crank rod play

i have a 650 rfvc motor with the top end removed.when i grip the small end i can get some movement from side to side.if you imagine loose valve clearances at tdc theres the same amount of freeplay.is this normal or is my big end on the way out?

sounds normal, you need end play.

As long as the rod has no up or down movement you're fine, sideways movement is a must.

Now if you're talking about a rocking movement you should consult the manual for spec limits that's done with a dial indicator.

Edited by pwrpapa

Side to side there will be slack and is normal but more importantly as mentioned above if you have no up and down slack your crank is more than likely in good shape but there must be none at all. Not even the very slightest up and down slack (similar to when you push up and down on a valve rocker) or the crank is toast.

pwrpapa, can you elaborate on this rocker movement you mention? If it's what I think it is, it may have been responsible for seizing my recently rebuilt engine.

I could angulate the rod on the crank pin to a fair degree, and the bearing cage showed a wear pattern consistent with this.

I could not however see any specs in the manual for this which is kinda why I missed it when I first rebuilt the motor.

from the FSM connecting rod big end side clearance 0.050 - 0.450mm, service limit 0.060mm

use your feeler gauge and take some measurements and then you will know what is going on and if you don't have any up and down movement you are good.

pwrpapa, can you elaborate on this rocker movement you mention? If it's what I think it is, it may have been responsible for seizing my recently rebuilt engine.

I could angulate the rod on the crank pin to a fair degree, and the bearing cage showed a wear pattern consistent with this.

I could not however see any specs in the manual for this which is kinda why I missed it when I first rebuilt the motor.

Some manuals have specs for rod wobble or a rocking motion, mostly two stroke motors a few four strokes. Four strokes don't like wobble, two strokes seem to be more forgiving in this area.

If the rod wobbles or rocks what I do is set up a dial gage and check to see how many mm's or thou. it rotates back and fourth, most four strokes will only tolerate .012 of rock before the bearing and pin starts to cause engine failure, If it's under that say .005 or less than I will run the crank.

I've seen as much wobble as .035 before..

When you measure rock the total movement should be .005 not .005 one way and .005 the other way, that would be a total of .010.

To tight is not good either for side play,I like to see a total of .012 total play there.

from the FSM connecting rod big end side clearance 0.050 - 0.450mm, service limit 0.060mm

use your feeler gauge and take some measurements and then you will know what is going on and if you don't have any up and down movement you are good.

i already checked with the feeler gauge and its within specs.read the other posts its the rocking movement i am cocerned about bud.i do not have a dial gauge to measure but it seems like a lot of movement.

Some manuals have specs for rod wobble or a rocking motion, mostly two stroke motors a few four strokes. Four strokes don't like wobble, two strokes seem to be more forgiving in this area.

If the rod wobbles or rocks what I do is set up a dial gage and check to see how many mm's or thou. it rotates back and fourth, most four strokes will only tolerate .012 of rock before the bearing and pin starts to cause engine failure, If it's under that say .005 or less than I will run the crank.

I've seen as much wobble as .035 before..

When you measure rock the total movement should be .005 not .005 one way and .005 the other way, that would be a total of .010.

To tight is not good either for side play,I like to see a total of .012 total play there.

this is what i am concerned about.i do not have a dial gauge but the top of the rod seems to rock a couple of mm total.not cool.thanks for the reply that makes sense to me.

Time for a crank rebuild.

i already checked with the feeler gauge and its within specs.read the other posts its the rocking movement i am cocerned about bud.i do not have a dial gauge to measure but it seems like a lot of movement.

i read the other posts and in your first one you stated "side to side" not rocking side to side.

when i rebuilt my rfvc engine i had the same concern because it had that rocking movement too, but it never knocked before the rebuild and the measurments were in spec so i put it back together and haven't had an issue yet. if your worried about it then replace it and have some peace of mind.

I'll look at some of my manuals to see if I can find the spec for it.

i read the other posts and in your first one you stated "side to side" not rocking side to side.

when i rebuilt my rfvc engine i had the same concern because it had that rocking movement too, but it never knocked before the rebuild and the measurments were in spec so i put it back together and haven't had an issue yet. if your worried about it then replace it and have some peace of mind.

my mistake regarding side to side details. .my engine had no rattles or knocks before it was stripped and like yourself i am concerned.holding the small end i can get a couple of mil side to side movement.was yours this bad?how long ago or miles ago did you do your rebuild?a crank rebuild costs alot of dosh here in belfast so i am not sure what to do.

I'll look at some of my manuals to see if I can find the spec for it.

thanks papa,i would be greatful for any help as i am not sure if its worth gambling wrecking a new topend.although i would prefer not to do the crank as theres a lot of money involved.

i rebuilt the engine about 7,000 miles ago. at the top of the rod it had less than .5mm of movement, but the side clearence was below the service limit so i ran with it. i've put 4,500 miles on it now and everythings is still working the way it is suppose to. i think you would be ok putting it back together.

i rebuilt the engine about 7,000 miles ago. at the top of the rod it had less than .5mm of movement, but the side clearence was below the service limit so i ran with it. i've put 4,500 miles on it now and everythings is still working the way it is suppose to. i think you would be ok putting it back together.

He has 2mm's of rock. .080 umm that's to much.

thanks papa,i would be greatful for any help as i am not sure if its worth gambling wrecking a new topend.although i would prefer not to do the crank as theres a lot of money involved.

Up and down movement service limit is .002, rod rock service limit is .008..

He has 2mm's of rock. .080 umm that's to much.

well then he needs to make a judgement call. it wasn't knocking before, side clearence is below the service limit and it has no up and down movement, if it was me i would run it.

well then he needs to make a judgement call. it wasn't knocking before, side clearence is below the service limit and it has no up and down movement, if it was me i would run it.

The deal is, we really don't know if he has no up and down movement because it takes a dial indicator to measure it and he has not done that yet.

As for the rocking motion of the rod he said he has 2mm's but again it may not because it takes an indicator to measure that too.

So it's possible that the crank is fine or it may be toast, without the proper tools one's only guessing.

Something you don't do when building motors.

Pop's

if you grab the connecting rod and try to move it perpendicular to the crank and you don't feel anything then thats your answer. you would be able to feel if there was any bit of play.

Your arms and hands are not dial indicators or micrometers.......service limit is .002 on the radial movement.

I guess I could stick the piston in the bore and wiggle it and that would tell me my piston to wall clearances too.:smirk:

I'm done with this thread sonny.:prof: you're starting to be a stinker..:smirk:

You win ok.........

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