68 replies to this topic

[cheapracer]

Quote

Originally posted by Ray Bell
While on this subject, has anyone got any idea about using narrow rods on wide journals?

I know there are rods which are 'piston guided'... I don't know to what extent. But with the modern shift to tiny big ends, many cranks can be ground down to the journal size, but the journal width is often too great.

Has anyone ever addressed this issue? I mean found an answer to it without replacing the crank?

Ray I was involved with Toyota's a lot in the late 70's and the 2T/3T series had something like a 16mm wide big end shell from the factory but all replacement bearings both OEM and other were about 12mm wide and never a problem was had. Looked strange in the con rod though.

[Kevin Johnson]

Quote

Originally posted by cheapracer


Ray I was involved with Toyota's a lot in the late 70's and the 2T/3T series had something like a 16mm wide big end shell from the factory but all replacement bearings both OEM and other were about 12mm wide and never a problem was had. Looked strange in the con rod though.

That's really interesting. Did you ever hear of issues with the wider shells (leading to the narrower ones)?

[bobqzzi]

Quote

Originally posted by Ray Bell
While on this subject, has anyone got any idea about using narrow rods on wide journals?

I know there are rods which are 'piston guided'... I don't know to what extent. But with the modern shift to tiny big ends, many cranks can be ground down to the journal size, but the journal width is often too great.

Has anyone ever addressed this issue? I mean found an answer to it without replacing the crank?

You can narrow the entire big end of the rod so it is no longer being guided by the sides of the crank journal. You'll need the small end of the rod made to about .010" narrower than the piston wrist pin bosses. If you already have rods and pistons, you cna use spacers as long as you make sure the wrist pin bosses are 90 degrees to teh pin.

[Ray Bell]

I don't understand that one, Bob... 90° ti the pin?

While we're here... the issues that are raised by this seem to include a serious one about oil containment in the bearing. Normally there is a restriction, the narrow clearnance between the cheeks of the crank and the sides of the big end, that helps keep the oil in the bearing, keep up the oil pressure.

I had a thought about making some form of spacers, which would have to be in two pieces, of course. My idea is to make pieces that dovetail together sideways and are slipped onto the journal prior to assembling the rod on the journal. Perhaps with laser cutting it might not be too hard to make these with the required precision?

[bobqzzi]

Quote

Originally posted by Ray Bell
I don't understand that one, Bob... 90° ti the pin?

While we're here... the issues that are raised by this seem to include a serious one about oil containment in the bearing. Normally there is a restriction, the narrow clearnance between the cheeks of the crank and the sides of the big end, that helps keep the oil in the bearing, keep up the oil pressure.

I had a thought about making some form of spacers, which would have to be in two pieces, of course. My idea is to make pieces that dovetail together sideways and are slipped onto the journal prior to assembling the rod on the journal. Perhaps with laser cutting it might not be too hard to make these with the required precision?

Ray, rod side clearance does not control oil flow- it is controlled exclusively by the bearing clearance, pressure and viscosity of the oil. If you calculated the area of exposed bearing edge, and then calculate the area at side clearance, you'll see that the latter is much larger and cannot impose a restriction.

The spacers go on the wrist pin and take up the space between wrist pin boss and the sides of the small end. The side of the small end of the rods and the sides of the wrist pin bosses must be parallel so as to get a consistent .010" clearance all around.

[Kevin Johnson]

Quote

Originally posted by bobqzzi


Ray, rod side clearance does not control oil flow- it is controlled exclusively by the bearing clearance, pressure and viscosity of the oil. If you calculated the area of exposed bearing edge, and then calculate the area at side clearance, you'll see that the latter is much larger and cannot impose a restriction.

No, this is not correct. It is true that the area is normally much larger and that in itself would not present an oil restriction. The restriction comes into play when the side of the rod contacts the side walls at the pin. When this happens, the instantaneous oil release rate from the bearing is cut in half -- think about it. If you look at a used connecting rod (I am looking at a Porsche 928 one right now) the sides of the rods have burnished wear patterns indicating full contact. The question is what percentage of the time it happens but if it happens at all (which it does) oil flow is reduced. With a V8 or V6 with two rods sharing a pin the effect is magnified.

When there is no possibility of contact at all the oil flow is greater and controlled by the parameters you list (perhaps others as well -- radius of the junction between pin and wall, for example).

It would be interesting to learn if the rod normally cycles back and forth on the pin dependent on the thrust stroke or the oil pressure fluctuation from side to side. It would seem that an equilibrium would be favored but from the wear pattern this cannot always be true.

