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Terry Haines

Conrod Bearing Info

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...in light of some of the info I have posted in the budget 3.0 post Ref...

 

www.fordcontour.org/30-budget-build-t10928.html&st=20&start=20

 

...would owners and DIY engine builders like some more detail on 'how it works' etc.??? :unsure:

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...so lets start off this way.

Forget rod and main bearings are in two halves..lets look a a plain round shaft that we want to run in a support plate with a round bearing bush...made of a common bearing material.First off the hole in the non moving part needs to be big enough to allow for the thickness of the bush we decide to use..Depending on the load on the shaft , we want the bush to stay fixed in the support plate and not turn with the shaft. To do this the hole is usually slightly smaller than the outside diameter of the bearing bush we intend to use...and is pressed in.This gives an interference on the bush (nip) so that it will remain fixed...As we press it in the INSIDE diameter of the bush will get a little smaller (crush) but must not be so small to prevent the shaft rotating...all with me so far?

 

 

..OK then.Lets switch to a sleeve bearing but in two halves, as in rod and main bearing liners. We STILL need the same features as in a full sleeve bearing...

The nip....If you fit half the bearing in half the rod or main bearing you will notice and feel 1)It has to be 'sprung in 2)It's TOO BIG...the edges cannot be fitted 'flush' at each end of the housing(s)..It MUST feel this way...The liner is slightly bigger/longer than the housing it fits into...think back to 'why' in the plain bearing.

So we have two half bearings that fit snug but are still proud at one end....

When the two halves are bolted together the ends of the bearings TOUCH FIRST before the cap is fully seated..

This is also a 'must have' condition and is the start of the 'crush' phase of assy.

The crush...

As the bearing halves are pulled together with the cap the radial load forcing the liner into the housing goes UP..

This is the same as pressing in the plan bush in a plain housing. This added pressure of the crush IS ALL THAT RETAINS THE LINERS INTO THE CAPS...

 

...all clear so far before we go to the 'issues' list....??? unsure.gif

 

 

 

 

 

...before I get too involved and deep with measurements..Let me state what the 'average' DIY builder who is fitting bearings ,main or rod,does....NOTHING!!! rolleyes.gif

 

Most all 'assume' that the bearings, new or otherwise are OK...also assume that the 'parent bore'(the housing without liners) is also OK and in spec...Most will not even use Platigauge to check running clearences!!

 

...so this, to me , is half assed!...Get that out of the way first eh!

Assume NOTHING...at least use plastigauge so you know ONE dimension is in spec!!!

 

To the question on 'liner proud', no, most FULL sets of engine data don't give this, it's established by measurement of...rod or main housing parent bores without liners fitted, parent bores with liners fitted and liner thickness..All of wich require micrometers, bore gauges etc to establish.A 'GOOD' engine builder will use these AND record mulitple measurements along the way..VERY involved 'IF' you do it 100%.

The liner proud on a 'cracked rod' is almost impossible to measure as you do not have a flat reference face to measure from.

Main can be measured but the proud reading is almost never listed in engine build specs...Next stage.... IF we did it 100% what dimensions would we use to help establish nip and crush ...to be sure the liner will not spin???

 

 

bearing_crush.jpg

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...in light of some of the info I have posted in the budget 3.0 post Ref...

 

www.fordcontour.org/30-budget-build-t10928.html&st=20&start=20

 

...would owners and DIY engine builders like some more detail on 'how it works' etc.??? :unsure:

sure, although I don't know if I would. only because I don't need to. I will read on and am capable of math equations. If I need to I would refer to this thread. Thanks Terry!

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The diameter of the hole or housing for the bearing. The liner must be slightly larger than the hole or housing interfearing with the movement of the bushing, but not too much that it won't allow the rod to move.

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....so measurements,other than running clearance.

To establish crush etc we would need...

*Average internal diameter of the housing WITHOUT bearings fitted.

*Average diameter WITH bearings fitted.

*Average thickness of each bearing liner.

