With the engine now containerized like some dodgy artifact in a museum, it’s my turn to review the pile of bits and start plotting the grand rebuild. First order of business: order the parts we think we know need replacing—because nothing says “confidence” like putting quotation marks around the word know—and then begin the glamorous job of cleaning decades of grime.
Some of the parts are on backorder, of course, but only 2–3 weeks. Which, in the world of vintage motorcycle restoration, is practically same-day delivery. Either way, they’ll arrive well before reassembly—assuming I don’t lose half the bolts down the garage floor cracks first.
Piston Rings
I wish my Father-in-Law was still around to fill me in on the “stories.” Funny how time slips by and it’s the little details you miss—the daft repairs, the shortcuts, the why on earth did you do that? I’m left piecing it together like some forensic mechanic, reading the tool marks and spotting the non-standard bodges. The piston, for instance, was definitely swapped at some point. Back in the early ’60s, that was no big deal—most bike owners knew how to change pistons and rings. Getting more than 30,000 miles out of one was pure fantasy. These days, engines happily chug past 100K, and most owners think “maintenance” means changing a lightbulb or trading the whole bike in for a shinier one. The art’s been lost, along with the smell of burnt oil baked into your fingernails.
Here’s Mario checking the piston-to-cylinder gap with a set of shims. We could squeeze the piston in with 0.003″, but it refused to slide through. At 0.005″ it was just loose enough to remind us we weren’t working with Swiss watches. That gave us our working range.
The plan now is to hone and clean the cylinder, which, of course, will probably nudge the gap a little wider. Nothing like a bit of suspense to keep the rebuild exciting.
And this is where the “science” (or, more accurately, years of stubborn trial and error) begins—with the rings. The piston is a +020, the first step up from factory STD. Back in the day, BSA gave you STD, +020, and +040, before politely suggesting you re-sleeve and start again at STD. Since this engine was running well enough before, this is really more of a maintenance check, made necessary by the 55+ years it spent sulking in a shed. The cylinder only needed a light deglaze/hone, and the piston itself was still in good nick. But either way, there was no way we’d reuse the old rings—they’ve had their day.
This piston has three grooves: the top two take identical rings, and the lower one is for the “oiler,” complete with a dainty little spacer. (See linked piston image for your daily dose of vintage metallurgy. – My piston diagram)
Here is a link to a youtube that explains a lot, but you may not have time or interest as it’s getting deeper into the advanced mechanic / engineer details.
Piston RING GAP – HOW and WHEN to adjust it + GAP CHART – BOOST SCHOOL #6
The rings, in case you missed piston school, do two very simple but life-saving jobs: they dump excess heat into the cylinder wall (since this engine is air-cooled and couldn’t chill a pint if it tried), and they keep combustion gases from sneaking past the piston like freeloaders at a football match. Trouble is, metal isn’t really solid—at least not when it’s getting roasted alive—so you have to allow for expansion between “sitting pretty” and “running like hell.” That gap will close up as the engine heats, and if you don’t leave one, the ring ends will crash into each other, buckle up or down, and shove against the ring lands. I’ve seen the aftermath: entire pistons turned into confetti. The last thing an engine wants is chunks of feral metal bouncing about in the combustion chamber.
So, instead of +020 rings (which might leave us with a comedy-sized gap), Mario suggests we go with +040 rings and file them down to fit. Custom job. His rule of thumb is about 0.002″ per inch of bore diameter. The ring manufacturers, naturally, insist on 0.004″ per inch. Who’s right? Neither, both, and somewhere in-between. Like most things with these bikes, it’s a cocktail of theory, trial and error, and the grumpy wisdom of mechanics who’ve been at it since the ’60s.
For now, I’m holding off on ordering the rings until we’ve cleaned and honed the cylinder, so we can actually get accurate measurements instead of guessing and praying.
To the left, you’ll see the clutch pressure plate—posed with four driving plates stacked up like pancakes and five driven plates sulking in the background.
I gave the driven plates a spa day with some very fine 140-grit sandpaper, just enough to scrape off 55+ years of fossilized muck. The driving plates still need their cork thickness checked, but at a glance they look fine—like they could actually do their job without bursting into tears.
Word on the street (or at least on the internet, where truth and nonsense live side by side) is that finding good-quality clutch plates these days is a right pain. So the fact mine cleaned up nicely feels like winning the lottery—minus the money, glamour, and champagne. The bright side? They’re dead easy to remove, replace, and adjust… or so I say now, before future-me learns otherwise the hard way.
Shiny Wheels (Rims + Spokes + Hubs)
When I was scrounging for parts, I hauled my wheels up to Walridge to see if any of the $109 specials they had lying around would fit my bike. Spoiler alert: they didn’t. But after poking about with Mike, we confirmed both of mine are WM2-18 rims. Naturally, they’ve got different center hubs and probably demand different spoke packages—because BSA never missed an opportunity to complicate a simple job. Forty spokes apiece, just to keep me busy swearing.
A quick bit of math (or masochism) tells me new rims are $149 each, plus about $85 for a spoke set. That means I’ll need to strip the hubs, restore them, and then rebuild the wheels from scratch. And let’s not forget to measure the hub offsets—usually 3/16″ or 1/8″ from centerline—because nothing says “fun weekend” like trying to true a wheel that’s fighting geometry itself. All in, it’s about $250 per wheel, plus elbow grease and a steep learning curve. The old rubber might do for a couple of trial runs, but after that I’ll need new tires unless I fancy sliding into a ditch on 50-year-old rubber.
Tools – Construct
Up front, the job is to replace the seals; in the rear, it’s mostly about polishing up the chrome “decorative” bits—which is just BSA’s way of saying extra parts to rust and annoy you. Of course, both ends require some bizarre, special tool to disassemble. Because why would you use a wrench when you can invent medieval torture devices instead?
So, off I went to the Metal Supermarket and picked up a couple lengths of pipe to sacrifice in the name of engineering. The plan? Start grinding and hack-sawing away until the tool I need finally emerges from the scrap, like Excalibur from a block of steel. More details to follow—assuming I don’t saw through something vital, like my patience.
The Adventure shall continue….