Da hab ich doch was interessantes von Seager Engineering gefunden, viel Spaß mit:
TO BE A BUSH OR NOT A BUSH THAT IS THE QUESTION.
We are doing quite a few BSA twins all of which have a mains bush that not only supports the crank but also delivers the oil to the internal crank oil ways to pressurise the big ends…
This includes A7/A10 and A50/A65 engines
The gear pump supply’s a reasonable flow of oil around the engine but only at around 40-50 psi.. in a an ideal world this is consistant if the bush remains in good order. If it or the crankshaft wears so the clearance goes much over 0.005” quite a lot of that oil which should go to pressurise the big ends escapes and falls into the crankcase… at what point it causes a big end failure is debatable. In my experience it’s normally the drive side rod that fails. Obviously A65 engines are more powerful and are affected more… a seizure on the end normally will break the rod and it sticks to the crank and the moment in the bike forces it around and the fact it can’t rotate, they snap and the bit still attached to the crank flails the crank case, busts the skirt on the barrels and the piston stops up the barrel and the rider comes rapidly to a stop possibly covered in hot oil from the broken case… but not always
Now I know loads of these engines that have done thousands of miles… some 50k but they all have been looked after… clean oil( changed at 1500 regularly) helps tremendously and so does good carburation.
Un burnt petrol in the oil does untold damage as it dilutes the oil and petrol has virtually no lubricating ability…
It buggers the bush and the big ends and bores… more oil is burnt , more petrol in less oil, more wear and it goes on till “bang!!!”
Remember mini engines that were ruined by people leaving the chokes on or mark 2 Ford Escorts with VV carbs with an accelerator pump… these ruined there engines at 40-50 k.
Austin did a “Gold seal “ engine because it was common place..
So most people believe it’s debris that cause wear but in reality it’s petrol.
If you clean the engine thoroughly and the oil tank and replace all the oil ways, recondition the engine to a high standard and change the oil at 500 miles and then every 1500. Use a basic 20/50 and then I use silkolene Comp4 20/50 which is a dedicated motorcycle semi synthetic oil…. Which can be used with a clutch… but it stays in grade and maintains oil pressure. The spill track of the bush is only around 3/8… that’s the distance the oil traces from where it’s pumped in till it falls out into the crankcase… thicker oil takes longer and more goes to the ends… don’t forget the crank spinning makes centrifugal force which spins the oil out quicker reducing oil pressure…so keep you tank fuller, that means cooler thicker oil, a multigrade means it has the viscosity of a 20 weight oil when cold and a viscosity of a 50 weight oil when hot…. I do NOT recommend using a mono grade on plain bush engines which have no end feed!
Now here’s the point, it’s has been mentioned why we don’t fit a bearing like a needle roller…here’s why
Firstly it wasn’t our idea… and I never wanted to compete with the specialist and we all know who they are!
Secondly the it’s not quite as simple as fitting a needle roller… you have to redirect the oil so it heads out of the crankcase and into the outside cover and goes into the end of the crank via a quill and oil seal all of which we have to drill/weld and add metal to achieve this.
It’s not a quick fix but it is a big improvement if you’re going to do a lot of miles!
Now the 64million dollar question is shall we create a process and all the fixtures to achieve a this redesign modification.
I believe what we do now will suffice for the majority of our customers…
We certainly make a good job and have done several hundred over the years , probably at least 300 and only had 2 back for the bush siezing… almost definitely due to not warming up and using a straight 40 oil.
I would be interested to hear peoples thoughts… because the Triumph 350/500 could also have the same treatment…. I did do one of those once and that’s still bombing about in Essex.
Ain’t that right Perry
Kommentar von Mr.Jim Motoren:
Young Pat
We’re both seasoned but I would like to add my 10 cents.
There is absolutely nothing wrong with the design BUT there are a few key factors.
As you correctly stated the use of multi grade oil is essential. Single grade oil requires a lengthy warming up period as it isn’t protective until it reaches optimal working temperature.
Secondly material choice is critical. Vandervell developed VP56 lead bronze specifically for this application. It’s a butter soft lead bronze alloy.
This used in combination with a hard steel shaft provides excellent wear properties.
Your in house bushes are undoubtedly of a similar composition.
Personally I prefer hard chroming the crank pin back to as close to standard as possible or press fitting a needle bearing inner sleeve to increase the diameter when the crank is below -0.040” and Donald ducked.
If you calculate the surface are of the crank pin at std size compared to the surface area at -0.040”
the difference is staggering!
Very few “dealers” were equipped to do bush replacements and consequently engine life was often reduced dramatically after the first rebuild.
Non factory bushes were fitted and cranks were sent out for grinding but the lack of line boring equipment or a king pin reamer with a guide sleeve meant these “ rebuilds” were doomed from the get-go. They hand reamed, scraped or flex honed the bushes until they could shut the cases and turn the crank. They didn’t have internal micrometers so god knows how much clearance the crank to bush was by the time they were done?
I have known A7’s and A10’s reach 50k miles and more from new but after rebuilding they were lucky to reach 10k before failing.
Not through bad design but bad engineering.
Have you ever heard or read of the timing side crank pins being case hardened at the factory? The reason I ask is the accelerated wear experienced after the first grind. I have a sneaky suspicion they were.
Keep up the good work and informative posts.