Connecting Rod Big End Bearing Replacement

[Vintage Engine Repairs]

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



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!)



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.

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



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!

 

[Shane¹]

The hardest part of doing this is finding the bearings to replace them with.

 

[Ronie]

Nice write up, I'm going to give it a try with a 372 crank.

 

[Vintage Engine Repairs]

The hardest part of doing this is finding the bearings to replace them with.

I get all my bearings from my local bearing supplier.

 

[Gullet]

I get all my bearings from my local bearing supplier.

Care to share that source?

 

[066 redeye]

Hi Tom
So I could get say an 066 big end bearing from my local bearing shop?
Cheers Chris.

 

[drf256]

Very nice job Tom. Now you just have to clamp them together and offset drill the pin hole for a stroker.

 

[wcorey]

I've done this swapping parts around making one out of two in a situation were one has a bad bearing and the other a broken off flywheel or clutch end. Another situation was with a difficult to find/expensive m'cycle crank with only marginal wear and oversize rollers were available in .0001"
increments.
But...
Also at issue here is whether or not you'd need to additionally replace the crank pin and/or connecting rod.
Those are of course the inner and outer bearing races and if it was run worn/loose for very long the rollers hammer the pin and leave chatter marks much like can be seen on the one in your first pic. Additionally if the bearing has actually failed rather than only being worn, damage the the rod and pin is even more likely.
Very slight, difficult to see/measure damage/wear can drastically reduce bearing life...

 

[Vintage Engine Repairs]

Care to share that source?

Any bearing or seal manufacturer, I use statewide bearing in Australia. You’ll need to ask for a metal cage needle roller bearing and give them the I.D. of
the connecting rod big end, the OD of the crank pin and the width of the connecting rod big end.


Edit:

Needle Roller Bearings - Statewide Bearings

Statewide Bearings distributes a range of single and double row needle bearings Australia wide from leading brands. Find your local branch!

www.statewidebearings.com.au

 

[popopboat]

Well, bearings used are probably one of the standardized sizes.
But what bothers me more is, usually when a big end bearing fails, connecting rod hole becomes oval and should be replaced as well, but ive never seen a chansaw conrod being sold separately.

 

[Stump Shot]

I too at one time wanted to try this, problem was the crank was built with the pin made into the one half of the crankshaft and pressed into the other. As hard as I tried I could not get that crank apart without damage as it could not be cradled properly. I was inspired after having watched a snowmobile engine crankshaft being trued with a lead hammer, put back in a lathe and rechecked repeatedly until the runout was gone(it was constructed like the one pictured above). He would hold it in his hand and depending on how the gauges read would give it a smack with the lead hammer. It was impressed upon me verbally the importance of getting the job true for success. I've yet to see a crankshaft constructed like this yet to give it a try, as it sure would be interesting to me to see exactly what failed and if repair would be possible and go through the process myself if so.

 

[Al Smith]

Its a chore to align a Harley crank shaft and those are tapered .I only assume like when Falicon fabricates an 084 Stihl rod to a Husqvarna 3120 crank shaft for a stroker it 's a guarded secret .As to if they use an offset pin I have no idea .It's all way above my pay grade .
This I do know when you see that bluish color on the crank shaft throws that's caused by an induction heater used in the assembley process .It's not a bearing starting to fail as many might think .

 

[Al Smith]

Now then off on one of my tangents .I worked on induction heaters for the 29 years I worked in an automobile engine plant ,they are simple as well as complicated .Uses high frequency AC voltage to heat the metal so much they almost act like micro wave for metal .Works on the I squared R losses on the metal.the complicated part. The machines can kill you ,cooks you from the inside out .Safety is paramont Depending on the design can heat the end of a connecting rod up to red or harden a valve seat on a cast iron cylinder head . The bearing surfaces on a crankshaft and any number of things depending on the application .

 

[Mastermind]

Show me where to buy the crank pin and rod.

I made some alignment jigs to do this, even made a couple of 066 cranks from swapping parts around. At the end of the day, the lack of new replacement parts, and time invested make attempting this a poor choice.

 

[Shane¹]

You can get rods and pins for some models from this site. I used a 08 pin and bearing in a 7900 crank turned out great still going strong

 

[Shane¹]

Here is the stand I made to true mine a little more crude but worked just fine

 

[Mastermind]

You can get rods and pins for some models from this site. I used a 08 pin and bearing in a 7900 crank turned out great still going strongView attachment 372573

That's cool !!!!

 

[Al Smith]

My bud Randy (RIP ) ,a tool maker made pins for Harleys .What grade of steel it was I do not know but it was heat treated .The Harley fork and blade rods were lapped for size and roller bearings could be found in various sizes .Single rollers not caged .Where Falicon gets the 084 rods I haven't a clue either .Like I said it's way above my skill set .Sadly the booze got Randy,pickled his brain .

 

[isaaccarlson]

I straightened a crankshaft a few years ago with a ball peen hammer and rounded chisel. It worked very well. I didn't have anything to lose, so I gave it a try. A few whacks and test, then a few more....over and over until it was straight. It took a lot, but came out close enough that the texture on the stub end was more than the runout.

 

[Al Smith]

To the question of where to find a bearing .I went through a couple of my bearing catalogs from Bearings Inc .One of them listed open cage needle bearings in metric sizes both SKF and slacker brands .I'd have taken a picture but both my camera batteries are shot. New should be here by weeks end then I'll snap a pic of the sizes I found .