 

From this we can establish the CRUSH or amount of interference that HOLDS the 2 bearing liners in the housing..

 

CRSH is MOST important to retain the liners,same as in a plan bush bearing.Amount of crush must be enough to overcome the shaft trying to turn the bearings in the housing...do we get that bit?

 

 

 

...math for the crush.(assumes we carry out all the measurements I have listed).

We 'need the 'outside ' diameter of the two bearing liners.

We have the inside diam(av) when they are fitted,we have the average thickness of each liner.

So we can establish the outside diam of the two liners...

 

This will be BIGGER than the diam of the rod less liner...take one from the other and we establish the 'crush' or interference that holds the liners tight into the rod/housing...easy eh!

 

 

Say the fitted diam is 20MM

av thickness of each liner is 1.5MM (x 2) for two liners...

That will give a liner outside diam of 23 MM

If the rod/housing dim WITHOUT liners is say 22.5 MM

then the 'crush is 0.5 MM....

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What sort of prep is advised for the outside of the bearing-inside of the rod to improve contact/grip?

 

Gar

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....the rod parent bore must be 100% clean, oil & varnish free...The back of the bearing liner...same, 100% oil and varnish, dirt etc free(the back of the line is a main heat/cooling path for the bearing). There are 'ol sweat engine builder tricks that can improve the nip/crush etc..The main point I'm leading to here is how the nip & crush AND the parent bore size of the conrod affect bearings spinning..

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Would it be out of the ordinary to find that the hole sizes for the bearings are different? Also then I would think that the bearing liners would have different thickness to accomidate this issue. Or would the holes get shaved to the same diameter? Say on a complete tear down and rebuild project.

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Would it be out of the ordinary to find that the hole sizes for the bearings are different? Also then I would think that the bearing liners would have different thickness to accomidate this issue. Or would the holes get shaved to the same diameter? Say on a complete tear down and rebuild project.

 

Does "line boring" ring a bell?

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.....any material removed from the parent bore of the rod or m/b housing will REDUCE the crush and take the housing (usually) outside it's specs/limits. Selective fit liners can't always be had for all bearings ...so you can't get back the crush...and as I said, it is the crush we need to prevent the bearing spinning so....what is the 'best' compromise for say the size of the parent bore(less liners) of a conrod...Top end of spec(biggest bore), bottom end(smallest bore but still in spec) or nominal???

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Guest KAOS_2.5M
.....any material removed from the parent bore of the rod or m/b housing will REDUCE the crush and take the housing (usually) outside it's specs/limits. Selective fit liners can't always be had for all bearings ...so you can't get back the crush...and as I said, it is the crush we need to prevent the bearing spinning so....what is the 'best' compromise for say the size of the parent bore(less liners) of a conrod...Top end of spec(biggest bore), bottom end(smallest bore but still in spec) or nominal???

 

is the idea here that machining the parent bores is a bad idea in general? what if there was a spun bearing, wouldn't the parent bore be unsuitible for liner fitment?

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NEVER m/c the parent bore unless oversize OD bearings are aval...which in the V6 they are not...If the bearing has spun the rod is shot...DUMP IT...and if one has gone the rest are not far behind ....new rods always...when they are PM.We will get to the why etc later..

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Guest KAOS_2.5M
...don't jump the gun here!. I will get to PM rod issues later..lets first FULLY understand nip & crush and how the bearings work OK...

 

no doubt, just felt like getting that out there.

 

 

Say the fitted diam is 20MM

av thickness of each liner is 1.5MM (x 2) for two liners...

That will give a liner outside diam of 23 MM

If the rod/housing dim WITHOUT liners is say 22.5 MM

then the 'crush is 0.5 MM....

 

so here, the ID of the liners is 20mm right?

 

and to establish average thickness, should we take more than one thickness measurement and actually average them? or do we just trust that the liner is uniform? I guess I'm wondering why you say "average thickness" rather than just "liner thickness"?

 

and is 1/2mm of crush a standard value, or just an example?

