Monday, March 19, 2012

Snowmobile Cylinder Honing

Someone brought me the cylinders from their Polaris 550.  They had burned a piston and one of the cylinders was a little scuffed up.

This engine has traditional aluminum cylinders with cast iron bores.  These are very robust and can take a lot of abuse.  Modern (or high performance) engines have cylinders that are all aluminum with some kind of chrome or Nikasil plating on them.  Plated cylinders allow for greater engine performance (I think it is due to better heat transfer ability), but they don't seem to be as durable when things blow up.  Once the plating starts to flake off the bore you need to have the cylinder replated or replaced.

For this job I am using a cheap "glaze breaker" type tool.  This tool is inexpensive and available at any auto parts store.  It is not a true hone.  It is only used to fixing minor surface defects.  If you have a cylinder that needs to be resized or you need to fix an out of round condition you need a much more complicated and expensive hone like this.

I normally clamp the cylinder to the edge of the work bench and place a pan under it to catch any cutting oil that drips out.

Put the hone in a variable speed drill and insert it into the bore.  Make sure to coat the bore with a light weight oil.  The hone should be spun at a slow speed and moved up and down while it is spinning.

On a properly honed cylinder you want very fine scratches left on the bore.  These scratches hold a little bit of oil and make the rings seat properly.  The scratches should cross each other at about a 30 degree angle.  You have to get the right combination of RPM and up and down movement to achieve this.  Always check the edges of the ports when finished honing.  The edges should have a very slight radius to them.  You do not want a sharp edge that will catch a piston ring when it comes by.  Normally a small piece of emery paper can be used to clean up these edges. 

Friday, March 16, 2012

Arctic Cat Sno Pro 800

Someone brought a new Arctic Cat Sno Pro 800 by the shop recently.  It was only running on one cylinder and needed the spark plugs changed.  I think that they have a habit of warming it up for a very long time before driving it and then hardly getting above idle speed when they do drive it.  This leads to fouled plugs. This machine was built to run fast, not go across town to get groceries.

This second photo show about the best view you can get of the spark plugs.   Like a lot of newer machines this one has part of the chassis structure over the top of the engine.  It may make for a great light weight and stiff chassis, but it sure is a pain to service anything on the engine.  There really isn't a hood to lift up, just two little side covers that give you a peek at the engine.  To make the job a little more time consuming there is 4 plugs for this 2 cylinder engine.

With 4 new plugs in it it ran great.  The power delivery on this machins reminded me of an old 2 stroke moto cross bike from back before they had power valves.  There is a little rumble at low speed and then when you hit the right RPM the thing takes off screaming and stretches your arms out. 

Saturday, March 10, 2012

Rev XP Brake Replacement

For many years almost all snowmobiles were put together in the same way.  It didn't matter what factory it came from , when you opened the hood the pieces were all in the same place.  In recent years there has been a big push to make the machines lighter and more compact.  This has resulted in a lot of changes in the layout and arrangement of the chassis and drive train parts.

The red arrow in the first photo is pointing to the cover over the brake disc on a newer Ski Doo Rev XP machine.  This ones happens to have a 550 fan cooled engine, but all the Rev chassis machines have the brake in this location.  I have seen a few of these get stuck from a build up of snow and ice around the brake rotor.  While riding fine snow can blow in on the hot brake rotor and melt, when you stop for a coffee break it cools off and freezes.  When you try to take off the machine won't move because the brake is frozen in a block of ice.  If you have one of these keep an eye on it.  Make sure you have the correct size Torx wrench in your tool kit to get the cover bolts off.

This particular machine is in the shop to get the brake rotor and caliper replaced.  Someone drove the machine around with the parking brake set and the brake heated up so bad that everything was destroyed.  This is the second time I have seen this happen this year!

Here is the new brake caliper/drive shaft bearing mount.  As part of the effort to save weight and package everything smaller Ski Doo has made the bearing mount part of the brake caliper.  The bearing sits in the large opening, and the four smaller holes bolt the assembly to the side of the tunnel.

This is the bearing and the end of the drive shaft where it sticks out of the tunnel.  In the old days almost every machine used the same 1" bearings for the drive shaft and the jack shaft.  As you can see in this photo the drive shaft is hollow and uses a large diameter bearing.

Before installing the new bearing mount I heated it up with an electric heat gun.  This causes the metal to expand and increase the size of the hole that the bearing fits in.  On many bearing mounts heating like this can increase the size enough to eliminate any pounding or pressing to install the bearing.

The brake line is connected to the caliper with a banjo bolt.  Make sure you install it with two copper crush washers, one on each side of the fitting.  Once the line is connected the air must be bled out.  See this post for info on how to do that.

Here is the new brake all assembled and waiting for the cover.  Remember if your machine feels a little sluggish make sure you don't have the parking brake set.

Sunday, March 4, 2012

Polaris 120 Snowmobile Rebuild

I have an older Polaris 120 snowmobile that I am rebuilding.  The engine had been neglected, so I decided to tear it down and inspect everything. While I had it apart I also removed the governor.  This is the first step to making these machines run faster.

With no governor to control engine speed the motor will rev up until the valves float.  To increase the RPM that this happens at I decided to install a set of "high performance" springs from Recreational Motorsports.  These springs are little bit longer and stiffer than the stock springs.  After the governor elimination this is the simplest way to make more power. The spring on the right is the new one, the original spring is to the left.

Whenever you are assembling internal engine components be sure to coat all the moving surfaces with some kind of assembly lube.  I normally use Lubriplate.

On these tiny motors I use my simple homemade valve spring tool.  You can see this tool and a manufactured one in these old posts here and here.

These small motors motors have a splash lube oil system.  There is no oil pump to distribute the oil around the engine.  The bottom of the connecting rod cap has a long thin "dipper" that sticks down into the oil reservoir.  As the crank spins this dipper splashes oil up onto the cam shaft and around the crankcase. 

Make sure the timing marks are lined up when the cam shaft is installed.  The small punch marks on the edge of the gears are the timing marks.

This photo shows the main parts of the governor assembly.  The governor can be bypassed by messing around with the springs on the throttle linkage, but this puts a higher load on this plastic gear that will eventually lead to it breaking and making a big mess.  If the governor is not being used it is best to remove this gear.  This photo also shows the governor control rod.  I cut it in half and remove the part that goes inside the crankcase.

When installing the spark coil it must be mounted as close as possible to the flywheel.  I have found that the easiest way to do this is place a piece of paper between the coil and the flywheel when tightening the bolts.  When you turn the flywheel you are left with a paper thin space between the parts.

This covers some of the high lights of small engine rebuilding.  When I get the rest of the machine together I will have more photos of chassis work.