to turn the engine over one revolution with power being taken off for exhaust, intake, compression, friction and drive loads. The average is not very high but the instantaneous is and the bearing has to support this load.
Plonking around at road speed does not put a big strain on the bearings but when you hammer it, it does. I think that was the start of this thread - horsepower.
Fixing the bearing dimensions, the only remaining variables are load, oil inlet temperature, viscosity, pressure and RPM. If the RPM is reduced from 7500 to 6500, the bearing oil volume decreases by 9.5% with 180F, SAE50, 50 PSI oil.
The A65 pump volume was measured by scanning the gear, drawing a curve around the edge of the tooth space, measuring the area in CAD, multiplying by the gear width to get one tooth volume and multiplying by the number of teeth. It is optimistic as the meshing teeth do not fully clear the volume and there is no leakage. I think the pump was a '70.

"Plonking around at road speed does not put a big strain on the bearings but when you hammer it, it does."

Lugging the engine actually is the worst case scenario on loading. Good way to murder an A65.

I did it the old fashioned way to calculate pump flow. Did measurements and calculations and used a 70% efficiency factor. I was also a late pump (DD or cast iron gears as I remember). The flow is adequate for the bearing size and typical RPM of a street engine. The pump is on the edge at 7500 RPM for sustained running. Everything better be perfect to run at a sustained 7500 RPM.

Yes, but I said "plonking", not "lugging".
I was not aware that they used cast iron gears in any of the pumps. The '70 and '71 had steel.
If even the early pumps were as bad as some people claim when used under normal riding I doubt BSA could have been able to advertise that "One in four is a BSA" unless they were referring to dead bikes found on the side of the road or FORD. Racing is a different bowl of cuttlefish.

I was refering to body material or stampings, D, DD, or cast iron. The gears were the same on all late pumps.