Hitchcocks Forum

I'm doing some bottom end work on my Redditch Bullet and replacing the main bearings as well as the big end bearing. I am thinking of using a single sided sealed bearing on the outermost drive side bearing (with the seal on the outside of course). Does anyone see a problem with this?

You might want to check the operating temperature of the seals?

Should be fine. Later Bullets had a proper neoprene seal on the drive side shaft so all you'd be doing is giving this seal some back up. Good idea I'd say and I'll try it on the next Bullet engine I build, thanks!

REgards, Mark

I did this with my 1964 Royal Enfield 350 motor and I have had no trouble in 2,800 KM's.and looks like going on forever. This engine is an old one that still has the leaky old cork seal at the end of the drive shaft where it goes into the primary case. I put a sealed bearing in with the inner seal taken out and the outer seal against the old cork seal and I have had no engine oil leakage into the primary case at all. It worked perfectly.

It would be a good idea to check the technical sec of the bearing you intend using, I think you will find the seals are to stop anything getting into the bearing and the crankcase pressure will be on the wrong side of the seal and may push the seal out causing more problems.

To Revband the seal will not be pushed out. You have to take the engine side bearing seal out so the bearing is oiled by the engine oil.
The engine breather means that there is no great pressure in this area. Maybe 5 psi. I used the bearings that have the metal seals because the hot oil in this area may have distorted the plastic of the bearings with plastic seals.

Binary, it's because the inner seal is removed that can cause the problem, the seal is designed to withstand present from the outside not the inside the metal seal is far stronger and may well be OK that's why I said check the spec, 5 psi in the crankcase?, Really how did you measure that, A 71mm piston flying up and down thousands of times a minute plus piston blow by all going out through a very small vent hole/breather.

Both Rubber seals (bearing suffix RS, designed to keep oil/grease in) and metal Chip-shield (bearing suffix Z, which are not actually seals at all, but shields to keep stuff out) are retained in the same manner. Either will/can be blown out by similar pressure pulsations.


The simple old RE Duck-bill breather is adequate to maintain an AVERAGE net NEGATIVE crankcase pressure up to about 4500/5000 rpm. Only at higher rpm (or bad rings/bore/oil pump) does net positive crank case pressure start to become an issue.


The std Bullet with a breather vent of no larger than 2 x 1/4 breather holes can pass a max of 8cfm
before back-pressure in the crank case commences. That gives you some idea as to how little 'air' is actually being moved via the breather vent in relation to the amount displaced with each cycle of the piston from TDC back to TDC


Crankcase Pressure is also being communicated into the Oil tank (pulsation dampener effect) and to a lesser degree, into the timing chest.


The odds of a Drive-side Bearing outer Rubber/chip seal being displaced are very low, (infinitesimal) as there is another 'seal' to back it up.


Excessive Crank-case pressure entering the chain-case? It will leak oil.

Tim, that post has left me very confused. "Either will/can be blown out by similar pressure pulsations.". "The odds of a Drive-side Bearing outer Rubber/chip seal being displaced are very low, (infinitesimal)"

TimNZ. I am interested in your above comment regarding the std bullet breather of 2 x ¼. Does this mean it had 2 x ¼inch breathers? Mine (1996 535) has but one and I find issues with engine breathing above ~3500rpm. If I understand your note correctly and there is supposed to be another one, where should it be?

I know that BW has shown how to turn the tappet adjuster cover stud location into another breather, but I’d need to have the engine apart to do that (next time!).


Mustaphapint – apologies for my sideways diversion from your thread.

Thanks, ChrisD