Hitchcocks Forum

It is always interesting to get someone else's perspective on the design of the bullet engine, especially when that person is deeply involved with motorcycle engines of a drastically different sort. In this case it relates to my employer who is an avid racer of Japanese and Austrian short stroke singles for the motor-cross (I might be wrong, but I believe KTM is an Austrian based firm). He has a great workshop at the back of where I work, and is often busy rebuilding the engines of these bikes which fail due to a combination of the lightness of the components and the stresses of competition use. He actually has to replace the pistons of these machines after every 20 hours of use, before they crack. Anyway, he offered to take my Electra X crank to a specialist 'crank' workshop he uses for his bikes, to see what needed doing. When I eventually got the crank into work, and showed it to him, he was astounded at how large and heavy the flywheels were. My employer made two comments, I thought worth mentioning. The first was how badly the crank had been treated in the factory in India, as there were dents on the flywheels where heavy handed Indians had hit them (with a sledge hammer?) to align them. Secondly, he mentioned that the main bearings on the bullets were too small in relation to the enormous size of the flywheels. Even if they were only slightly unbalanced, they would put enormous strain on the bearings. His bikes have in comparison, tiny flywheels, but larger main bearings than we have in our bullets. He took my crank to the specialist, where the flywheels were found to be unbalanced. They were rebalanced to Jap racing specs, and the big end bearing and pin was found to be in excellent condition. I was strongly advised to reuse the existing big end, pin and rod. I Will attempt putting the crankcases back together again this weekend, and will surely post here soon if I'm in need of excellent advice.

Hi Robin, that all makes sense to me. I have rebuilt a couple of Honda CRF 250 / 450 MX engines and the cranks are tiny, as you say and the pistons are like digestive biscuits stuck on the small end of the conrod. Things like valves wear out very suddenly after low mileage as well.
In many of the Bullet engines I tune, one of the main features is that I split the cranks, lighten the flywheels, rebalance them and retrue them. A few pounds off the rotating mass is good for horse power and accelaration, as well as giving the main bearings an esier life. Paul.

Hi Robin. It is those big heavy cranks in British singles that give them that low down plodding charm and chugability. It’s true that lighter flywheels will give better acceleration but you loose that flexibility which is possibly the reason that you want an old style single. I’ve ridden Rotax powered bikes and they were amazingly fast by comparison but you had to constantly change down a gear once the revs dropped even slightly as the chain snatch was terrible, caused by each firing stroke’s power hitting the drive and not being absorbed by a heavy flywheel. It was exciting to ride but would not be liked by someone who wants a more leisurely ride. Yes the Bullet bearings are small by modern standards, but they are better than many classic bikes that often have only a bronze bush on the timing side. Make sure the replacement bearings are high quality ones, and try to find a high capacity version of the ball bearing (6305) (it will have extra balls) but it might be hard to find and be very expensive.....Here is another idea with the main bearings and I wonder if Paul (BW) has tried it? Fit two NU 305 rollers bearings on the drive side (perhaps even "Superblend" types where the rollers are slightly barrel shaped and can work better with slight offsets and crank flexing) and a high capacity ball bearing on the timing side instead of the roller NU205. Thinking is: that the drive side of the crankshaft will have increased bearing capacity and the high cap ball on the timing side will be sufficient and still allow positive crankshaft location.

Hi Les - I have already received new bearings from our hosts. The cages of these bearings look a lot sturdier than the originals fitted in the crankcase, which is good as it was the cages of the NU 205 and NU 305 on timing and drive side which disintegrated and ended up in the sump.

Hi Robin - just to add a bit to what Les has said, remember these little engines sometimes have the burdon of pulling a sidecar and three people uphill against a head wind! I'd like to see a KTM do that. It's the big flywheels that allow a relively low powered engine to keep pulling when the going gets tough. Personally I think the characteristics of these old engines are very satisfying and give a really pleasing riding experience - something the Japs have never understood.

I rode a Yamaha SRX600 single daily for six years. Excellent, fast, reliable (although sometimes hard to start) and an underated bike of the time and a million miles away from the bullet I ride now. It was however very snatchy around town an actually quite stressfull and annoying to ride slowly. Although an inferior bike I now much prefer the bullet engine for tractability, character and simplicity.

Yes, I'll stick with the heavy flywheels too thanks. About 8 years ago I tried a Rotax 500 engined bike an found it snatchy, peaky and lacking character.
I stayed with the Kawasaki GPZ500 that I had at the time and then tried a Bullet.
The rest is history and I'm on my second one now.
Interesting also that Robins crank specialist could find no issue with his AVL Electra crank. Only on this forum have I seen or heard any issues raised regarding the big end. The two Enfield dealers that I know and the UK importers don't have significant issues. Now a crank specialist says they are ok too!
Far better than the old floating bush I think.

Hi Les, I don't know about trying to use the timing side main for crankshaft location, but I have built two 500 race engines with a 4th main bearing of the needle roller type, in place of the bronze bush on the timing side. The corresponding section of the timing side mainshaft had to be reduced in diameter slightly to match the smaller I.D. of the roller bearing than the bush. It seems to work ok and I haven't had any wet sumping problems as a result.
As for lightening the flywheels, it should be remembered that the Bullets [British and Indian] lend themselves well to pulling sidecars, albeit at a liesurely pace. 2 or 3 lbs off a 22 lb crank isn't enough to make a complete transformation of how the engine behaves. Our 350 racer can plod along at 30 in top gear and pulls harder than any standard 500 Bullet from 2000 rpm, but can spin up to 8000 rpm without shaking itself to bits. I guaruntee that no one would want to drop one of my 'lightened' cranks on their foot - they are still heavy compared to modern stuff !
Regards,Paul.

Hi Paul. Thanks for getting back on what I suggested. Just to go over some details. The reason I suggested using a max capacity ball bearing on the timing side is that I suggested using 2 rollers on the drive side, instead of a 1x roller and 1x ball bearing so to have some lateral crankshaft location the ball bearing could be shifted to the timing side. The lateral location is required on this engine due to the fact that the load of the oil pump worm pulls the crankshaft over to one side. However for even more capacity the timing side bearing could be changed for an NJ205. This bearing has one side open and one side flanged (same as a "Superblend") which would allow easy assembly of the cases. I have since found out that these NJ flange bearings can have its axial location offset by up to 1.5mm and also allow reasonable axial loads against the side flange of the bearing's inner race which will take care of the oil pump side thrust. The end float would have to be accurately set perhaps .010" would be a decent amount. The shims will obviously be positioned behind the detachable inner race that fits on the shaft. The engine would then have three roller bearings on the crankshaft, two on the drive side instead of one roller and one ball type bearing. Just some ideas if one wanted to maximise the crankshaft bearing capacity.