BSA Bantam - Rebuilding the Ignition Timing Plate

The last major repair for the engine was to fix the worn-out points and points plate bushing. The reproduction points cam that I purchased had not been correctly finished, and had a sharp edge at the beginning of the cam lobe instead of a smooth curve. 

The points arm, literally eaten away by the sharp edge.

The points timing cam. The sharp transition is clearly visible.

A points cam should have a much smoother transition, such as the one seen here on a CB450 cam:

Smooth with no sudden transitions

I had a machine shop carefully smooth out the edge with a fine tool grinder.

Refinished cam lobe

The sharp transition is completed smoothed out

After fixing the cam, I had to replace the bronze bushing in the points plate. This bushing supports the end of the crankshaft and keeps it dead center relative to the points arm. It also prevent the crankshaft end from wobbling, which has been known to cause metal fatigue failures and ruin the crankshaft. I pressed out the oil bushing with a large screw and hammer:

Successfully removed

Brand-new bushing

Bushing installed.

After pressing out the bushing, I installed the new bushing in its place, using a large amount of Loctite 680 to lock it into place. I reassembled the points, re-timed the engine, and now it starts on the second kick every time.

DIY Engine Repair Part 2 - Spark Plug Thread Repair

It was a beautiful Spring day! Perfect for pulling the bikes out of storage and taking them for a quick shakedown run. And of course, when everything is going well, something must go wrong! :-P

While riding the Bantam, it suddenly lost power (and then had to push it 12 blocks home). I pulled the plug to check for carbon buildup, found nothing, and re-installed it. And then, the dreaded feeling of a threaded part tightening, tightening, and suddenly loosening >_<. I pulled the plug out and found this:

Look closely...

Closeup of the threads... no longer in the head of course

Mangled

I purchased a thread repair kit from my local auto parts store and got to work. First, I used the provided tap to chase the original thread and use it as a guide for the larger thread.

Once the plug hole had been retapped to the correct oversize, I applied the required silicone to the outside of the insert, screwed it onto an old plug, and placed it in the hole.

The final step is to permanently set the insert into the head. The kit comes with a steel punch that spreads the top three threads (which are serrated on the outside). By hammering the punch into the top of the insert, the serrated heads permanently lock into the head alloy material.

BSA Bantam - Rebuilding the Top End

The Bantam that I rebuilt developed two air leaks during the shakedown runs, so it was necessary to strip the top end and repair the problems. The leak on the exhaust side was caused by an incorrectly installed exhaust gasket (easily repaired). The leak on the intake side was caused by two factors: The rubber carburetor manifold had become dry-rotted, and the mating surface on the cylinder head was heavily pitted from the rust that I had sandblasted off. Replacing the manifold was easy, but smoothing out the mating surface required a bit more work.

Condition when purchased

When I purchased the bike, the cylinder was heavily rusted, and required sandblasting and machine work to repair. It was rebored, and the head surface was skimmed. However, I never had the carburetor mount cleaned up, and thought I would just seal it with silicone when the time came.

Pitted surface

To seal, the rubber manifold has a machined groove on its underside, and a rubber o-ring is glued into it. However, this system only really works on a perfectly flat surface, so I added HondaBond sealant to try to solve the problem. Eventually, this was sucked out by the engine vacuum, so it was necessary to find a more permanent solution. 

I disassembled the top end of the engine, cleaned off all the oil and sealant from the mating surface, and applied a coating of MarineTex 2-part epoxy to build up the surface. I scraped it flat, allowed it to dry, and then repeated that step two more times. Once the epoxy had fully set, and sanded the surface flat with 600 grit wrapped around a block. This filled in all the pits, and gave me a perfectly flat surface to seal out the air. I then installed the new manifold, and went to the next step.

Sanding the surface flat

Ready for reassembly

Piston and rings

Because of the air leak, the piston had suffered some minor scuffing on the skirt (soft seizure). Before I could reinstall it, I had to smooth the skirt to prevent further damage.

This can be fixed by lapping the piston. Using 1000 grit sandpaper soaked in motor oil, I gently sanded down the damaged section, and also smoothed out all the rough edges on the rest of the piston to cut down on friction.

Light scuffing

Smoothing the entire piston with 1000 grit, with focus on the seizure marks.

Fully smoothed and ready for reuse

Rings reinstalled

Back together 

BSA Bantam - Testing the 12 Volt Ignition System

I was finally able to take some video of the BSA running with the new ignition system. Enjoy!

BSA Bantam Ignition Test

BSA Bantam - Repairing the Engine Cases

Backtracking on the BSA project a bit, the biggest obstacle to repairing the engine was the enormous crack that had developed in the crankcase.

 

No one I spoke to had ever seen a crack in that position before, and our best guess was that, over the years of sitting without a piston or cylinder head, the case had filled with water, froze, and split at the seam. Regardless, The case could certainly not be used like that, especially since 2-strokes rely on a pressure-tight bottom end to function correctly. I brought the case to a local welder to have the crack patched up, but the results were not...encouraging.

 

 

 Either there was still oil contamination in the crack, or the welder wasn't familiar with the material, but the results were extremely sloppy and porous. I ground down the inner weld only to find numerous blow-holes and voids. It was only a lack of available donor cases that kept me from scrapping this part entirely. I spent a week hand grinding and filing the weak welds down:

 

 ...and then was able to find a different welder who specialized in cast aluminum repair. His results were a huge improvement, and wiped away most of the irregularities left by the previous guy. I fixtured the case into my ancient milling machine to clearance the inside of the case for the crankshaft:

and decided to leave the outer weld bead alone. The case is so thin at this point that I decided the extra strength and material was more important.

BSA Bantam - Homemade 12V Ignition System

When I bought my Bantam Trail Bronc it was, lets say, a bit rough:

50 bucks, but the tetanus was free!

At that point, maintaining originality wasn't my concern. Just getting it running again would be a big enough hassle, so I left it mostly as is, and gave it a full engine rebuild and some new tires.

I won't get into the details on the engine build today, because that could take up several posts. I could write for pages about that job, so instead, I'm gonna focus on the modernized electrical system that I installed.

The original system was a low output flywheel ignition coil. I replaced all the bits with aftermarket parts, and was able to get the thing running:

BSA Bantam First Start

And then, after that? Nothing. The bike never started again. It sat in my garage for a year while I rechecked, modified, and adjusted every possible part. I never suspected the coil because, well, it was brand new, and the plug sparked whenever I held it against the case. Then, 3 weeks ago, I tried the same process yet again, holding the plug against the case and kicking it over. Amazingly, at that exact moment, the coil which had apparently been internally shorting all along completely died and produced no more spark. Eureka!

It turns out that if the output is too low, a plug that sparks freely when held in open air may not necessarily spark inside the engine, where the gasoline wets it down. Before rebuilding, I wanted to precisely set the spark timing, but do away with the need for a timing disc. I modified a used spark plug with an adjustable screw and a soft aluminum cap and then inserted into the plug hole.

I carefully turned the crank until the piston hit the plug, and drew a reference line on the flywheel aimed at tab on the case. I then repeated this in the other direction, and punched a reference line between the two pencil lines. This marks top dead center.

 Next, I marked the location of the proper firing position, about 16.5 degrees before TDC, and marked that, as well as labeling it.

Next up was the electrical bits. I stripped out the coil and magneto core, and reused the stock points and condenser with new wiring.

I used these to trigger an aftermarket 12V Yamaha ignition coil that I mounted next to an original Bantam battery tray. I wired it all up and used a miniature hobby 12V battery to power the whole system.

Now it will reliably start on the second or third kick. I will have to take some video as soon as possible.