main bearing help
Turbo Mini Forums

9.85 @ 145mph
202mph standing mile
speed didn't kill me, but taxation probably will

It has more load capacity - nowhere near 80% more - but unless the main bearing part of the crank is cross drilled, they starve the big end bearing of most of its oil for 50% of the revolution of the crank.

It's swings and roundabouts.....

Also, most decent engines are 5 bearing, not 3 - that affects the bottom shell loading considerably as the crank flexes.

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

My LS engine has flat/grooved. Works for me.

As did the same setup on my old Mini and even using cheap oils, engine bearings were pretty much one thing that never really gave any trouble.


Can you actually attribute a single problem to having a flat lower shell ?

9.85 @ 145mph
202mph standing mile
speed didn't kill me, but taxation probably will

got the proper bearings today. 3 with central tang :)

13.95 1/4 mile on a 2.95 fd. Carnt be that hard to beat!

If you need to cut a new slot and wish to do it accurately, i'm told that a Woodruff cutter, number 610 is the ideal tool:

http://cgi.ebay.co.uk/RAIZO-H-S-S-WOODRUFF...=item4157f8d7e4

You will need a lathe or mill to drive it though.

As it is difficult to get centre tang, solid lower bearings for the later 998s, then I'm going to have to convert mine to the offset tang grooved setup.

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 just don't understand this crave for flat bearings. Fit old ones if you can, they've never been an achilles heel.

like i say got the proper ones so no need to mod there half grovved and half flat

13.95 1/4 mile on a 2.95 fd. Carnt be that hard to beat!

I can't bear it! I think I would like to use groove/groove on my engine tbh.

If something is worth doing, it's worth doing half of.

groan

Colin, does your twink turbo engine contain groove/groove bearings by any chance or did you revert to plain/groove?

reading back through this topic, I said things then that I was un educated on. I have since learned a lot and I can now say that in areas of this topic, my opinion at that time, was wrong.

The plain bottom bearing does significantly increase the load carrying capacity of the bearing. whether this is 80% more may still be debateable, but it is signifficanty greater that the 20% more I suggested though that 80% figure quoted in TBASE does seem plausable.

With that in mind, I will always use the plain bottom bearing in any highly loaded application. There was a point I installed the grooved bottom bearing, but went back to the plain bottom bearing before running the engine after the above realisation.

The debate as to whether this plain bottom bearing starves the big ends of oil (again in my humble opinion) is unfounded.

It would appear that the main bearing will fail first, yet the big ends show little signs of damage other than incidental as a direct result of contamination from the failure of the main bearing itself. This suggests that the big ends are receiving plenty lubrication even with the plain bottom main bearing.

Everyone talks about crankshaft cross drilling, to reduce the centrifuging of oil towards the big ends that can starve the main bearing of supply. Well, using the groove bottom bearing will only accentuate this?........

Also, if the cross drilling is done incorrectly, IE where the drilling is infront of the highest loaded point during the power stroke, increase wear rate of the bearing might be experienced. You only need to look at expensive billet cranks designed by big name crank manufactureres that get it right, to see where others get it wrong.

One other thing to think about here is whether cross drilling is actually required in the first instance. The big end bearings will use only the oil they can flow. what is significant though is that the center main bearing of the A series feeds two big ends, while the other two only supply one big end, yet the center main bearing has the same diameter oil feed as the outer two. perhaps that in itself is significant enough to cause probems.

Is the centrifuging action that significantly less with a cross drilled crank compared with a standard drilled crank?.... what happens with long stoke cranks?.... what happens in other engines with longer stroke cranks?... (granted the A series is an oddball long stroke affair). A certain engine builder and auther in a certain book suggests that cross drilling is an insurance, he also said that he has built engines without it without any problem. After talking with this Auther in person, it was said to be put across that way in the book, as with much of his other theory, that to do it any other way would have been controvertial from what would be considered by everyone else to be THE thing to do, which when it comes down to the actual physics of it, may be totally un necessary. Telling all the engine builders doing it one way that they are wrong apparently doesnt sell books lol................ Basically, there is enough information between the lines in print in his books for you to make an educated decission rather than being led down the generally accepted route just because some one said so. The question remains, do you really need to drill your cranks full of holes just to move an oil hole?

One other factor to consider here regarding bearing surival is the bearing clearance. Again, certain figures quoted in a certain book are a little miss leading. Bearing clearances should be as tight as 0.001".

A Nice detailed post. I have always been of the opinion the plain bottom is better, but was hoping you were planning to test the water with groove/groove.

