Con Rod Journals

[carlt]

Can you use Narrow journal rods on a wide journal crank

just been offered a long 85mm stroke crank [wide journal]
I only have steel narrow journal rods

[Chancer]

No!

Well if it was life and death the engine would run for a while but I reckon the big end shells would displace laterally and it would all end in tears.

I am sure that someone somewhere has done so in ignorance, anyone care to own up?

In a similar vein a young friend built up a new engine from all new bits for his Talbot Sunbeam, I was given the job of balancing the carbs and noted that whilst there was no blow by to speak of it seemed to be burning a bit of oil and it would gradually choke itself up on oil fumes at idle, there was no way that I could actually balance them and it got pretty choky in the garage.

I asked lots of questions and when we got down to how had he fitted the oil control rings, the expander and the scraper rings he replied "what scraper rings".................

[rgh0]

The rod is positioned by the crank journal and is free to float on the piston wrist pin at the piston end. The crank journal holds it in the right position versus the piston in the centre of the wrist pin.

Even if the bearing shells stay in place which I think they would due to their clamping by the big end journal the narrow rod would float to one end of the wide journal or other and this would then mean the piston would not be correctly supported and you would get accelerated wear if not outright breakage of the piston due to the offset loads.

Overall not a good idea especially if building a competition engine which is why you would typically be using narrow journal rods. If its a road engine 125E standard rods are pretty cheap.

cheers
Rohan

[Witofthestaircase]

Carl I've just had two sets of wide journal standard length I beam steel rods checked for true, rebushed, honed to size and balanced. PM me if you're interested in having a set and I'll send some pics etc All the best Russell

[carlt]

The rod is positioned by the crank journal and is free to float on the piston wrist pin at the piston end. The crank journal holds it in the right position versus the piston in the centre of the wrist pin.


cheers
Rohan

I am going to have to have custom pistons -
how about pressed pins and ptfe buttons to centre the rods on the crank journal ?

[rgh0]

I guess its possible to do something like you say to use the piston to centre the rod on the crank but your now starting a fundamental redesign of the core of the engine so i would proceed carefully and try to do research on any other engines that have taken this approach to see how they did it. Piston temps may get to hot for PTFE in some circumstances but you could set up spacers ( maybe bronze?) on the wrist pins between the rod and the bosses on the piston to hold the rod in the centre.

Personally I would just get some new rods that suited the crank you have or a new crank that suited the rods you have the cost is small compared to the cost of an engine failure as you work to develop a unique solution

cheers
Rohan

[carlt]

I guess its possible to do something like you say to use the piston to centre the rod on the crank but your now starting a fundamental redesign of the core of the engine so i would proceed carefully and try to do research on any other engines that have taken this approach to see how they did it. but you could set up spacers ( maybe bronze?) on the wrist pins between the rod and the bosses on the piston to hold the rod in the centre.

cheers
Rohan

Apparently the Peugeot M16 engine uses the piston boss to centre the rod - with lots of BE clearance on the crank

[Lotus14S2]

You should not run narrow rods on a wide rod crank. It is not just the positioning of the rod in the piston, but there is also side thrust that is supported by the relationship of the rods and crank. Different engines have different side thrust clearances, and this is to prevent the possibility of the rod thrust through the side of the piston, forcing the piston into the cylinder wall, which will oval the bore. Teflon buttons will only hold a full floating wrist pin in place, but will not prevent the thrust of the rod into the inside surface of the piston. There are some engines with large side clearances, as much as .06", but normally it is around .01" or so. There is the normal thrust of the piston by the rod because of angularity, but this thrust is at right angles to that.
If you have a very good wide journal crank, then see if you can trade it for a narrow journal crank, or your rods for wide journal rods.

[rgh0]

You should not run narrow rods on a wide rod crank. It is not just the positioning of the rod in the piston, but there is also side thrust that is supported by the relationship of the rods and crank. Different engines have different side thrust clearances, and this is to prevent the possibility of the rod thrust through the side of the piston, forcing the piston into the cylinder wall, which will oval the bore. Teflon buttons will only hold a full floating wrist pin in place, but will not prevent the thrust of the rod into the inside surface of the piston. There are some engines with large side clearances, as much as .06", but normally it is around .01" or so. There is the normal thrust of the piston by the rod because of angularity, but this thrust is at right angles to that.
If you have a very good wide journal crank, then see if you can trade it for a narrow journal crank, or your rods for wide journal rods.

I agree with the conclusion for practical reasons of cost and development risks as I have said in above posts but I am not sure you have the engineering detail of your explanation correct - maybe I am interpreting it wrong

One end of the con rod will always be free to float longitudinally along the crank axis. In the twin cam as in most engines float is at the wrist pin end of the rod and longitudinal location is provided by the crank. Alignment of the rod at right angles to the crank is always maintained by the width of the small and big end journals (not by the longitudinal location) so the rod thust is across the engine at right angles to the crank in plan view and this side thrust is created by the rod angle though its stroke cycle. The pistons skirts are designed to take the rod side thrust in this direction across the engine through its cycle.

Designing an engine where the logitudinal location is by the piston rather than the crank is possible but not normally done and would be an engineering exrecise to introduce it into a twin cam. The end result in terms of the major crank, piston and rod loads is the same.

In some engines piston wrist pins are offset from the centreline sideways to reduce the rod side thrust on the power stroke and so miniise bore wear in this direction which is where most wear occurs. I dont know of any engines where rods are offset longitudinally from the centre of pistons but I am sure someone somewhere will have tried it at sometime

cheers
Rohan

[Quart Meg Miles]

rghO wrote:

In some engines piston wrist pins are offset from the centreline sideways to reduce the rod side thrust on the power stroke ...

Like the TC and almost all other engines. It's about 1/16" offset on a used piston I have.

PS How should I be doing these quotations, I can't find a method in FAQ?

[Lotus14S2]

I am not talking abut thrust caused by connecting rod angularity.
It is my opinion that factories, builders, and rod manufacturers all seem to place rod side clearance service limits for a reason.
If you have, for example .040" clearance or side float of the rod, and .010" clearance between the inside shoulder of the wrist pin boss and the connecting rod small end, you can have a thrust along the longitudinal axis of the crankshaft, which will allow the connecting rod to press against the inside boss of the wrist pin, and thus force the piston into the wall of the cylinder. You can also, with a full floating wrist pin, hammer the retainer out of place, which can be caused by the rod twisting or a slightly out of alignment rod. From what I?ve read, this is possible, and can cause ovaling of the cylinder wall, and thus a loss of compression. The connecting rod does get compressed and stretched; it can also twist under the forces it sees.. There is also an argument that excessive side clearance will cause over oiling of the cylinder walls.
If the car is just an ordinary street car, then the loss might not be enough to cause concern, but it is possible wear, and could lead to expensive part replacement at some inconvenient time in the future. If on the other hand you are building a race engine, then I would think it is best to err on the side of accuracy and precision to begin with, and have a good baseline which would allow for experimentation.
I don't know if my engineering is in error, but the above comes from reading a few texts and popular books on the mechanics, construction, and assembly of engines. I have been a mechanical engineer for the past 50 years; although this is not a requirement -- and might even be a detriment -- to build engines.
Good luck with your project.

[nomad]

If there are manufacturers that locate the rod in this manner I'll bet they use a full skirt piston in bores that are not radically over square. Seems that as the engine wears misalignment problems would amplify themselves so that I doubt the engines would have a long life.

Just my .02 and I is trained as a ingineer 2!

Kurt.