[McGuire]

If the rod side clearance is say .015 inch, the rod can shift to one side or the other or ride in the middle, but the net clearance will remain .015 inch. Meanwhile the journal clearance is in the neighborhood of .0015 inches, so side clearance has no significant effect on system (gauge) oil pressure.

However: Gauge pressure aside, side clearance will still affect the local oil flow around the big ends. (Two different things hmm.) On some engines, Olds and Ford V8s to name two documented examples, too little side clearance can result in insufficient oil cooling of the big ends. The tell is coking (blackened oil residue) in the crevices of the connecting rod body and around the fasteners.

Within reasonable limits, from the the connecting rod's point of view it can't have too much side clearance, though it will increase windage, and also tax the skirts and rings. Like many engines, the SB Chevy was designed to oil the cylinder walls via connecting rod throwoff. Unless you are running piston-guided rods or some other exotic setup, there is no reason on earth to depart from the factory specs. You can swap the rods around on the crank until you have the clearances you need, matching the narrowest rod to the narrowest journal, etc. Then you may have to narrow the widest rod(s), which can be easily done by lapping the big end on a surface plate with 180 paper. But even more important, at no point can the bearing insert be allowed ride into the journal radius.

[Kevin Johnson]

Quote

Originally posted by McGuire
If the rod side clearance is say .015 inch, the rod can shift to one side or the other or ride in the middle, but the net clearance will remain .015 inch. Meanwhile the journal clearance is in the neighborhood of .0015 inches, so side clearance has no significant effect on system (gauge) oil pressure.

However: Gauge pressure aside, side clearance will still affect the local oil flow around the big ends. (Two different things hmm.) On some engines, Olds and Ford V8s to name two documented examples, too little side clearance can result in insufficient oil cooling of the big ends. The tell is coking (blackened oil residue) in the crevices of the connecting rod body and around the fasteners.

Within reasonable limits, from the the connecting rod's point of view it can't have too much side clearance, though it will increase windage, and also tax the skirts and rings. Like many engines, the SB Chevy was designed to oil the cylinder walls via connecting rod throwoff. Unless you are running piston-guided rods or some other exotic setup, there is no reason on earth to depart from the factory specs. You can swap the rods around on the crank until you have the clearances you need, matching the narrowest rod to the narrowest journal, etc. Then you may have to narrow the widest rod(s), which can be easily done by lapping the big end on a surface plate with 180 paper. But even more important, at no point can the bearing insert be allowed ride into the journal radius.

Thanks for the confirmation. One way to help ensure avoidance of the cooling problem would be the addition of directed oil ejection slots directed at either the thrust face of the cylinder wall (example Chrysler) or the piston underside (example BMW). The Porsche rod I have here has about 8mm^2 of clearance including both sides of the rod (using .0508mm bearing clearance and 52mm journal diameter). If you add 4mm^2 openings to both sides of the rods I think that this would help avoid a potential rod cooling problem ala the Olds and Ford.

This is different from adding the oil squirter in the body of the rod fed from the interior of the bearing like with the Porsche 924 (Audi based engine). That becomes an additional controlled leak in the circuit. I have not seen the specific oil squirter design for the 928 that had to be withdrawn because the excess windage. [This pushed the engine into overheating and exacerbated the windage based oil aeration problem.] I am pretty sure the design was not slots on the sides of the rods. I think if Porsche had used the slotted faces that would have avoided the heightened windage problem and allowed the additional piston cooling they were after.

[bobqzzi]

Quote

Originally posted by Kevin Johnson


No, this is not correct. It is true that the area is normally much larger and that in itself would not present an oil restriction. The restriction comes into play when the side of the rod contacts the side walls at the pin. When this happens, the instantaneous oil release rate from the bearing is cut in half -- think about it. If you look at a used connecting rod (I am looking at a Porsche 928 one right now) the sides of the rods have burnished wear patterns indicating full contact. The question is what percentage of the time it happens but if it happens at all (which it does) oil flow is reduced. With a V8 or V6 with two rods sharing a pin the effect is magnified.

When there is no possibility of contact at all the oil flow is greater and controlled by the parameters you list (perhaps others as well -- radius of the junction between pin and wall, for example).

It would be interesting to learn if the rod normally cycles back and forth on the pin dependent on the thrust stroke or the oil pressure fluctuation from side to side. It would seem that an equilibrium would be favored but from the wear pattern this cannot always be true.

Actually it is correct- and it is the burnished wear that you are trying to avoid as it represents a frictional loss. The benefits piston guided rods are the reduced friction due to the replacement of 360 degrees of rotating motion with the much smaller sweep or 12-15 degrees at the small end of the rod. Additionally, since the big end of the rod is narrowed, it weighs less and is generally accompanied a narrower rod bearing which reduces friction. Using piston guided rods would probably not be the thing to do for a street engine, but it is virtaully free horsepower in a race engine and not hard to do. Most manufacturers will make them for you, or you can convert standard pistons by milling the wrist pin bosses parallel and then using appropriate spacers.