I have a SA 1100 crank that needs to go to 0.30, and while I will not commit this to a new engine build (hence being part of the current group buy for a std one), it would be nice to have on the shelf as a spare just in case. I just question if the time comes how long the groove/groove bearings will last at over 9k rpm with 20+ psi going through them. Very tight clearances and a bit of luck may well be the answer, though lets hope I don't need to find out.

Edited by minimole23 on 4th Sep, 2013.

good valid points Colin & well put over buddy, so to further the debate on plain main bearing failures on a nice note:
in your opinion would through drilling of the centre main of the crank aid the oil flow to a plain bearing helping it to survive on the centre main only so it receives oil every 180 degrees instead of 360

Edited by BENROSS on 4th Sep, 2013.

Just read the section on bearings, interesting read. There seems to be a lot of pros and con for for grooved non grooved bearings. I would of thought that if localised heat was an issue on the big ends, then using a bearing that has a larger oil carrying capacity (grooved) would be better suited for high rpm engines. It mentions running larger clearances on the mains to allow for more oil flow, but I would of thought increasing oil pressure would be a better option to increase the flow of oil through the bearings?

That also lead me onto the section about rod clearances...12-14 thou is mentioned for high revving engines, that seems massive to me I'm used to using 6-8 thou? Is that what people run? Surely if the crank were to flex enough to need that kind of clearance you would need to run a larger clearance on the big end to allow for the amount of rock in the big end, and wouldn't that result in excess side loading of the piston/little end?

Just trying to make sense of all this, these clearances quoted are complete out of the norm for me!...lol

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1/4mile in 13.2sec @ 111 terminal on 15psi

Matty, I put the specific question about bearing clearancies on high reving high performance A series engines to a world renowned championship winning engine builder.

His reply was a complete contradiction of what is said in TBASE. bearing clearancies were said to be 0.0005" to 0.001". with that I questioned Vizard on why he suggested such large clearancies, and he got real shirty. His basic reply was that he said that because he cannot make sure that the average engine builder would build the engine to the perfection required to achieve clearances as tight as 0.0005"

Side clearancies......nothing much was said but in the main it is what it is and usually larger than anything specified factory tollerence.

End float should really still be within the factory tollerence.

Big clearancies mean the crank and rods will bounce arround and introduce loss of power through piston skirt side loadings and crank/ rod vibrations. 'Large' bearing clearancies can introduce oil whirl, oil whip and dry whip, basically instability of the oil film and bearing capacity as a whole.

Oil film...... The larger the bearing surface, the stronger that oil film becomes. Increasing the oil pressure will increase oil flow through the bearings, and as you say, reduce the bearing temperature, but it does nothing to increase the oil film strength. That is down to the surface area and the qualities of the oil itself. Modern oils should work better, but oils degrade quickly. Increasing the bearing surface has an imediate effect permenant.

So considering that most of us on here are pushing the boundaries of what these little engines can put out, some more than others, going from a plain bottom bearing that clearly has a higher load carrying capacity, to a grooved botom bearing can only a backwards step with regard to bearing load carrying capacity and reliablity?......

You can calculate the load carrying of the bearing assembly, but I'm not that clever

I completely agree with all of above. When tease starts talking about grinding journals within 10th alarm bells started ringing, as those tolerances seem very unrealistic to work to, and almost irrelevant when crank flex of 15thou is mentioned, that would make grinding journals within 10ths completely pointless!

I've been in contact with the company that do all our line boring and supply shells for our cranks and they cannot see any plain bearings available in +30 so it looks like groove/groove bearings it is then. Apart from running a good quality oil the only other thing i can see that could possibly help keep an even film across the bearing and help prevent it from biasing one side of the bearing is to get the shell to sit as centrally as possible on the journal by adjusting the crank thrusts.

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1/4mile in 13.2sec @ 111 terminal on 15psi

I'm not sure how centering the journal in the bearing will have any effect on the oil flow bias across each bearing 'land'. The pressure either side of the bearing will ultimately be pretty much the same, so the pressure drop across each land and ultimately oil flow will be the same?..................

I've seen on several engines that, where the bearing is biased to one side of the journal the wear tends to be towards the side where there is smaller clearance. These are generally on engines with minimal endfloat and the the shells run close to the radii of the journal. It could of been down to too much endfloat causing the bearing to ride up on to the radii but as the wear seemed to go further in than the width of the radii. I can only assume that this has caused an area where the oil can't escape and be replenished as quickly with fresh oil.

I guess it's just something I've been concious of lately.

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1/4mile in 13.2sec @ 111 terminal on 15psi