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Con rod assessment question
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Hi all,

I am trying to re-join the fold.

I haven't posted here for a few years now, but am now trying to get my blue tanked TTR back on the road.

After getting a very low compression reading from my hot engine (90psi) I had hoped that a rebore and lapping in of the valves would be all that was necessary to revive it, and with the help of the excellent guides on this site I have stripped away the top end. But now that the con-rod's small end is on show I felt that I really should have a look at that too.

There is some scuffing in there, but not enough to be felt with a finger nail. I tried putting one third of the gudeon pin back in the piston and checked if I could rock it. I couldn't so I concluded that there s no significant ware in the gudgeon pip. I then put the same part of the pin into the small end eye and I could feel a bit of rock. I had hoped to feel none.

I have looked in the Yamaha workshop manual for small end bearing wear limit dimensions, but non are quoted.

I looked in the Clymer manual for XT600s and there is no small end dimensions quoted in there either. I also have a tech book that is aimed at students training to become motorbike mechanics in the 1980's and the only small end check mentioned there is to check for rock on the gudgeon pin, if there is any, replace the pin.

I asked an engine reconditioners / machine shop for their opinion and they said "there is about 1 thou of an inch clearance between the unworn part of the gudgeon pin and the small end,  probably OK but what s Yamaha's spec?"

So finally my question to you: Is there a spec for this clearance / is it OK to reuse my con-rod?

(I will be fitting a new piston / gudgeon pin if I go ahead with this rebuild, if the cost doesn't get excessive. I.e. if I don't need new valve guides, valves, valve seats, con-rod, etc)m

Any advice much appreciated,

Thanks

Lama



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Welcome back Mark!
I think the little end is so narrow that any tiny wear can be felt as rocking motion, but as the piston only travels in the up/down plane (more or less) it will be fine. I agonized over this when I did my first rebuild, but put it back together anyway as the time and expense of a bottom end rebuild made it worth the risk to me and the result was no nasty little end rattles.
I'm not aware of an official wear limit in the manual - that's not to say there isn't one, just that I haven't trawled through it to find one...
Good luck,
Simon.

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I go along with Simon's assessment method!

If thee is no discernible up and down movement in big or little ends and the little end has no obvious scoring or "bluing" from getting hot (usually when the engine is run with low or no oil) the I consider it's good to go. This "method" hasn't let me down yet on a score or so of engine rebuilds wink

Brian

PS And yes, welcome back!



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Hi Simon, Brian,

 

Thanks for the welcome back, and for your thoughts on how to check the little end for ware.

I will have another go at checking for up and down movement of the original gudeon pin in the small end, but I am told the middle part has about one thou' ware and trying with just an unworn end inserted makes it hard not to rock it. I have noticed that if I compare the amount of rock with no oil verses with the pin oiled, adding the oil reduces the amount of rock to about a third or a quarter. I think I read somewhere that engine oil would take up about one thou' so I am taking that as encouraging. I've pretty much decided to re-use the con-rod, or at least to go ahead and order my top end parts. I will then check it with the new gudeon pin that will come with the oversized piston. Just waiting for the machinist to tell me which oversize piston to order and then will be sending a shopping list to Totally TTRs.

I have noticed blueing where the flywheels are thinnest around the big end pin. But I have looked at images of a few second hand ones and they all seem to have it to the same extent so I'm assuming this if from the original manufacturing process.

Thanks again,

Lama



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blueing overheating or lack of oil getting hot bad oil supply not to the extent of a seizure but close to

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May be due to the heat treatment process.

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I have noticed blueing where the flywheels are thinnest around the big end pin. But I have looked at images of a few second hand ones and they all seem to have it to the same extent so I'm assuming this if from the original manufacturing process.

Thanks again,

Lama

 

They are all like that, so spot on.



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