A65 EN­GINE AD­VICE

Real Classic - - Letters - Barry Huron

I have sev­eral com­plete BSA A65s plus en­gines. I am piec­ing one good one to­gether for a Tribeezaha project; a 500 Tri­umph frame, BSA en­gine with RZ Yamaha wheels and forks, and I’m most im­pressed by the BSA unit 650 en­gine. How­ever, I can’t get the logic be­hind the com­monly-dis­cussed en­gine mod, that chang­ing the tim­ing side main bear­ing is good idea. Why is it that putting in a weaker bear­ing is con­sid­ered an im­prove­ment which will stop the left con­nect­ing rod from seiz­ing?

Con­sider some ba­sic laws of physics here. If the oil sup­ply to the crank fails then both rods will seize. A plain bear­ing is sev­eral times the strength and load bear­ing ca­pac­ity of a roller or ball type; all car en­gines use them. I have heard all the sto­ries about how the end float needs to be dead-nuts blueprinted, and how BSA’s de­sign of the plain bear­ing is all wrong. The sug­ges­tion is that you need to spend an amount of money equal to the value of the bike on a con­ver­sion to make it re­li­able enough to ride any dis­tance. And don’t even start the en­gine if you don’t have the spe­cial steel oil pump…

One of my en­gines has about .065” end play and the main bear­ing clear­ance is .009”. It must have run for quite a while to wear that much! I was look­ing at the en­gine with the head off and could not help but no­tice that the tops of the pis­tons did not match up...there was a .009” step be­tween them – but I’m more con­cerned with how much more I can cut off them to lighten them.

I got the en­gine cheap enough be­cause it had thrown a chain and bro­ken the cast­ing, which had then been cut away. So I got out my power grinder and cut, cut, cut and then clean, clean, cleaned. The motor looked like it might have had its oil changed some­time around 1970, so I made sure to blow out the gorp and grot to get the crank­case as clean as I can. Look­ing down in the crank­case, there are funny gouges. Hmm. The guys who busted these cast­ings out of the sand were a lit­tle long at the pub that day, eh wot!

Next I groove on the de­sign of the mounts (like, peace out man) be­cause I need to make a mount for the bench so I can jig the re­mains of the 500 Tri­umph frame around the BSA en­gine. This will use the front and top down­tube and cen­tre­stand mount, so I can use a bolt-on hard­tail sec­tion. (No it’s not a bob­ber. It’s a su­per light­weight street racer, geared for red­line at 80mph.) Look­ing at the bot­tom and rear mounts and the mount-mounts, I have an­other cold one while I ad­mire the en­gine de­sign and the chal­lenge it will of­fer to my en­gi­neer­ing skills. It’ll ei­ther be a tes­ta­ment to my abil­i­ties or per­haps I will prove Dar­win right…

Mea­sur­ing the bot­tom mount­ing bosses, I see num­bers stamped on each side of the cases and they are dif­fer­ent. I check my other Beez­ers and the num­bers on those cases are all the same. The trou­ble light (all me­chan­ics know why it’s called that) comes on over my head just like in a comic strip. I think about the step be­tween the two cases, the funny gouges in­side which, upon closer in­spec­tion, re­veal small cracks out­side. This motor has been grenaded and they re­placed only the left crank­case half. And even with a step like that it

still ran long enough to wear the main bear­ing. To me this sounds like a fan­tas­tic en­gine de­sign!

Get­ting fur­ther into the A65 en­gine, the trans­mis­sion ap­pears about 50% stronger than the ones in my Com­man­dos... and on this one there wasn’t even a spring fit­ted on the kick­start. Some­body had put a lot of hard miles on this BSA and it stood up to the abuse.

The cru­cial point for any­one re­build­ing an old A65 is that you should se­ri­ously con­sider pulling the en­gine apart and clean­ing the crank­shaft sludge trap. To my way of think­ing this is the cause of the bad rep­u­ta­tion of the A-se­ries en­gines. They were so good that no­body did the main­te­nance, so the trap filled and the left rod seized and some­body came up with the idea that they could make a liv­ing off of of­fer­ing a bear­ing con­ver­sion they said would cure it. I chal­lenge any­one to show me a BSA where only the right side seized!

This is not pe­cu­liar to BSA en­gines ei­ther. I re­call a re­cent ar­ti­cle in RC about a ma­roon 500 Tri­umph that took out the left side rod. All twins are sub­ject to this if peo­ple don’t do the main­te­nance. ‘Oh we won’t be rid­ing far... I change the oil of­ten... It’s so orig­i­nal… I wouldn’t want to spoil the patina... Fac­tory orig­i­nal, never been opened up...’

Well I hate to tell you: they need to be clean in­ter­nally. I am go­ing to take a close look at the drillings in my crankshafts to see if they can be mod­i­fied such that the de­tri­tus does not ac­cu­mu­late in the sludge trap. In­stalling a mod­ern fil­ter should do what the laws of physics in the form of cen­trifu­gal force have done in the past, with the ul­ti­mate re­sult be­ing that the bike re­minds the neg­li­gent user that they didn’t clean the fil­ter.

It is through bit­ter ex­pe­ri­ence that I re­late this wis­dom, hav­ing gleaned it by the cat­a­strophic fail­ure of the left side rod in my first su­per­charged Com­mando, a fail­ure I at­trib­uted to ex­ces­sive wrist syn­drome at the time. In ret­ro­spect and with retro-in­spec­tion of the crank this was not a fail­ing of the en­gi­neer­ing of the bike. It was a fail­ing of the pre­vi­ous own­ers who never cleaned it out.

These old Bri­tish en­gines were de­signed by peo­ple who were steeped in the tra­di­tion of pulling off the oil pan (sump) of their car en­gine, and pulling a few shims out of the bear­ings to tighten them up every cou­ple of years. By the 1970s that main­te­nance had dwin­dled to get­ting a tune-up every six months when they changed the points. Re­mem­ber get­ting a tune-up? We don’t even change oil now! This is a safety thing: I know what it’s like to be rolling on the throt­tle and groov­ing on the sen­sa­tion at 80mph – and then hav­ing the rear wheel lock. It is a path­way to a great spir­i­tual ex­pe­ri­ence…

Please con­sider this level of main­te­nance. No one wants to buy a wrecked bike from an es­tate sale.

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