Big End Connecting Rod Bearing Replacement

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Hi,

So I have heard multiple times on the forum that the big end bearings can’t be changed. To buy a new crankshaft.

I didn’t like the idea that you have to spend hundreds of dollars just to replace a bearing if it fails. So I did some digging and looked in to how I could do so from home.

I got a cheap crank to test it out

Firstly I pressed out the pin from between the webs

9B5CAE34-9E7E-45E3-A71A-F3055E7B72CC.jpeg

Then aligned it back up and pressed it all back together as if I had put a new bearing in.

Then came the truing…

I needed a way to support the crank without needing to spend hundreds of dollars on some specialist jig.

I came across a 3d printable trying stand and had asked a friend if he could print it.

He charged $20 AUD the bearings were $7 AUD and hardware $10 AUD (note the choice of stihl colours!)

9B7237BE-7245-49AB-B678-336A8D7512BB.jpeg


I then went through the process of truing it. I managed to get one side to less than 0.005mm runout on the dial indicator as in the video below. However the other side was more. Then when I went to tweak the other side the first side went out.

Video of less than 0.005mm runout.

View attachment IMG_4408.MOV

The closest I could get was about 0.03mm runout. That’s certainly within acceptable tolerances..

738BF4A8-CA12-4664-9317-D91A90F81E71.jpeg

so with a small investment in a dial indicator (a cheapie will be fine) a stand and some way of holding the crank, it’s certainly possible and far cheaper than hundreds on another crankshaft, just to replace the bearing!
 
Hi,

So I have heard multiple times on the forum that the big end bearings can’t be changed. To buy a new crankshaft.

I didn’t like the idea that you have to spend hundreds of dollars just to replace a bearing if it fails. So I did some digging and looked in to how I could do so from home.

I got a cheap crank to test it out

Firstly I pressed out the pin from between the webs

View attachment 1071311

Then aligned it back up and pressed it all back together as if I had put a new bearing in.

Then came the truing…

I needed a way to support the crank without needing to spend hundreds of dollars on some specialist jig.

I came across a 3d printable trying stand and had asked a friend if he could print it.

He charged $20 AUD the bearings were $7 AUD and hardware $10 AUD (note the choice of stihl colours!)

View attachment 1071312


I then went through the process of truing it. I managed to get one side to less than 0.005mm runout on the dial indicator as in the video below. However the other side was more. Then when I went to tweak the other side the first side went out.

Video of less than 0.005mm runout.

View attachment 1071314

The closest I could get was about 0.03mm runout. That’s certainly within acceptable tolerances..

View attachment 1071317

so with a small investment in a dial indicator (a cheapie will be fine) a stand and some way of holding the crank, it’s certainly possible and far cheaper than hundreds on another crankshaft, just to replace the bearing!
Very good work there .
 
Having done this press out and realignment a few times I found the crank pin would be worn or damaged out of spec and new needle bearings would not last long. If a crank has enough hours on it to have worn out the needles or broken them, then just replacing them will not be a long lasting repair.
 
Having done this press out and realignment a few times I found the crank pin would be worn or damaged out of spec and new needle bearings would not last long. If a crank has enough hours on it to have worn out the needles or broken them, then just replacing them will not be a long lasting repair.
Well the trick is to install both new, I am doubting lets say STIHL would offer these parts. I have done a few dirt bike cranks, and they aren't exactly easy to get in perfect true. I hear a lathe is the best way to true them.
 
Well the trick is to install both new, I am doubting lets say STIHL would offer these parts. I have done a few dirt bike cranks, and they aren't exactly easy to get in perfect true. I hear a lathe is the best way to true them.
I want a mini lathe so badly lol.
 
Having done this press out and realignment a few times I found the crank pin would be worn or damaged out of spec and new needle bearings would not last long. If a crank has enough hours on it to have worn out the needles or broken them, then just replacing them will not be a long lasting repair.
Thanks Jerry, of the times you’ve done this, how many hours roughly on average did you find you’d get before they failed again? When you say “it would not last long” i remind myself the hours you use to put on a saw in a year is more than I would in a lifetime LOL.
 
a) the interference fit of the pin to the crank wheel may not be the same as it was the first time. Enter the discussion about welding. There was a thread on here years ago about porting a twin cylinder I think echo and where to start and end the weld. That was for strength though.
b) who is offering rod and pin kits for these?

In vintage motorcycles earlier roller cranks were made up from a bunch of different sized rollers to choose from. Later on manufacturing tolerances were better and that was no longer done. A true Harley fanatic probably knows this date. For BMW they stopped the roller crank and went to pressurized oiling in 1970 but if I recall correctly an assortment of roller diameters were available. Some outfit must do this for vintage restorations, Bley who I had their literature has stopped. They made a slightly larger pin and heat treated it. I believe centerless grinding can be used to make the rollers.

