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Author Topic: Worn Connecting Rod  (Read 1622 times)

Offline Ken Culbertson

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Worn Connecting Rod
« on: April 26, 2018, 01:54:50 PM »
I have an OS46FX that appeared to have bad bearings.  I have replace them but the problem remains.  Turns over freely with good compression when you "flip" the prop but when you cycle it through there is a substantial bump at TDC.  The head shows no signs of contact so that leaves three possibilities.  The Connecting Rod, the wrist pin and the crankshaft.  What little I remember about my machinist days is that bearings are designed to wear out instead of the part they are attached to.  The crankshaft connection has substantial slop so I am pretty sure this is the problem.  My question is - are these a matched set?  Will I also need to replace the crankshaft and or wrist pin?  The wrist pin shows no slop but it doesn't get anywhere near the movement of the connecting rod connection.  ???

Thanks - Ken
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Offline RandySmith

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Re: Worn Connecting Rod
« Reply #1 on: April 26, 2018, 03:00:36 PM »
Quote from: Ken Culbertson on April 26, 2018, 01:54:50 PM
I have an OS46FX that appeared to have bad bearings.  I have replace them but the problem remains.  Turns over freely with good compression when you "flip" the prop but when you cycle it through there is a substantial bump at TDC.  The head shows no signs of contact so that leaves three possibilities.  The Connecting Rod, the wrist pin and the crankshaft.  What little I remember about my machinist days is that bearings are designed to wear out instead of the part they are attached to.  The crankshaft connection has substantial slop so I am pretty sure this is the problem.  My question is - are these a matched set?  Will I also need to replace the crankshaft and or wrist pin?  The wrist pin shows no slop but it doesn't get anywhere near the movement of the connecting rod connection.  ???

Thanks - Ken


Ken
It could be  the  piston sleeve fit is  tight at the  top, this is  a normal thing, so check that

Randy
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Offline Ken Culbertson

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Re: Worn Connecting Rod
« Reply #2 on: April 26, 2018, 04:44:22 PM »
Quote from: RandySmith on April 26, 2018, 03:00:36 PM

Ken
It could be  the  piston sleeve fit is  tight at the  top, this is  a normal thing, so check that

Randy

It is as smooth as silk with the head off.  No hint of a bind.  Is there another way to check this?  Were my assumptions about the crankshaft correct?

Ken
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Offline Brett Buck

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Re: Worn Connecting Rod
« Reply #3 on: April 26, 2018, 04:55:05 PM »
Quote from: Ken Culbertson on April 26, 2018, 04:44:22 PM
It is as smooth as silk with the head off.  No hint of a bind.  Is there another way to check this?  Were my assumptions about the crankshaft correct?

  That's a really bad sign. In proper condition, with the head off and the piston/liner in, I would expect it to at least get tight/squeaky ad TDC and possible freeze there. If it turns over with no drag at all, that suggests that the piston/liner is worn out.

   Brett

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Offline RandySmith

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Re: Worn Connecting Rod
« Reply #4 on: April 26, 2018, 09:30:55 PM »
Quote from: Ken Culbertson on April 26, 2018, 04:44:22 PM
It is as smooth as silk with the head off.  No hint of a bind.  Is there another way to check this?  Were my assumptions about the crankshaft correct?

Ken

If the sleeve is  NOT moving up and  away  from the piston, then   yes  you have a  problem some where
look at the  backplate and see if you see  swirling  cuts in it
How  new is  the motor? or can you tell
can you post pictures?
how many head shims are in the  head?
You should  NOT have  play in the rod, more than about 2 thou inch
is the  wrist pin keepers  there ?
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Offline Ken Culbertson

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Re: Worn Connecting Rod
« Reply #5 on: April 27, 2018, 06:14:17 AM »
Quote from: RandySmith on April 26, 2018, 09:30:55 PM
If the sleeve is  NOT moving up and  away  from the piston, then   yes  you have a  problem some where
look at the  backplate and see if you see  swirling  cuts in it
How  new is  the motor? or can you tell
can you post pictures?
how many head shims are in the  head?
You should  NOT have  play in the rod, more than about 2 thou inch
is the  wrist pin keepers  there ?

Ok - Swirling on the backplate but it is more like polishing than cuts.  One head shim.  I rechecked the piston and it will lift the sleeve at TDC.  Con rod is a disaster at the crankshaft connection.  Wrist pin clips in and no slop at the piston end.  I have ordered a new one.  If that doesn't work I will let you know.  Thank you so much for responding.

ken
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Offline Ken Culbertson

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Re: Worn Connecting Rod
« Reply #6 on: May 03, 2018, 09:49:00 PM »
Quote from: Motorman on April 27, 2018, 06:15:23 PM

If you pull it through slowly it can squeeze the oil out and feel sticky and get a clunk as it pops loose on the way down.
Motorman 8)

Got the new connecting rod installed and the beast runs really well now. #^  It does have that "pop" when I cycle it through slow.  Glad to hear that is "normal". y1  How much break in do I need with a new rod and new bearings.  Same piston and sleeve but it has been out and reinstalled.  I have never had an engine with this much compression.  Now all I have to do is find the right pipe and design a ship to house it. S?P
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Offline Ken Culbertson

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Re: Worn Connecting Rod
« Reply #7 on: May 03, 2018, 10:10:20 PM »
Quote from: RandySmith on April 26, 2018, 09:30:55 PM

You should  NOT have  play in the rod, more than about 2 thou inch
is the  wrist pin keepers  there ?

Randy/Brett:

Thanks for all the advice.  Thanks to you I now have a working OS46FX!.  I installed the new connecting rod (putting those wrist pin clips back in resulted in some censored comments).  Put in some oil and cycled it through till it flipped smooth, hooked up a tank and just like every other MAX I have it started on the first flip.
It has colossal compression.  Should I consider an additional head gasket?  I pretty much subscribe to Brett's advice that the factory knows best.  I am just not used to this much compression, maybe it is normal.  I probably need to run it through an abbreviated break in cycle since the piston has been out of the sleeve and all new bearings.  Now to start the search for an appropriate pipe.

Ken
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If it is not broke you are not trying hard enough.
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Offline YakNine

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Re: Worn Connecting Rod
« Reply #8 on: May 06, 2018, 10:29:47 AM »
Hey Ken , I had a similar issue with a veco .61 I got off eBay a few years ago. I don't really know what they did to it as it looked new on the outside. I took it to a club meeting and a couple guys felt that someone beat it nearly to death with a starter. Larry Scarinzi offered me a parts engine with no top end ,I put the Rod in and it has become a great engine, I ended up buying a crash damaged engine and with the parts I had left over put together another one.TJ
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