[Kevin Johnson]

Quote

Originally posted by bobqzzi


Actually it is correct- and it is the burnished wear that you are trying to avoid as it represents a frictional loss.

No, it is not correct. The fact that the wear does occur -- irrespective of its deleterious nature -- indicates that the oil flow is being restricted from that side of the rod.

Quote

Originally posted by bobqzzi

The benefits piston guided rods are the reduced friction due to the replacement of 360 degrees of rotating motion with the much smaller sweep or 12-15 degrees at the small end of the rod. Additionally, since the big end of the rod is narrowed, it weighs less and is generally accompanied a narrower rod bearing which reduces friction. Using piston guided rods would probably not be the thing to do for a street engine, but it is virtaully free horsepower in a race engine and not hard to do. Most manufacturers will make them for you, or you can convert standard pistons by milling the wrist pin bosses parallel and then using appropriate spacers.

I am not arguing that wonderful things cannot result from narrow rods rather one of their consequences with respect to oil flow, ceteris paribus.

[bobqzzi]

Quote

Originally posted by Kevin Johnson


No, it is not correct. The fact that the wear does occur -- irrespective of its deleterious nature -- indicates that the oil flow is being restricted from that side of the rod.




I am not arguing that wonderful things cannot result from narrow rods rather one of their consequences with respect to oil flow, ceteris paribus.

Hmm..so the journal is, for example, 1" wide. You have 2 rods on it totalling .950" wide leaving a .050"

How does the rods sliding back and forth decrease this opening? It doesn't, so total oil flow through the journal is unchanged. Since this area is already much larger than the bearing clearance, gauge pressure is unaffected.

If you are suggesting that the side of the rod experiences reduced flow, then that is correct- but this doesn't mean the bearing itself gets less oil.

The bottom line is that at any "normal" side clearance (ie, enough to keep the rod from coking up as McGuire describes) oil pressure and total flow are unaffected by increases in this clearance.

[Kevin Johnson]

Quote

Originally posted by bobqzzi


Hmm..so the journal is, for example, 1" wide. You have 2 rods on it totalling .950" wide leaving a .050"

How does the rods sliding back and forth decrease this opening? It doesn't, so total oil flow through the journal is unchanged. Since this area is already much larger than the bearing clearance, gauge pressure is unaffected.

Each rod bearing shell pair describes a cylinder in three space. The journal passing through it makes the incremental openings at both ends washers with zero height -- think back to calculus integration/area exercises. Each "washer" will flow a given amount of oil under the parameters you described. The total flow is dependant upon both washers being free to flow oil.

When the rod is up against the pin area or the adjacent rod then one or more "washers" are closed off and the flow stopped. In the extreme example with a rod pair only one of four "washers" would be exposed.

It can be argued that the space at the side of the rod up to the bearing shell could act as a reservoir and the incremental flow rate would never be enough to charge this void to the extent that flow would stop. However, empirical evidence suggests that this is indeed the case.

Quote

Originally posted by bobqzzi

If you are suggesting that the side of the rod experiences reduced flow, then that is correct- but this doesn't mean the bearing itself gets less oil.

Part of the bearing surface does not experience less flow. The temperature gradiant across the surface of the bearing would change, however, since the side of the shell where flow has been blocked needs to have its heated oil traverse the width of the shell to depart -- that path also contra to the prevailing oil supply feed. This additional time would allow the oil to absorb even more heat and reach the temperature at which coking would result. This is what the empirical evidence suggests.

Quote

Originally posted by bobqzzi

The bottom line is that at any "normal" side clearance (ie, enough to keep the rod from coking up as McGuire describes) oil pressure and total flow are unaffected by increases in this clearance.

No, that's not totally correct. Pressure -- for all practical purposes -- is unaffected [presuming that the flow rate is within the capacity of the pump -- generally a given under normal operation]. Flow is affected and the anecdotal evidence is well known of additional entrained oil in the windage volume.

There will be a point/area of inflection where additional side clearance will not produce any additional flow -- this would be some increment past where the rod cannot physically meet the side of the journal area or its rod pair partner. In this case, as well, no wear pattern would be observed, of course.

[cheapracer]

For who mentioned side spacers at the little end, you obviously didn't work on 2 strokes in the early 70's!

I have seen no end of motors destroyed from spacers - as soon as they have the slightest excess clearance they pound quickly into an elongated shape and let go soon after leaving rather a mess.

Gaaawd, I just remembered the other method of centralising the rod in those days, bolt and clamp small end (shudders).