Just v blocks is what I have seen in manuals not the bearing thing you created. That key way cut out makes it just about impossible to put the indicator where it should be.
 
a) the interference fit of the pin to the crank wheel may not be the same as it was the first time.
It’s something I asked a mechanical engineer who worked at the ministry of defence in the Uk. If you take it apart multiple times I’m sure you would go beyond the allowable tolerance. Once, I wouldn’t think so. It took considerable effort on my 6 ton press to seperate it.

F1EF9ABF-EF44-41D6-939A-027FAE63A1BA.jpeg

If the pin isn’t damaged reuse it, bearings can be brought from a local bearing supplier.

Crankshafts are often rebuilt (though you’d normally use a new pin) on bikes.
 
Thanks Jerry, of the times you’ve done this, how many hours roughly on average did you find you’d get before they failed again? When you say “it would not last long” i remind myself the hours you use to put on a saw in a year is more than I would in a lifetime LOL.

First off I would like to say that fixture you had made looks like a slick outfit, while I prefer the between two centers method so I can eliminate any slack or movement from the bearings like on your fixture, just call me ,Old school.
I measured the pin diameter and found it between .001 -.002 undersize and could see striations or scratch like lines on the pin. At 13,000- 15,000 RPM the bearing went out within two weeks of cutting, say an average of 6-7 hours per day by 5 days a week so ten days of production cutting or around 60-70 hours of run time. My close friend, a master machinist said he could produce me a new pin and case harden it precise enough that the bearing could last much longer but by just putting in new needle bearings on a heavily used pin puts a lot of pounding on those needles if there is any slack at all. On the vintage saws I learned on the big end rod bearing was made of two pieces, the needles were easily replaced but in many cases they would not last long term either. Careful examination along with measurements often turned up damage and wear to the crank pin that would translate to short bearing life.
 
First off I would like to say that fixture you had made looks like a slick outfit, while I prefer the between two centers method so I can eliminate any slack or movement from the bearings like on your fixture, just call me ,Old school.
I measured the pin diameter and found it between .001 -.002 undersize and could see striations or scratch like lines on the pin. At 13,000- 15,000 RPM the bearing went out within two weeks of cutting, say an average of 6-7 hours per day by 5 days a week so ten days of production cutting or around 60-70 hours of run time. My close friend, a master machinist said he could produce me a new pin and case harden it precise enough that the bearing could last much longer but by just putting in new needle bearings on a heavily used pin puts a lot of pounding on those needles if there is any slack at all. On the vintage saws I learned on the big end rod bearing was made of two pieces, the needles were easily replaced but in many cases they would not last long term either. Careful examination along with measurements often turned up damage and wear to the crank pin that would translate to short bearing life.
Thanks for the info Jerry! Much appreciated :) For a home owner that could be many years, so not all that bad right?
 
Sorry ,Tom,, I type slowly with just one finger, it takes me much longer to put up a reply before you fall asleep or head off to bed.
all good mate, I appreciate all the info you share with me! I edited my above post. 60-70 hours, That could be many years of time for a homeowner, so not all that bad really!
 
Thanks for the info Jerry! Much appreciated :) For a home owner that could be many years, so not all that bad right?
Well, 60 -70 hours may seem like a lot and likely homeowner owned saws do not see high hours in a year but then again most homeowner saws will die from fuel related damage and not wear out crank big ends.
 
Well, 60 -70 hours may seem like a lot and likely homeowner owned saws do not see high hours in a year but then again most homeowner saws will die from fuel related damage and not wear out crank big ends.
Ain't that the truth, some homeowners can create so many multiple ways to kill any peice of equipment. Imagination is the limit.
 
Well the trick is to install both new, I am doubting lets say STIHL would offer these parts. I have done a few dirt bike cranks, and they aren't exactly easy to get in perfect true. I hear a lathe is the best way to true them.

I used to do dirt bike cranks when I worked at a cycle dealership.

We had a big press for the pin. To true up the crank I put it on a surface plate in machinist V-blocks, checked true with magnetic base dial indicator.

Not cost effective if you can get a new/used crank, but they were my own bikes, I got parts at dealer cost, and my time was free.

Tom has made a nice makeshift setup.
 
I used to do dirt bike cranks when I worked at a cycle dealership.

We had a big press for the pin. To true up the crank I put it on a surface plate in machinist V-blocks, checked true with magnetic base dial indicator.

Not cost effective if you can get a new/used crank, but they were my own bikes, I got parts at dealer cost, and my time was free.

Tom has made a nice makeshift setup.
Thanks mate 👍
 
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