K head jackshaft - thoughts on eliminating the middle bearing?
Turbo Mini Forums

Simple question:
Do we really need this huge hunk of metal in the middle of the jackshaft (the old camshaft) any more if all it's doing is driving the oil pump?
If it can be simply machined off, there is an obvious weight saving, but perhaps more useful is that the hole feeding the centre cam bearing can be plugged, which obviously assures a better flow of oil at the centre-main crank bearing.
I don't know how much drag is induced by a journal bearing like this, but obviously there should be less drag if there is one less journal?


Anyone think of a good reason why it should stay? Am i missing anything obvious?

I had the same thought, I also thought of making an aluminium shaft,pressing steel rings onto it for the bearing faces and using needle roller bearings

I seriously doubt it!

if you were chasing every single little weeny bit of weight & drag,,,then yeah i reckon get rid of most of it including the center bearing journal

but i havn`t done it cause i`m not chasing every little weeny bit

not sure about shaft 'Whip" at high revs either might be something to look at tho

www.miniman.com.au

"""LazyGoodForNothingSmartArseKnowItAllBackYardMiniMechanic"""

Check the critical speed.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

I had thought about making a shaft from tube with only the outer bearings, but then looked at just removing everything from a standard cam. My only concern was that vibration may be induced, but then with the small radius of gyration it might not be that significant. The other thing to consider is a standard cam is a bit random to begin with, I would want to give the shaft a really good clean up and have it balanced. Maybe even spin it up to 4500rpm and visually check, after all once its fitted you'll never know if its flailing about, I cant see that happening but for the sake of just leaving the center bearing there.

What about plugging the oil feed to the center bearing anyway with the bearing journal still compete ? Perhaps drill some oil feed holes at an angle towards the journal surface from the outer edge of the shaft. There is plenty oil being splashed about and centrifugal force of oil on the surface of the shaft would then feed oil to the journals. Maybe even remove the oil feed from all the journals in a similar manor.... Hmmm I've just had an idea

I was a little worried about the belt 'tension' when i looked at doing this. Maybe the end bearing takes most of that that force anyway but it seemed to me if you took the shaft down too thin it might allow it to flex too much and getting rid of the bearing wouldn't help either.

Sounds like a trial and error thing to me. I do like the ideas here tho.

The moment arm of the pulley over the distance that it would be acting on, makes that all but irrelevant IMO. I'd never attempt this if it were still pushing valves, but it really is just a torsional load, with the slightest of bending moment - something a larger, thin wall tube would obviously work better at...

Would an electric oil pump be a better alternative?

http://www.jbperf.com/

How about this:
(credit Paul for the thinking lol)
SKF (and others) make machined outer, and inner rings, which means you don't have to worry about grinding and heat-treating a shaft - which you can now simply make from tube...
The bore in the block near the pump is 1,50", and there are loads of needle bearings available for that one. The front-most bore is a bit of a bastard size as far as I can tell at 1-51/64"...
That could be easily bored out to a more "user friendly" size however... I'd actually put a cylindrical roller at this end, which will also constrain the shaft axially...

The interesting thing is that the oil feed to the head is actually a "pulse feed" from the front most camshaft lobe, so the head would definitely need an external feed coming from the front of the engine, instead of this location.

Hmmmmmmmm......

Edited by TurboDave16V on 8th Apr, 2015.

Whilst those bore/bearing sizes sound nice and large to suit a nice large hollow shaft (best for rigidity obviously), surely you are restricted to something much smaller in diameter ???

Correct me if I'm wrong but from the evidence of all those who have tried turning engines over without the cam chain, or have broken a chain or belt, the conrod big ends hit the cam lobes unless everything is running timed as it should be.

ie, you are probably limited to a shaft no bigger than the base circle diameter of the normal cam lobes ???

Schrödinger's cat - so which one am I ???

yeah, it'll hit the rod bolts,

my phase 2 took about 2-3mm out of the rodbolts, so you can go over the base circle a bit, unless your running some kind of stroker with non S 1275 rods.

dont forget youve got to factor in the bending load from the cam belt tension, not a lot, but could be ebough to make a long shaft whip at 4k...

I had a scrap cam that I put put in a lathe. The idea was to remove the lobes and use it for 16V conversion. However, the lobes (or what was left of them) are very hard to machine. Best would be to grind most of them away and then use a lathe to clean up the remains.

I suspect that getting rid of the middle bearing might create some interesting vibration or whipping. But that is only a guess...

Could you pistol drill the cam all the way through to save weight ?
I know some of the newer cams have oil feed to the lobes, so it must be possible.

Metric is for people who can't do fractions.

I'm no tribologist but how much would the drag from an unloaded and hence probably fully hydrodynamic bearing actually be ?

Main losses from what I understand would be the tappets and the mixed lubrication regime at lower rpm.
Would it not be easier just to limit the oil flow to the centre cam bearing if you want more oil for the main

Edited by Sir Yun on 9th Apr, 2015.

That sir, is not rust, it is the progressive mass reduction system

http://aseriesmodifications.wordpress.com/