1971 BSA Thunderbolt

[fredintoon]

- - - The motor seems to be free, so I'm going to drain and refil the oil and see if I can get it to fire on some starting fluid. Hope to have a chance to work with it this weekend, but we'll see. I'm hoping to get it up in decent running condition and eventually tear it down and give it a nice restoration. Nothing award winning, but I think it'll be something to be proud of when it's done!

Hi Slow Hand,
when you get the bike running, be gentle with it, the bottom end is kinda frail.
The A65 engine was developed from the earlier A10 engine; the top end was re-worked to give another 20HP but they didn't bother to upgrade the A10 bottom end.
Same (admittedly large) ball race drive side main bearing.
Same lead-indium rod end shells.
Same plain bushing timing side main bearing.
Same massive unsupported central flywheel.
Same really pathetic oil pump.
And while I've never ridden an oil-in-frame BSA, if the frame's oil filler cap is hidden under the dual seat nose like an oil-in-frame Triumph they've scooped the seat base out to clear it so the seat will pound out your prostate as you ride.

[James England]

Hi barrelguy,
we used to install Lucas electrical equipment on Bristol aero-engines and that stuff worked perfectly because we were willing to pay top whack for top quality.
For the sniveling pittance that British vehicle mfrs were willing to pay it's a tribute to Lucas that their vehicle electrics worked at all.

Exactly! Short term thinking and cheapness. Hardly a recipe for success...

[James England]

Hi Slow Hand,
when you get the bike running, be gentle with it, the bottom end is kinda frail.
The A65 engine was developed from the earlier A10 engine; the top end was re-worked to give another 20HP but they didn't bother to upgrade the A10 bottom end.
Same (admittedly large) ball race drive side main bearing.
Same lead-indium rod end shells.
Same plain bushing timing side main bearing.
Same massive unsupported central flywheel.
Same really pathetic oil pump.
And while I've never ridden an oil-in-frame BSA, if the frame's oil filler cap is hidden under the dual seat nose like an oil-in-frame Triumph they've scooped the seat base out to clear it so the seat will pound out your prostate as you ride.

No amount of routine maintenance can compensate for that lot, eh? Add 20hp at the top end and leave the bottom alone. Sounds about right.... You reminded me of the oil pump thing.... They are pathetic on the Bonnevilles too..

They are now re making Matchless engines in the UK. They cost at least $3,000, I believe.....but is roller bearings, caged needle rollers etc etc..... They've dragged the engine into the 21st Century...at a price.

[James England]

Years ago I was talking to a former Matchless dealer from Toronto, this is his story:-
" I was in England at the Matchless/AJS factory.
I asked the boss what he knew about Yamahas as someone had opened a Yamaha dealership just down the road from my place.
He said, I gave an apprentice money to go buy one and we brought it back here to tear it down and check it out.
That little bike is a jewel and it'd cost us more to build it in our factory than what they are selling it for retail.
I think we're f**ked."

How right he was. In our inimitable British fashion, the British bikers referred to the new kids on the block as "Jap C**p". I remember a British bike spares place, Barry Ladlow of Laceby, putting a sign out on the front of his building saying "No parking for Jap C**p". Having bought spares from him for years, he grudgingly acknowledged that my XS1100 parked where it shouldn't be "wasn't bad". Praise indeed

[Slow Hand]

Yeah, I wasn't planning on beating it up at all. Figured its be a fun, cool old bike to tool around on. I have heard the BSA 650's needed to be revved higher to keep oil flow up. I've also heard that the bush vs bearing deal comes into play on higher mileage motors. Unfortunately, with no gauges I have no idea how man miles this one has on it. He rode it for ten years before he wrecked it. He also had a '78 gdwing he had for a out ten years and only put 24,000 miles on it, so I doubt the BSA is super high mileage.

Hi Slow Hand,
when you get the bike running, be gentle with it, the bottom end is kinda frail.
The A65 engine was developed from the earlier A10 engine; the top end was re-worked to give another 20HP but they didn't bother to upgrade the A10 bottom end.
Same (admittedly large) ball race drive side main bearing.
Same lead-indium rod end shells.
Same plain bushing timing side main bearing.
Same massive unsupported central flywheel.
Same really pathetic oil pump.
And while I've never ridden an oil-in-frame BSA, if the frame's oil filler cap is hidden under the dual seat nose like an oil-in-frame Triumph they've scooped the seat base out to clear it so the seat will pound out your prostate as you ride.

