BMW M42 connecting rod modifications

[Boyracer]

I have bought new con rods for my BMW M42 engine project (1886 cm3 16 valve 8500 rpm) but there are several things I would like to get help with.

1. OEM rod big end has oil escape notches on both sides of the rod. These are missing from aftermarket rods. I think they should be machined to retain oiling system as designed by factory?

2. Pistons are originally meant for BMW S50B30 M3 engine which has 2 mm narrower big and small end widths than M42 engine rods. Now aftermarket con rods barely fit to the piston, there is maybe 0,1 mm lateral movement when piston pin is in place. On OEM M42 piston and con rod there is something like 3 mm.

I realize 0,1 mm might be bit too little if piston bore centerline and crankshaft journal center are not exactly inline. My thinking is that I should get some material removed from aftermarket rod small end sides rather than from the piston to allow more space for con rod to move if necessary. Or is it better to take material off from piston?

Thank you in advance.

[Guy Croft]

What make are those rods?


G

[Boyracer]

No idea about the original manufacturer I'm afraid, they are just general purpose BMW M4?/M5? engine family rods. I bought them from local shop which got them from Swedish company who got them from don't know where. Bolts are ARP 2000.

My untrained eye has not yet spotted any serious flaws (although cutout for rod bolts could have been neater) so far but I have not made any really accurate measurements, I only have digital caliper.

[Rich Ellingham]

Boyracer, I would make the estimate that these are another unknown Chinese origin rod. My advice would be not to use them, you have no traceability of true knowledge of the material they are made from. I would ask you to use a bore gauge and measure the big end bearing circumference to see if it is even round. I know previous test on similar rods by Guy Croft showed them to be 'out of round', with a varying diameter in different orientations in the big end bore. When you consider how important the connecting rod is and the stresses it encounters there is no way I would go to such an effort of engine building with something so unknown. I like the fact that the rods in my race engine (cunningham) are made by a company who cares about my engine as much as I do!

Rich

[Boyracer]

Yes, I too think they are originally from far east. And I also understand how important rods are for engine...

But this is a budget build and pretty much only alternative would be using heavier original (forged) rods which are from engine that suffered a broken timing chain and large number of bent exhaust valves, incident that might have done something to rods too. Also, I have no idea what kind of life they lead before that and for how long.

I will check the important measurements very carefully next!

[Guy Croft]

http://guy-croft.com/viewtopic.php?f=25&t=2124

GC

[Boyracer]

I am sorry, I just noticed from the rules that questions regarding no name con rods are off limits. I have no intention to break the rules.

Please delete this discussion as I cannot do it myself.

[Guy Croft]

It's OK!

Thank you for your respect. But there is no harm done and no need to delete it, we can all learn from this. Not your fault if the rods aren't right,I blame the folk selling them really.

BTW - I just added that rule today.

G

[Boyracer]

Ah, that's why I did not remember seeing the rule earlier... It is quite understandable rule. I work on telecom business and much of our work has been outsourced to china where they then directly copy our products and then sell them on western markets undercutting our prices and gaining marketshare. It is just stupidity from our shortsighted company management and western shareholders.

If it is ok for you, I will first get some proper measurement tools and try to measure the rod dimensions and then maybe later post here how they match factory specification? Might be interesting story and atleast people get a good laugh from my total inexperience regarding engine building and taking accurate measurements :)

[Guy Croft]

sure!

G