[McGuire]

In regard to oil slots in the thrust face of the connecting rod... the first automotive application I am aware of is 1949 Cadillac OHV V8. There it was used in lieu of a drilling in the rod beam to oil the pin. But it was surely used before that.

There are a lot of ways to move oil around in an engine... the splash and semi-pressurized systems of the distant past are especially interesting to study. A favorite of mine is the 1919 Overland Four, designed to compete head-to-head against the Model T Ford. The timing gears served as oil pump.

[Kevin Johnson]

Quote

Originally posted by McGuire
In regard to oil slots in the thrust face of the connecting rod... the first automotive application I am aware of is 1949 Cadillac OHV V8. There it was used in lieu of a drilling in the rod beam to oil the pin. But it was surely used before that.

It looks like they are a holdover from splash lubricated rods like on the Model A Ford. I see them in some pictures of period rods. Kohler uses the same patterns currently on its splash lubricated rods.

Quote

Originally posted by McGuire

There are a lot of ways to move oil around in an engine... the splash and semi-pressurized systems of the distant past are especially interesting to study. A favorite of mine is the 1919 Overland Four, designed to compete head-to-head against the Model T Ford. The timing gears served as oil pump.

Lots of sharp engineers back then.

I found PN 2287735 Halford 1942 (for Napier) for the drilling in the beam for piston cooling. Probably came up before that in some form but older patents often don't cite prior art extensively.

[McGuire]

Not Model A Ford. They did have an integral oil scooper in the cap, however.

[Kevin Johnson]

Quote

Originally posted by McGuire
Not Model A Ford. They did have an integral oil scooper in the cap, however.

It may be that the rods I saw were rebabbitted and the slots were added then.

Here's the pic (from ebay) [rod on left]

[McGuire]

A characteristic feature of a super deluxe Model A Ford babbit job is the X groove. First a radial slot is cut in the bearing at the dipper hole using a Woodruff cutter. Then a long, arcing "X" is cut in the babbit that proceeds from the dipper hole out in four directions, the idea being to distribute the dipper oil across the width of the bearing. This requires a special tool that is rather interesting... and probably susceptible to googling.

I believe the Halford patent cited above is somewhat misleading. The novel feature there is a method of timing. Connecting rod drillings have been around forever, along with squirt holes, spit holes, and spurt holes of various kinds. Historically, inline engines generally employed a spit hole in the shoulder of the rod at 10 o'clock or so to oil the cylinder wall (some Toyotas and others still use it) while V8s often used a hole in the parting line of the rod to oil the opposite bank's cylinder wall.

EDIT: if you think about it, the oil slot in the thust face has the opposite purpose in the Model A vs. the Cadillac OHV V8. Since one is pressurized and one is splash, one is to let oil in and the other is to let oil out.

[Kevin Johnson]

Quote

Originally posted by McGuire
A characteristic feature of a super deluxe Model A Ford babbit job is the X groove. First a radial slot is cut in the bearing at the dipper hole using a Woodruff cutter. Then a long, arcing "X" is cut in the babbit that proceeds from the dipper hole out in four directions, the idea being to distribute the dipper oil across the width of the bearing. This requires a special tool that is rather interesting... and probably susceptible to googling.

I believe the Halford patent cited above is somewhat misleading. The novel feature there is a method of timing. Connecting rod drillings have been around forever, along with squirt holes, spit holes, and spurt holes of various kinds.

The Halford patent is useful because it provides a hard reference to the oiling method. It is frustrating because the general method is referred to but not formally cited as in later patents. No worry, here is a 1921 report on the 300 hp Benz aircraft engine [translation of a contemporary German engineering journal article]. See page 6.

http://naca.central..../naca-tn-34.pdf

That gives another data point in the time line.

Quote

Originally posted by McGuire

Historically, inline engines generally employed a spit hole in the shoulder of the rod at 10 o'clock or so to oil the cylinder wall (some Toyotas and others still use it) while V8s often used a hole in the parting line of the rod to oil the opposite bank's cylinder wall.

Chrysler (Neon 2.0) and BMW (see S50) are two relatively contemporary examples of the slots being on the big end faces, directed at the thrust wall of the cylinder and underside of the piston respectively.

Quote

Originally posted by McGuire

EDIT: if you think about it, the oil slot in the thust face has the opposite purpose in the Model A vs. the Cadillac OHV V8. Since one is pressurized and one is splash, one is to let oil in and the other is to let oil out.

Yes, I am sure that occurred to an engineer at the time of transition. The novel application here (just guessing) is the realization that the grooves can serve as a sort of safety valve for the overheating of big ends due to too little side clearance.