[fredintoon]

Yeah, I wasn't planning on beating it up at all. Figured its be a fun, cool old bike to tool around on. I have heard the BSA 650's needed to be revved higher to keep oil flow up. I've also heard that the bush vs bearing deal comes into play on higher mileage motors. Unfortunately, with no gauges I have no idea how man miles this one has on it. He rode it for ten years before he wrecked it. He also had a '78 gdwing he had for a out ten years and only put 24,000 miles on it, so I doubt the BSA is super high mileage.

Hi Doug,
the oil is pumped into the timing side bushing then enters drillways in the crankshaft to feed the rod end bearing shells. If the oil pump is worn &/or the timing side plain bearing is worn there won't be enough oil to keep the rod end bearings properly lubricated.
Drive it gentle and she'll be fine.
There are (appallingly expensive) aftermarket upgrades, a better oil pump and a roller bearing & seal conversion, to improve matters.
Then you can worry about the brakes; pretty good for back then, barely adequate for modern use.

[mharrington]

I will say the BSA engine is wonderful looking! So is the bike. My other bike is an XS650. Cool fun little bike. I just learned the XS650 twin is a German design- Yamaha acquired from Showa whom acquired from a German company. Its nice to have a lumpy twin and a gutsy big inline 4! I hope the BSA brings you lots of enjoyment!

[James England]

I will say the BSA engine is wonderful looking! So is the bike. My other bike is an XS650. Cool fun little bike. I just learned the XS650 twin is a German design- Yamaha acquired from Showa whom acquired from a German company. Its nice to have a lumpy twin and a gutsy big inline 4! I hope the BSA brings you lots of enjoyment!

my mate bought an XS650 in 1976. I had my Bonneville. I never told him but.......his XS650 was so much better than my Bonnie! Back then, the XS was so inexpensive too. A lot of well designed bike for the money. Have you heard of the mod you can do to make the pistons not synchronised? Was reading about it the other day.

[jetmechmarty]

Have you heard of the mod you can do to make the pistons not synchronised? Was reading about it the other day.

Typically, they switch them from 360 to 277. It's a common modification. The 277 likes to rev.


This one belongs to a friend of mine. It's a fun ride!

[James England]

Typically, they switch them from 360 to 277. It's a common modification. The 277 likes to rev.


This one belongs to a friend of mine. It's a fun ride!

like it. So what does the mod actually do then?

[jetmechmarty]

like it. So what does the mod actually do then?

With the 360 engine, the pistons go up and down together. They fire on opposite strokes.

With the 277, one cylinder fires while the other is at about mid stroke.

Riding the XS650 with the 360 engine you get more and more vibration as RPMs increase. With the 270 engine it get smoother and smoother as you add RPMs. They just have different characteristics is all.

[James England]

With the 360 engine, the pistons go up and down together. They fire on opposite strokes.

With the 277, one cylinder fires while the other is at about mid stroke.

Riding the XS650 with the 360 engine you get more and more vibration as RPMs increase. With the 270 engine it get smoother and smoother as you add RPMs. They just have different characteristics is all.

The Bonneville is the same...pistons move together and fire opposite strokes. Which is what makes them shake to bits. Didn't the XS650 have a counter rotating shaft driven from the crank? No such thing on the Bonneville of course. I wonder if the same mod could be done on the Bonneville then?

[jetmechmarty]

The Bonneville is the same...pistons move together and fire opposite strokes. Which is what makes them shake to bits. Didn't the XS650 have a counter rotating shaft driven from the crank? No such thing on the Bonneville of course. I wonder if the same mod could be done on the Bonneville then?

No counter shaft. The Bonneville shakes because it is very light. The XS650 is heavy. Everything is rubber mounted to the frame. That's the big difference.

Also, just for fun, the Triumph has pushrods, overhead cam on the Yamaha. The Triumph is dry sump, the Yamaha is wet sump.

[James England]

No counter shaft. The Bonneville shakes because it is very light. The XS650 is heavy. Everything is rubber mounted to the frame. That's the big difference.

Also, just for fun, the Triumph has pushrods, overhead cam on the Yamaha. The Triumph is dry sump, the Yamaha is wet sump.

Aha! Big differences. Hell yes, I'm so used to seeing rubber bushes etc that I forgot how the engine was metal to metal in my Bonnevilles. Shudder....

[jetmechmarty]

Aha! Big differences. Hell yes, I'm so used to seeing rubber bushes etc that I forgot how the engine was metal to metal in my Bonnevilles. Shudder....

The XS650 engine is solid mounted too! The handlebars are rubber mounted. The foot pegs are rubber mounted. The seat is rubber mounted. Rubber bits everywhere! I'm not even kidding!