how often to change a conrod
 

hey guys i have always wondered how often do u change the conrod in the engine any suggestions as to how often and under any circumstances ie. after run in, after how many litres, and so on any help is appreciated

Most factories suggest changing the initial conrod after break-in, and after wards that connecting rods be subjected to a measured operating life, and replaced maximum after 5 liters of fuel under normal operating circumstances.

It is also very important to check the condition of the connecting rod periodically during their operating life. This check should be performed after each race day, and the easiest way is to remove the rear cover from the engine (raise the piston up into a compressed position first! before removing the rear cover) then gently rotate the crankshaft in both directions to determine the amount of play in the connecting rod.

If the amount of play seems excessive (you can detect movement / play with your eye), carefully disassemble the engine and measure the crankshaft pin and connecting rod, a maximum differential of 0.04 and 0.05 mm in diameters is acceptable.

The fact that the crankshaft pin must be absolutely round (as must be the hole in the connecting rod) is one of the most overlooked aspects of engine wear, and can result in damage even with a brand new rod. The proper way to measure this is to measure with a micrometer on one axes (in line with the direction the rod would travel) and then again on the axes perpendicular to the first. The difference of these two measurements should be < 0.002 if any.

Also consider replacing the rod if the engine has seen any trauma such as a sustained over revving of the engine, or a significant over heating.

As a general rule engines that are run hard should have a rod replacement at least every 1.5 gallons of fuel regardless of visual inspection.

It is very hard to standardize how hard people run their engines, or what kind of gearing or temperatures these engines see, but is impossible to rebuild and engine with a hole in it, so error on the conservative side is in order.


AFM

It's too bad that manufacturers don't put out any specs on Mean Time Between Failures (MTBF) for conrods based on RPM and fuel consumption.

Question(s):

1. Why raise the piston to the compressed position before removing the rear plate?

2. What would cause a crankshaft pin to go out of round?

3. Sustained over revving for how long?

4. What would you consider significant overheating and for how long?

As always, thanks so much for your time and knowledge Alfonso. :)

1.- You could do it with the backplate off too. In that position the piston is catched by the pinch and by just moving the crank side to side (very little) without the piston moving, you are able to see the play on the crankpin end.

2.- The crankpin will allways go out of round with time, because the greatest stress over it occurs with every change in rotation of the rod/crank after explosion more or less at 45º, which you'll see why when doing the wear/play inspection...ther is a sudden clank on the pin.

3.- For example you loose radio and the car goes loose overrevving until it stops or gets stopped by someone.

4.- Overheating.... for lets say one tank...

AFM

AFM - Great information and many thanks for your site on this...

I'm new to Nitro and this really helps in knowing what to look for... I have learned a lot, but have a lot more to learn.... :nod: :nod:

The frequency will vary on the materials , how its run in and quality of oils in the fuel you use. In my experience ive had motors last over 35 litres before needing a rod change , others needed one after 15 litres .. the 35 litre one was a tiwanese motor and the 15 litre one was an italian motor.. both were punished equally just one wore more quickly..

thanks for the replys just a couple more questions

why is it recomended to change the conrod after run in?

wats the effects that run in has on it that normal day to day use doesnt?

cheers kosta

Its the fuel, i've put at least 15 litres through my TZ and its still tight! ;)

I change rods when they break? :lol: I have a pet hate for getting those little clips off the gudgeon pin so i prefer to just leave my conrods. :weird:

Rods
 

Rpm is the rod killer. Onroad motors i would change them after a gallon to a gallon and a half. Offroad motors will last double that because they don't see the rpm of an onroad motor.

Because when the engine is new, all tolerances are so tight that it places extra stress on the conrod....many times piston gets stuck at initial starts, so it places lots of loads on conrod bushings, and that is why we do break-in process on engines.

As I said before It is very hard to standardize how people run their engines, so error on the conservative side is in order.

AFM

I save a rod just for break in purposes. Take the new one out, slap in the break in rod and have at it. When you're done breaking in, replace the original (new rod). I use the break in rod for 2- 3 engines- possibly 4 depending on how much fuel it takes to loosen them up.

The top to bottom crank pin measurement will always be smaller after running for a while.... maybe only 0.005 smaller, but thats all it takes. Most good cranks will be 0.196" all around and probably slightly less up and down after running a few gallons- like 0.1955". If you get to 0.195" it's time for a new crank because even a new rod will be loose.

If you don't run the engine too hot, overrev or have a bad break-in (too cold), you should get 2.0gal on a rod. Just inspect frequently ;)

Now, currently I have 2 engines with 1.75gal each on original R5 rods. I will run practice and maybe a heat or two more with each at the WinterNta's before I replace rods.... no matter how they feel/look/measure, etc.... 2.5gal. is my cutoff on a rod.

Avoid cold break-ins to prolong rod life;)

thats a realli good idea well so i think :rolleyes:

is there anyway of gettin those stupid pins out that hold the conrod to the piston without them launching across the room never to be seen again :flaming:

You could try removing the mini clips (that you love so much :D )inside a plastic bag so when you favorite clip :sneaky: launches it won't go far ;)
A simple idea but it should work :) oh, and use a clear bag and not the white or blue shopping bags :D :lol: :lol:

That is a great idea!!! :nod:

funny man u are itchy b but great idea, wat tool do u use to remove them?

i would say the biggest rod killer is cold temp run in. The rod cops a beating when its new, tight and cold run in rich. If you can immagine a sleeve tapered at the top and a piston non tapered. if the sleeve dosent get to expand through heat then the piston smashes into the taper. This shock then gets transferred to the rod bushing, crank pin and bearings.. Both on and off road engines punish the rod / pin relationship , there is a lot of free revving in off road , with less resistance it can really speed up wear. Im amazed that engines las as long as they do...

I use a jewelers flat blade to carve it out. the bag trick is a good one too or just line it up with the flat blade and place the thumb over the clip so it cant fly off into the vally of the lost clips .. You know theyre trying to escape ...

is that jewelers flat blade thingo 1 of those reali tiny screw drivers that u get with bout 6 other drivers in a small packet if u know wat i mean :confused:

woo hoo got out those stupid pins i_:)_i but now how do i put them in :confused: lol

trail and error......

Ehh, but it is better to use new clips.....

got no new 1s on hand but ill order sum to have if i ever do need em cheers

To put them back in a have a modified bent long nose plier, its got a small pointed nose with really flat even surface, i grip the tail of the clip ( end that curves in ) and push the end into the groove, i then twis the clip to make it smaller as i push it into the groove, i then cover the hole with my finger and push the gudgeon pin towards it to make sure its clipped in. I then rotate it in the groove so that the tail is facing sideways. Beware the tail is not bent outwards in any way... it can catch when hot and total your piston