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Centaur crank lubrication

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AJW08/03/2019 21:59:41
289 forum posts
117 photos

Had my new build Centaur engine running for a bit now and seems to have freed up well as I had made the fit of parts a bit snug!

Upon dismantling for final painting I noticed a couple of points which I am hoping someone can help with.

The big end bearing looks as if it has not had sufficient lubrication, oil is supplied to it via a banjo fitting on the crank web which supplies oil from a main bearing. Mains bearings are a good fit with excess oil supposedly entering the banjo and on to the big end but I don't think sufficient oil is leaving the main bearing.

Could it be the bearings are too good a fit or would the main journals benefit from a groove somehow? Oil used was a fully synthetic 5-30 grade so quite low viscosity.

Also all though I assembled the piston rings with the gaps opposite each other and not on the loaded side of the piston they have both turned to end up on the loaded stroke side? Do they turn in normal operation?

Both are a good fit in the grooves with no obvious clearance although the gaps ended up wider than I would have liked.


MichaelR09/03/2019 09:10:46
376 forum posts
71 photos

There is a modification for the banjo oil feed, I also fitted a oiler to the conrod bearing, not sure why your piston rings are moving.

annotation 2019-03-09 085529.jpg

AJW09/03/2019 09:36:10
289 forum posts
117 photos

Thanks Michael, is that an Edgar mod? Haven't found any reference to any mods at all although have discovered a couple of discrepancies.

Your conrod lubricator I am assuming is small and a closed cup affair?


Jeff Dayman09/03/2019 13:11:55
1827 forum posts
45 photos

Piston rings can and do rotate on the pistons in engines, unless they are pinned as done in two stroke engines. Rotation in non pinned ring engines usually does no harm at all.

If the rod end is not getting lube, I'd suggest making an oiling hole in the cap and oiling by hand at startup. If you are running it for more than a half hour at a time then re-oil periodically. In model engines I would not trust any sort of hokey gravity fed tiny passage banjo gizmo figment of Westbury's imagination. Engines need oil. Unless there is a pressure lube system with a pump in a model engine, I'd hand oil things regularly.

MichaelR09/03/2019 14:37:06
376 forum posts
71 photos
Posted by AJW on 09/03/2019 09:36:10:

Thanks Michael, is that an Edgar mod? Haven't found any reference to any mods at all although have discovered a couple of discrepancies.

Your conrod lubricator I am assuming is small and a closed cup affair?


Hi Alan, The Centaur mods are from a article in Model Engineer 19th November 1976, Volume 142 No 3549 and titled Experiences With The Centaur By R.R.May.

My conrod oiler has a screwed cap, as photo. Michael.

crank pin oiler.jpg

AJW09/03/2019 18:36:39
289 forum posts
117 photos

Thanks Jeff, not keen on Edgar's design then! Michael, how on earth did you know about that article! We're not the only ones to witness lubrication deficiencies then.

Priorto assembling my newly painted engine parts I took the opportunity to modify the crankshaft bearings.

Now as the oil flows through the top bearing caps it enters a 1/16 wide and 0.010 deep channel I have milled axially in the top bearings. On the flywheel side the channel stops just short of the bearing ends but in the banjo/gear end I have taken the channel to exit the bearing which should allow oil out into the banjo.

After assembling the engine and filling the lubricators I definitely had oil flowing through to the big end so fingers crossed it also does this while running!


MichaelR09/03/2019 19:04:59
376 forum posts
71 photos

Alan, If you haven't already seen the full article for the mods see Here

AJW09/03/2019 23:13:53
289 forum posts
117 photos

Michael fascinating article nice to hear of the engine being used and not just run.

I did uprate my crankshaft sizes, mains and big end are 5/8 on the engine.

I think I will have to produce the gas valve shown as I would like at least to be able to run it on gas.

Once again thanks for the link.


Howard Lewis10/03/2019 08:38:53
3370 forum posts
2 photos

Possible help for engine builders?

If it needs to be said, the finish on the crank bearing journals needs to be as good as possible.

Not that many of us have the kit to measure it, but in full size engines with pressure feed, no more than 16 micro inch Centre Line Average. So polish to as fine a finish as possible, probably oiled 600 - 1200 grit wet/dry paper.

Bearing clearance affects lubrication, for pressure fed white metal (Aluminium-Tin) 0.002 -.0004" , where the oil needs to flow to cool as well as lubricate. (White metal bearings don't like high temperatures. On Marine, main propulsion engines, bearing temperatures are monitored in the same way as oil pressure ).

Many full size engines, running at low speeds, use ring lubrication for the main bearings.

(A large diameter loose ring on the shaft dips into an oil bath, and as shaft and ring rotate, the ring brings oil onto the shaft, to feed into the bearing by capilliary action ),

Probably closer fits for bronze with wick / drip feed? The Oil feed should not be on the pressure or inertia axes.

Inertia loads can exceed the gas loads, especially as the speed increases, because of the loads imposed by decelerating/accelerating the reciprocating parts as the direction changes..

Where an engine has a helical cross hatch in the bore, or maybe even the turning marks on the piston rings, the rings will rotate, hopefully at different rates son that the gaps only line up from time to time. Ring gaps are normally no less than 0.001" per inch of bore diameter, to minimise blow by. (The ring ends must not butt, or the ring will break, and damage the Piston and Bore. Blow by, obviously, will decrease as the rings and bore run in.

If you are measuring power, it will increase slightly during run in. (With modern machining methods, a car engine can take 10,000 miles or more to bed fully ).


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