Tips On Bottom End Rebuilds

[Vintage Engine Repairs]

I hope this helps any new comers tackle
their first bottom end rebuild

Case splitting, getting the correct heat to install new bearings, and pulling the crank through without preload all take a little skill, careful heat control, timing, and patience to do properly.

If you’re about to split the cases on a 2-stroke, here are 3 tips that may help

1. Buy the proper tools to make the job easier. Don’t cut corners — chances are you’ll end up frustrated and in a more expensive position than when you started. Buying tools is still cheaper than paying someone and you'll have them for life. Matt Olson sells some excellent pullers and the Husky clone case splitter is versatile and affordable.

2. Whenever possible, avoid pressing bearings into or out of their pockets. It’s a tight interference fit, and forcing them can damage or loosen the pocket, potentially leading to a spun bearing.

Heat, speed, and controlled cooling are your friends! 185c / 365f allows them to just drop in, but don't dawdle with the heat.

Get in, keep the heat high, fast and local. Done correctly you'll be able to hold the cases the whole time. It shouldn't take more than about 15 seconds to reach the correct temperature. At 20 seconds the bearings should be in and you're starting to cool everything down with compressed air...

The only reason to freeze a bearing is reducing the heat absorption. A home freezer on steel is no where near cold enough to make a notable difference to the clearance or the max temp you'd need.

3. Preload is one of the quickest ways to destroy a bearing. Once the crank has been drawn through and the case bolts are tightened, a couple of sharp taps on each crank stub can help align the races and relieve side load on the balls. The crank should rotate smoothly with minimal resistance.

Lastly, if in doubt, find someone willing to let you watch — or better yet, check this video out! I explain the process, temperatures, and techniques in much greater detail Happy repairing!

 

[sherlock349]

Always wanting to hear how others go about this and appreciate the comments.
Your advice does contradict Matt Olsen and the tools he designed to pull the bearings
cold pressed.
So feeling a little confused.
I am in the UK and come from a background of rebuilding motorbike engines 50 years ago.

 

[hacskaroly]

Your advice does contradict Matt Olsen and the tools he designed to pull the bearings
cold pressed.

Some people like the heat/freeze method, others like using Matt's tools. I use Matt's tools and make sure everything is lined up straight before cranking down and go slow. I use oil on everything so there are no dry spots on the bearings or pockets. If I tried the heating method, I would probably burn myself.

The first time I tried putting in bearings, I was using a hammer and wood blocks and then getting the case halves back together with ratchet straps and a hammer...it worked on two saws, but then realized biting the bullet and getting Matt's tools, was a lot easier on the chainsaw and me.

I think it really comes down to giving it a try and find out what works for you or is most comfortable for you to do. I work on a lot of older saws, so if I screw up a pocket, it is not that expensive to get another case half or full case.

 

[sherlock349]

I have Matt’s and have done 4 saws with them without a problem (so far) as you say go slow and keep checking alignment.

 

[sherlock349]

I would be really interested to know if there is a definite right or wrong answer to this, that is backed up with science.
Have watched the video and both methods are relevant. No longer confused.

 

[Vintage Engine Repairs]

My method doesn't contradict Matt's you're mixing two different stages. I own Matt's tools and use them on every rebuild. His tools are to pull the crankshaft through the bearings, but the heat is to install the bearings in the cases .

I should clarify: The reason you can pull the shafts through the bearings cold is that you have hardened steel on both surfaces. When one is much softer than the other (hardened steel vs magnesium) you'll lose material on the softer side.

I have seen him in the past use washers and nuts / bolts to install bearings and I have had to at times. It's not ideal, sometimes you're forced to do so unfortunately!

 

[Basher]

The heat only method is what we used back in the days when all the bearings had metal cages, we could heat the bearings without damaging the bearing cages. The plastic like cages prevent as much damage to the rest of the saw if they were to disintegrate , the metal cages did a lot of damage but I only run metal caged bearings in my own saws and after 63 years have only had one bearing go bad , it was on a nearly new saw I was running on a commercial harvest , so was most likely a defective bearing from the outset. 40:1 mix in all the saws I run and none of them have an easy life on commercial work of all kinds,logging.pulpwood,firewood, dangerous tree removals and land clearing,they all earn their keep. Use whatever method you are comfortable with, knowing the consequences of the work you are performing is what should guide you, not what someone else says you should do.

 

[Stump Shot]