 

were would I find the crush spec for a specific engine?

 

[edit]my typing sucks

Edited by KAOS_2.5M

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...the numbers are just an example.If you are building an engine the standard way to measure liners in at around 10>12 points with a 1/2 ball micrometer and record all the readings...An average of all can be taken or you can do the math with max or min depending upon locations on the liners(s) Vs bore measurements on the parent bore of the housing less liners...

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Guest KAOS_2.5M
...the numbers are just an example.If you are building an engine the standard way to measure liners in at around 10>12 points with a 1/2 ball micrometer and record all the readings...An average of all can be taken or you can do the math with max or min depending upon locations on the liners(s) Vs bore measurements on the parent bore of the housing less liners...

 

 

so is this to say that liners arent actually uniform in thickness?

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Guest KAOS_2.5M
...not always..Measurement is the ONLY way to find out what you have and how the bearing was designed.

 

thats something pretty important to know then.

 

is a good stainless digital caliper good enough to take these measurements accurately enough to build an "acceptible" engine?

 

or is a nice micrometer a "must have" to do this well enough?

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...calipers will not work on a curved bearing liner.Correct tool is a micrometer 1/2 ball type or with a ball attachment...Bore gauges also have curved feet to measure a curved surface...Calipers or a stoc micrometer is only good on shafts...

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...well we have had 180+ 'views' but very little comment/ questions from the masses...???

 

....should we proceed or is the subject a bit too dry for most, or maybe too involved??? :unsure:

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Maybe you can discuss more about the PM process and it's nature in respect to the bearings, big end stretch, etc. I understand how "cracking" the big end to create the end cap creates more surface area for better grip versus machining. Does this inadvertantly affect bearing nip and crush?

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....to create the end cap creates more surface area for better grip versus machining.

 

...Not the real reason.Grip is established by the bolt torque etc...the cracking is to save any m/c cost's and make each cap unique so that it can only be fitted one way.(if you are looking!) and only on the same rod...

The PM issues/mode of failure/why it's NOT a lube issue etc...will come later. I want to be 100% sure that everyone understands nip/crush, why we must have it ant how it affects spun bearings...If we all agree that we understand I will do a quick recap then go forward. Crank bearings etc are NOT fully understood by most tech's let alone DIY owners..even the engine build books have a poor text that badly explain bearing liners..some even write that the 'tags' hold the liners!...A lost of bad info and BS out there....my aim is to clear up all the misinformation...

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..OK. So later today I will explain what can cause liners to spin. I should add here that in my early days in Ford Engine Engineering I was involved in proto engine builds and testing. Most all of this also involved review of engine build specs, clearances, special build to replicate 'out of limit' failures etc. Example being, engine built with excessive running clearences, out of spec conrods, oil starvation etc etc...All of these staged tests on engines were followed by a a teardown, examination, review etc etc. This covered, depending on the specific test, all internal engine components...incl bearing liners etc. Covered hundreds of such tests and simulation in my early days in Ford. I will explain more later but just as a question...

 

 

*What do you think can happen to say a rod bearing liner if 1)The 'crush' on the bearing is too low? or 2)The rod parent bore is too big and out of spec?

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...so now we all understand that the following are facts in bearing design and retention..

*Nip and crush are both critical parts of retention of the bearing in the parent bore, be it a main bearing housing or a conrod*

*The parent bore internal diameter also plays a major part in the degree of crush( retention) of the bearing liner in parent bores*The 'tag' or 'tang' on the bearing liner does NOT retain the liner, all it does is set the bearings position, side to side, in the parent bore.

 

So lets play 'chicken and egg'....

 

...An engine is in a dyno cell...running max load at WOT....revving it's guts out....exhaust manifolds glowing cherry red...

working hard at it's max HP....Connected to the oil pan or main oil gallery we have an auto drain.....We will carry out a 'Death Rattle' test... DUMP ALL THE OIL and OIL PRESSURE ASAP...with the engine still running 'wide open'...What could/ will happen???? :blink:

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