Building a new Bantam Pre-65 (Engine) by Rob Sloggett


Started on March 10, 2018 at 7:00 PM



That's it, time to start looking and find myself an engine. And this is where my learning really started! The engine decision actually came about hand in hand with the frame choice although this is where my first understanding of the pre 65 scene started to develop. My assumption was obviously that the frame and engine always came from the same marque but with hybrids not being frowned upon it left it open for me. So then next decision was based on ease of build, readiness of parts and club knowledge.

10/3/18

Looking at the event results, BSA Bantams are the most common marque and I wanted to start with a simple 2 stroke so asking around the club (and many thanks for Jim Droughton, Russ, Kev and Mark Newman for the pointers here) I was pointed in the direction of a D14. This 175, 4 speed engine was suggested as a good starting point and a look on ebay had a few donors with a frame attached. The £500+ starting point for an engine with no history and a battered frame seemed a bit of a risk, but it gave me a starting point.

A few days later and a nudge from a few people pointed me towards a good D14 engine via Wobblie Wills trials adverts and put me in contact with Keith (who has been a brillant source of information as well this week). The engine seemed to be sensibly priced against the stuff on ebay and it had a good history. Keith was well known in the club scene so it was a simple deal to be done. He also had a brand new ignition system at a sensible price (saving me a few pounds against buying on line) so the engine and ignition came in on budget.

All that was left to do now was to get the engine from the other side of the country and once again, this is where the friendly club scene came into play. I had at least 4 different offers from people using contacts and network links so a big thank you to everyone who offered. As I type this, my engine has been collected and should be with me at next weeks Aqueduct competition to start the rebuild (thanks Russ and Becky).

The next step once again was a bit of a learning journey for me and I have always wondered how engines and bikes designed for the road get adapted for competition. I had heard on the grapevine about changes to the gearbox and a clutch modification so once again asking around gave me some solid pointers. Once again, I would like to single out Paul Owen (and Keith) for spending some time really talking me through the must haves and nice to haves to get me started. Whilst these may be debated by others, my starting point for the engine rebuild is as follows:

Must have....

  1. Bore out to 185
  2. 12T primary drive gear
  3. 12T front chain sprocket (paired with 62T rear chain sprocket)
  4. Swap 2nd and 3rd ratios around in the gearbox to lower 2nd gear
  5. Whilst engine is being worked on get main bearings and crank case seals checked

Nice to have ....

  1. Custom Jim Pickering Exhaust
  2. Custom Jim Pickering Yamaha Clutch basket mod
  3. Modification to the clutch arm to bring it outside of the case

Reading up on the 12T primary drive gear change, comments also seem to suggest a modification to the primary drive chain tensioner with some also suggesting having an endless primary drive chain made up to reduce the risk of the link breaking. One to ask around about I think.

Its also been suggested that a modification to the cylinder head to move the plug from the offset position to the centre could be considered so maybe something to add to the nice to have list.

08/04/18

A big thank you to Russ and a colleague of his who arranged collection of the engine. Picking it up from Sundays event meant it all became a bit real and was no longer just money going out of the bank account, I now had something to show for it! You don't realise until you physically carry an engine in a box just how much of the machine weight comes from that mass of steel and aluminum. Now it was time to finalise the modifications and a phone chat with Jim Pickering on Saturday narrowed the first wave of modifications to the following:

  • Rebore out to 185cc and fit new piston (supplied by Jim)
  • Modify primary drive to 12 teeth and fit a new continuous link drive chain to suit (supplied by Jim)
  • Option to switch 2nd and 3rd gears around or fit a bespoke trials gear set which lowers 2nd even further and lowers 3rd to make it even more useable. A bit extravagant but the budget will take it (just) so I will get this off Jim as well

Once these parts are with me I can get the engine opened up, checked over and upgraded with any other bearings and seals that are needed.

Once again, getting the engine has allowed me to draw up the remaining shopping list and at present it looks like the following:

  • Exhaust nut - £25
  • Carb manifold - £20 est
  • Kickstart - £95
  • Gear lever - £25
  • Carb - £115
  • Airbox - £50 est
  • Exhaust - £200 budgeted
  • 12T front chain sprocket - £25

I will keep this page updated as the engine build progresses.


Engine Breakdown

Budget = £950

Spend to date = £855

broken down as

Engine = £450

Electex Ignition = £170

Internal upgrades (8/4/17 update) = £235