The heat only method is still a viable option even with plastic caged bearings with some adaptation to how the heat is applied, as well as the order of progression.
There's two camps when it comes to putting a bottom end together, the "Stihl way" of putting the bearings in the case first and the "Husqvarna way" of putting the bearings on the crank first. Both will get the job done in the end, however for this particular discussion I'll be talking about putting the bearings on the crank first, as this is what allows the heat process to be utilized. Also, both processes are not saw specific either work universally in any saw.
So, to heat the bearings first without the possibility of overheating the plastic is what we are concerned with here and totally possible using the microwave method. By wrapping the bearing in wet paper toweling and placing in the microwave for a few minutes the water in the towel is steamed off and gets the bearing to the perfect temperature of 212F or 100C to expand without hurting the plastic one bit. No, the microwave will not make sparks because the bearing is encapsulated with the paper towel. The crankshaft can be placed in the freezer to further the effect but is not necessary.
I like to keep a driver handy that will push on the inner race just in case I flub up and falter as the bearing is being slid on and have the ability to finish it while still hot and easy to move. Usually if I'm prepared for such a thing it doesn't happen and everything works out just so. After some attempts the proper amount of toweling and time in the microwave get to be old hat and the bearings will drop right on to the seat on the crankshaft.
After both bearings have been installed the unit can once again be placed in the freezer to cool and the first case half heated. This is where it be easy or difficult depending on which case half is chosen to be first. I like to drop it into the flywheel side half first as this has the stop built in for the bearing to rest on. You don't have to install the oil pump or use feeler gauges inside to get the correct depth, (unless both sides have blind holes such as a 266XP for example) everything works out automatically.
Then follow up with the PTO half which I like to put in a saw holder with the alignment pins in and gasket laying on to drop the other half in. With the aid of a plastic mallet a tap or two can be applied when it gets to the pins and be persuaded to go the rest of the way before cooling too much.
Lastly, the bolts installed and the bearing side tension relieved before it cools. Which brings me to my last crank install "hack" that I found out quite by accident. I have an old B&D battery impact driver that is kind of pitiful and won't over tighten a fastener past what I would tighten by hand. So, on one occasion, I decided I would use it instead of a T handle in the essence of speed. What I found was that the couple of "rattles" it made on each fastener had vibrated the case enough that the side pull tension on the bearings had been completely relieved and was ready to go without having to tap on the crankshaft with a brass hammer. I would caution that this might not be for everyone unless you are able to lessen the torque applied with your particular tool as to not over tighten or strip or otherwise break the fasteners. Again, I know my tool is not good anymore and won't even loosen up a thing unless done by hand prior, so if you have a new modern working impact driver it might be best left alone.
At any rate, I find that this is quite the simplest and easiest way possible to do such work without sacrificing the quality of anything in any way shape or form. If someone has an easier way, I'm all ears and if it turns out it's just easier for them and not me, that's okay too, we all have to do what works out best for ourselves at the end of the day.

 

[Chainmale]

The heat only method is still a viable option even with plastic caged bearings with some adaptation to how the heat is applied, as well as the order of progression.
There's two camps when it comes to putting a bottom end together, the "Stihl way" of putting the bearings in the case first and the "Husqvarna way" of putting the bearings on the crank first. Both will get the job done in the end, however for this particular discussion I'll be talking about putting the bearings on the crank first, as this is what allows the heat process to be utilized. Also, both processes are not saw specific either work universally in any saw.
So, to heat the bearings first without the possibility of overheating the plastic is what we are concerned with here and totally possible using the microwave method. By wrapping the bearing in wet paper toweling and placing in the microwave for a few minutes the water in the towel is steamed off and gets the bearing to the perfect temperature of 212F or 100C to expand without hurting the plastic one bit. No, the microwave will not make sparks because the bearing is encapsulated with the paper towel. The crankshaft can be placed in the freezer to further the effect but is not necessary.
I like to keep a driver handy that will push on the inner race just in case I flub up and falter as the bearing is being slid on and have the ability to finish it while still hot and easy to move. Usually if I'm prepared for such a thing it doesn't happen and everything works out just so. After some attempts the proper amount of toweling and time in the microwave get to be old hat and the bearings will drop right on to the seat on the crankshaft.
After both bearings have been installed the unit can once again be placed in the freezer to cool and the first case half heated. This is where it be easy or difficult depending on which case half is chosen to be first. I like to drop it into the flywheel side half first as this has the stop built in for the bearing to rest on. You don't have to install the oil pump or use feeler gauges inside to get the correct depth, (unless both sides have blind holes such as a 266XP for example) everything works out automatically.
Then follow up with the PTO half which I like to put in a saw holder with the alignment pins in and gasket laying on to drop the other half in. With the aid of a plastic mallet a tap or two can be applied when it gets to the pins and be persuaded to go the rest of the way before cooling too much.
Lastly, the bolts installed and the bearing side tension relieved before it cools. Which brings me to my last crank install "hack" that I found out quite by accident. I have an old B&D battery impact driver that is kind of pitiful and won't over tighten a fastener past what I would tighten by hand. So, on one occasion, I decided I would use it instead of a T handle in the essence of speed. What I found was that the couple of "rattles" it made on each fastener had vibrated the case enough that the side pull tension on the bearings had been completely relieved and was ready to go without having to tap on the crankshaft with a brass hammer. I would caution that this might not be for everyone unless you are able to lessen the torque applied with your particular tool as to not over tighten or strip or otherwise break the fasteners. Again, I know my tool is not good anymore and won't even loosen up a thing unless done by hand prior, so if you have a new modern working impact driver it might be best left alone.
At any rate, I find that this is quite the simplest and easiest way possible to do such work without sacrificing the quality of anything in any way shape or form. If someone has an easier way, I'm all ears and if it turns out it's just easier for them and not me, that's okay too, we all have to do what works out best for ourselves at the end of the day.

Great info there Steve!

 

[Tor R]

I mainly use factory tools for splitting and installing bearing replacements on the crank.

It is possible to heat up around the bearings and a hammer, and split the crankcase like that, I have done that, but after I bought most of the Husqvarna pullers, the latter is what I do.

Installing bearings, I use heat & cold technique, if the bearings are of the newer type, nylon cage or peekish caged, I leave them in the freezer over the night, it reduces the heating of the housing by around 30˚C before they slide into place in the pocket.
Classic bearings around 200˚C, then they easily slide right into the bearing pocket.
I have tried fancy tools to screw the bearing cold into the bearing pocket, for me it is a technique I do not use, high chance of making magnesium shavings from it. For me, cold pressing is a no go !!

Assembly of crank & crankcase, I use original factory tools, crankcase always with the bottom up, then the conrod will always lie perfectly straight, hanging down.
The rest is really just cranking it together, spending a little time so that everything falls together naturally.