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BSA A65 Thunderbolt

John Macdonald

We can't stop here, this is bat country!
Mar 18, 2022
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Looks like we don't have an A65 thread here

I have a bike in the workshop that needs to be sorted out and put on the road

Anyone here with BSA experience?


That was from the ride home from the spot where it had been sleeping for over 20 years

Currently it's apart, motor on the bench waiting on disassembly for cleaning out the sludge trap and a thorough freshen-up.
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I'm getting things squared away for a start on this project after a long wait

Ordered some British hand tools as well as some bits and things for going forward.

In the next couple of weeks I'll have the top end off the bike for a look, then back together to get it on the trail.
Tracking says that I should get the British hand tools today

Might even get the top end off that engine before the weekend

We shall see...
A little scraping work on the piston tops shows First Oversize, or 0.020" over original bore


The cylinder bores agree with this detail, and are well within spec for another go after a date with the flex-hone and a fresh set of rings
There was evidence of oil seepage from the head gasket, and it's appearance indicates blowby between cylinders and to the sides


The gasket is solid copper, and could be re used after an annealing

As I will be going further into the engine, I might end up with a fresh one in a complete engine rebuild gasket kit

Crankshaft end play is supposed to be within 0.0015" and 0.003"
Preliminary inspection shows more than that, so the cases are going to be split and the crankshaft is coming out for shimming, fresh big end bearings, and a clean and service of the "sludge trap"

More to come later...
Sometimes you have to make the tool

I got the clutch / primary drive off this afternoon


That clutch basket mounts on a keyed taper
Needs a special puller

There's more than one configuration for pullers for the A65 series, this is considered an early engine

The puller threads into the clutch hub
Thread for this early clutch is 1"x24tpi
Cool thing is that 1"x24tpi is common to some found items in 2-wheel piles of gear.
It's a standard for the steerer tube of bicycle forks (and some motorcycles too) - and as I discovered it's standard for the little push scooters that are littered around the ranch

I found one that the kids had grown out of and repurposed the threaded end of the steerer tube, took a long coupling nut and a mating hard bolt and made the puller here at the welding bench instead of ordering it and waiting for it



Once threaded into the clutch hub, the bolt is driven in to contact the shaft stub, then as it's tightened it drives the hub off the taper

Worked great
That's all I'm doing tonight, weather is hot out there and I'm calling it a night
Getting closer to a parts order
Today I had a bit of time to spend on getting the timing side apart


This is a good waypoint, as from here I can mount the engine in the milling machine for a solid and rigid setup with the gages to positively measure radial play and end float on the crankshaft and the camshaft
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Got back out there and set the engine into the milling machine vise


This gives me a solid mount for the engine while putting a bit of pressure across the frame mounting lugs, while also having a rock solid mount for a gage to indicate the crankshaft end float.


It's important to know exactly how much end float is present, so that figure can be factored into an adjustment of the crankshaft bearing shim

Allowable end float for a rebuild is 0.0015" - 0.003"

I got an initial reading of 0.0105"


I'll be revisiting this setup later, measuring again with a different holder for that indicator as I don't completely love those flex-arm holders. It was handy at the moment.

I'll also be setting up to measure radial play for that crankshaft at both ends, and also the camshaft.

So far I don't feel any radial play at all, and I hope to find the next measurements to be within spec.
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I got out for the weekend to help with the Virginia City Grand Prix races, doing corner marshall work at the 8-mile trail junction

That was a hoot and a half

Saturday was Open/Pro, Sunday featured the vintage class

I might just go full-monty on the Desert Sled mode of this service and facelift and take this bike out for next year's Vintage class race

That could be fun
I had a A65 back in 1965 so my BSA experience is forgotten. I traded it in on a Suzuki X6 Hustler which was so much more reliable than the BSA it was unbelievable.

Here I am in 1967 on my way riding from Houston Texas to Belize to visit one of my sisters living there.


Camped out most of the way down and back.
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