1956 BSA B31 Scrambler Rebuild – Part 9

Philip Rashleigh has worked for months to turn a big pile of bits into a replica Gold Star scrambler. Getting the engine sorted out hasn’t been entirely straightforward…

This is the interesting part. It’s a long and sometimes confusing story. I think I had better start by telling you how I had planned to build the engine in the first place, and then the changes that took place and why, and then we will move on to the actual build.

What did I start with? A boggy standard 1955/56 B31 350cc motor. I had planned to keep it this way while I built up the bike but… you know how it is; you get ideas and these eat into you. I have never been happy to keep any engine I have ever owned in a hobby vehicle as it left the factory. A T100, Mini and Land Rovers have suffered in my hands from getting more power out of them. So it seemed only natural to start thinking about getting the capacity up a little. The humble B31 can be taken out too 400cc or thereabouts — but why stop there? A B33 is 500cc, that’s 150cc louder, if not a lot faster.

I knew nothing about B-series BSA motors, thus I needed someone to chat to about them. After a few false starts I found Roy Shearwood and we started talking. He agreed that a B33 top end would be just the ticket for a slightly faster B31 and very little work was required. However… why stop there? Why not put some Gold Star parts in it and give it a bit of fire in its belly? I was quite shocked to find out that the famous Goldie was a highly developed B31/B33! Plan three was then hatched, keep the B31 bottom end, graft on a ZB34 head and barrel and with a few other choice internals it should be a little more interesting…

I found a ZB34 head and just need to find a barrel. These are made from unobtainium, totally off the scale for rarity. I had heard of a chap in NZ re-casting them but I never managed to get his details. After about 12 months of searching I found one, very beat up, in the States. This duly arrived and was sent to Roy for inspection, he declared it saveable but it would cost the same money to repair as it would to buy a new DBD barrel and liner and mod it to fit.

Hmmmm, DBD… the Holy Grail of Gold Star parts! It got me thinking and talking. DBD barrels are about two fins shorter than the ZB barrels so a spacer would be required or a shorter DBD conrod, and the fins would need cutting down to match the ZB head. The tappet adjuster would stick out and look odd, so I had some more thinking to do.

So I now had the parts to build a DBD spec motor! Umm… this has got all out of hand, hasn’t it?

I sold the ZB head and barrel on eBay, made a small profit and sat back and waited for the classic racing season to start( as Roy had a few bikes to get ready, engines to build and a 350cc Goldie racer to build from scratch in ten days, which won its first race…)

So that’s the story of how I got to a DBD34 spec motor. The cases are standard B31, cleaned up, checked and ready to go.

The crank tells the same story — it was put on a set of knife-edged grinding wheel balancing rollers and checked for true. It was 1.5 thou out, so that was OK. The big end had only done about 100 miles or so, so that was also ok. The B31 and DBD34 main bearings are different but if you know a man who knows a man then a mains conversion bearing set can be bought, if you don’t mind waiting for them to be made.

Timing side? Lighten the idler gear and fit a pair of late DBD34GS scrambles cams and that’s that sorted – well, kind of. Roy wants to time it spot-on, so I feel a little jiggery-pokery going on there, and possibly vernier adjustable ones at a later date. I discovered a CB34GS timing cover with tachometer drive hole at my local Brit bike garage (Monty’s in Plymouth) — I was building to Goldie spec now so what the heck — however Goldie Scramblers had this hole blanked off, so this will now require a blanking disc to make it look the part. Oh the vanity of it all!

Barrel? This had a cracked and repaired alloy sleeve (which is why it was taken off a very, very nice DBD34 and replaced with a new one, from Mr Phil Pearson; his perfectionism my gain). However on inspection it was found that the conrod in his engine must have had a slight bend in it as the bore was worn on one side. Not a problem; a re-bore and new piston, and tell the chap that his conrod was bent and he had better strip down his newly built motor to check!

This is where it really starts to get out of hand. I asked Roy ‘how’s the barrel?’

‘It needs a re-bore’, and I get the story about the bent conrod. I had two choices of piston; standard or slipper. Hmm, thinks me; standard will be cheaper. But Roy had the slipper pistons in stock so it would cost less… that’s done deal! I now have a 9.5:1 slipper piston at +20 thou! Not quite the standard thumper I originally wanted but, hey, that’s life.

In the cylinder head the inlet valve was a little recessed due to some over-zealous re-cutting. It also seems to have been fitted with Triumph valve springs and these have made it all coil bound. God only knows how they got the springs in, as they had to be cut to get them out.

Now this is where we had a heated but friendly discussion — do I want a new inlet valve seat? Well, no, not really; there is a chance of cracking the head and the little extra performance didn’t really seem worth it. Roy had a grumble about the head of the valve being slightly shielded by the seat, and I tried to explain that I didn’t want a race motor, I wanted a good usable thumper. Roy countered by saying he would clean up the inlet and exhaust port to make it flow a little better. As the head needed new valves, he would fit a DBD34GS Clubman inlet valve to help overcome the problem with the seat, as the larger valve will sit further out in the head. And as he has a new toy for cutting seats and grinding valve angles that does away with the hand-lapping process he is a little happier, though he has reserved the right to fit a new seat… I feel there will be more discussions at a later date.

I had intended to run a Mk1 Amal Concentric, but the hand of fate had other ideas. How would I like a nice new Mk2? Well to be honest they look a little crap and they are more expensive than a Mk1, then you need the mounting kit and such so the price goes up even higher. Well, says Roy, how about a Mk2 for less than the price of a Mk1 with mounting kit and if you like it I’ll ‘cosmetically’ alter it to look similar a GP when we strip the engine down next year. I’m not keen on the idea but I’ll go along with it as the advantages of a Mk2 over a Mk1 are quite good for throttle response and power higher up in the rev range. Although I must say I hadn’t planned on stripping the engine in a year’s time either!

For the valve gear, the costs of new DBD eccentric rocker spindles and rockers are prohibitive (used rockers alone are going for over £400 the pair). Roy convinced me that B31 rockers, shafts and pushrods can be made to work. As this is what the BB Gold Stars use, we too are going in this direction; the rocker box just needs to be ordered to accept them. Another hard to find part are the humble B31 / B33 pushrods, I have no idea why but these are getting rare.

The rocker box was becoming the bane of my life. Roy wouldn’t build the engine with out it as he likes to do things in one go. I can understand that — it’s the way I like to work if I am allowed. However I knew of another Gold Star engine builder waiting for his order of six from the same source and I was lower down the pecking order than he is! We just had to wait. The delivery duly arrived after three months or so of waiting and I now have a rocker box.

Any day soon, this engine is going to come together…