Fitting a con-rod the wrong way.

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Model T Ford Forum: Forum 2015: Fitting a con-rod the wrong way.
Top of pagePrevious messageNext messageBottom of page Link to this message  By Frank van Ekeren (Australia) on Tuesday, July 14, 2015 - 03:17 am:

On another thread, we had some controversy on the consequences of fitting a connecting rod backwards in an model T, usually not a problem that comes up to often as most rods are fitted per book instructions, they go in the right way.
Today going through a engine, with the owner, for what was needed for the rebuild, this was found.



This shows the centre line of a achieving maximum strength of the small end when fitted right, bolt facing the cam shaft.


And the weak point in a rod flip, is the strain on the bolt side on the inlet stoke, pulling on the top of the rod clamp.

It's like fitting a fibre timing gear,
it could last for the life of an engine or fail tomorrow!


Top of pagePrevious messageNext messageBottom of page Link to this message  By Richard Gould, Folsom, CA on Tuesday, July 14, 2015 - 09:55 am:

Hi Frank, I asked a question on the earlier post, but I couldn't find the post. Was it deleted?
I couldn't see see how placement of the bolt mattered. I understood how there was stress on the top of the rod on the intake downstroke, but failed to see how placement of the bolt was relevant. The more I think about it, I think I see the reason. It really has nothing to do with the downstroke, except to say that the intake downstroke is where the wrist area is being pulled down, not pushed down. The reason has to do with the direction of the circular motion of the crankshaft. With the bolt placed opposite the crank as in your illustration above, the rotation acts to pull the wrist opening apart whereas if the bolt is placed on the camshaft side, the rotation acts to push the wrist opening together. That being said, if the clearance between the wrist pin and piston is adequate so the wrist pin can rotate easily, there shouldn't be much stress on the bolt. If I've missed the boat on this, please enlighten me.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Frank van Ekeren (Australia) on Tuesday, July 14, 2015 - 05:48 pm:

Richard, glad to see someone understands it, yes you are right, but also you have the reverse thrust on this stroke as well, many reasons can over stress a gudgeon pin, we have seen several threads on the piston bosses being to tight in new pistons, engine running to hot, low oil etc, but as you can see that first photo is of the rod out of a very worn engine and at some stage has managed still to snap the bolt side like a carrot.

We know that a crank will break, not if but when, so we try to prolong that buy driving with out laboring or over reving, the rod around the wrong way is the same risk which is one that doen't need to be taken by simply fitting it right in the first place.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Ted Dumas on Tuesday, July 14, 2015 - 05:59 pm:

The rod is a two force member with a near frictionless pin joint at each end. The forces at each end are equal and opposite and in line with the centerline of the rod.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Ted Dumas on Tuesday, July 14, 2015 - 06:01 pm:

The rod is a two force member with a near frictionless pin joint at each end. The forces at each end are equal and opposite and in line with the centerline of the rod.


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