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How To Rebuild the Mighty Three
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How-To:  Rebuild the Mighty Three
Bruce Turk
 

This article originally appeared in the "NEWS," the official newsletter of the New England Sonett Club and later appeared in "NINES".  The information contained in this article is meant as a guide for owners who are mechanically adept and have some experience.

 
Rebuilding one of Saab's three cylinder, two stroke motors is much easier than a lot of people would lead you to believe.  After all, the motor really does only have seven moving parts. They are: the three pistons, three connecting rods and the crank.  Back in the good old days you would pop out the motor, hone the cylinders, replace the pistons and crank and be back on the road the same day! The good news is that the actual rebuilding is as easy as it ever was, the bad news is that you can no longer walk into your local Saab dealership and walk out with a new crank and over sized pistons. The best that you can do today is diagnose the problem, purchase the parts that you need from one of the very few two stroke specialists still in existence, and make your repairs.

The purpose of this article is to save you a bunch of money by doing all the "grunt" work yourself. Forget about doing a partial rebuild.  Replacing a piston here or there with a "good" used one is a recipe for disaster. Your rebuild is going to cost $1,000 to $1,500 if you do the assembly work yourself. This is a $1,000 savings over having someone else do everything for you.

Motor Disassembly: Separate the motor from the transmission, remove the pressure plate, clutch disc and fly wheel. Remove the distributor from the block by removing the clamp screw . Remove the nut (28) from the front pulley. Using a puller, remove the entire pulley/vibration damper assembly. Remove lock ring (19) and gather up any shims that might be underneath. Shims were sometimes installed at the factory to take up crank end play. Pry out the front cover (16) and clean out the old grease.  GT motors have a few parts on the end of the crank  that differ from standard motors (Figure 1), just keep track of the order that they are removed and reassembly will be very straight-forward.  

Unbolt and remove the cylinder head.  Do not use a screwdriver to pry the head off as this can damage the head gasket surface.  Remove the bolts that secure the motor mounts and lower water inlet to the block.  Remove the bolts that hold the crank case to the block.  Don't forget the two bolts that were hidden behind the flywheel and the ones that face downward at the rear of the engine. Using a rubber mallet, knock the crank case off the bottom of the block.


 

 

Now for the fun part. Flip the motor upside down and stick a suitable arbor inside the hole at the back end of the crank. The arbor acts as a handle to help lift out the crank/piston assembly (see Figure 2).  With a helper steadying the block, pull up on the crank until the pistons pop out of the cylinder bores. Sometimes the piston rings get hung up just as they near the bottom of the bore. This is caused by a ridge that develops from normal wear of the  cylinders. If the pistons are stuck, gently rock the crank until the pistons release.


Pack up the crank with the pistons still attached and send them out for crank rebuilding and piston replacement. I can go into detail on piston classification, piston needle bearing sizes, and determining piston type, confusing and boring everyone to death, but I won't. Your parts supplier will determine the correct piston size and type. They will also fit the pistons to the crank so you don't have to worry about fitting the correct piston needle bearings. If you really want all the confusing details on these subjects, consult with a copy of the Saab two stroke engine manual.

For the purpose of discussion, lets say you're using J & B Imports for your motor work. Call Bud at (714) 282-7378  or e-mail to jandbimports@abac.com and tell him that you're sending over a crank/piston assembly. Ask him to rebuild the crank and replace the pistons with the next oversize (usually 1/2 to 1 millimeter over). Bud will probably ask if there are any deep scratches in the bores that might require an unusual amount of boring to smooth out. If Bud needs the diameter of the bores or old pistons, no problem; your local engine rebuilding machine shop can measure them for you.

Once you receive the rebuilt crank and new pistons, its time to take them, along with the block, crank case and head, to your local machine shop. Tell the machine shop to boil the block, crank case and head, to remove all the old carbon deposits. Ask them to skim the head to insure it's flat and smooth. If you have a oil injected motor, have them check the oil passages to make sure they are clear.

I have the cylinders bored to .005 (five thousands) over the piston diameter. The correct amount of clearance between the pistons and cylinder walls are open to debate. Saab set them up at .002 and we all know the result: LOTS of seized motors! The consensus is to set them up from .004 to .007, your machine shop can help you determine how much clearance is needed for your type of driving.  

Make sure they match each piston to a specific bore (piston diameters can vary slightly). Ask them to round off the intake and exhaust ports so that the rings do not catch on them (see figure 3). If you're a little nervous about installing the crank/ piston assembly back in the block,  ask the machine shop to do it for you. Total cost for boiling, head skimming, boring and port grinding should not exceed $300 (my guy charges $250).

To install the assembly yourself, proceed as follows:  Oil up the crank, pistons and cylinder bores with plenty of two stroke oil. Line up the piston ring gaps with the small ring stop pins. Lower the center piston into the bore first, followed by the pistons on either side. Make sure the crank's split metal sealing rings (6) are turned so that the gaps do not line up with the seam between the crank case and block.
 
The piston rings will have to be compressed to allow them in the bores. Wrap some electrical tape over the ends of two long, thin, slotted screwdrivers. The tape will prevent scratching the rings. Using both screwdrivers, compress the rings, one at a time, while your helper lowers the crank into the block. I know that turning a screwdriver loose on piston rings sounds a little crazy, but it isn't.  Remember, you're not working on a sophisticated computer mainframe.  You're working on one of the "lowest tech," most forgiving motors ever designed.  If your cylinders  have never been bored, the factory installed taper at the bottom of each cylinder might be sufficient to compress the rings, making the use of screwdrivers unnecessary.
Once the crank is sitting happily in the block, rotate the rear cover (14) so that the cut out corresponds with the hole for the distributor. No sealing paste or "Form-A-Gasket" should be used between the crank case and engine block. Just clean the two surfaces and apply a thin layer of oil. Now you can bolt up the crank case. See the list at the end of this article for correct bolt torques.  Install a new head gasket with the broad side of the metal ring facing up.  Note: the head gasket must not be coated with any sealing compound. Tighten the head bolts in a star pattern (see Figure 4).


Fill the area around the distributor gear with grease.  Install the front cover (16) with the arrow on the front of the cover pointing to the T.D.C. timing mark on the block. Replace the shims, lock ring, vibration damper and any other goodies that were discovered during disassembly. Install the distributor, flywheel and clutch assembly.

Break in procedure: This is another topic open to debate.  Saab manuals list breaking in procedures that are as different as night and day.  One manual says not to exceed 50 mph for the first 2,500 miles, while another one says to accelerate from O to 50 mph as fast as you can, ten times, and that no other breaking in is required! I prefer to err on the safe side, so I use one quart of oil with six gallons of gas and take it real easy for the first thousand miles. A rebuilt motor will feel very tight for a long time. My 93B's motor didn't feel completely broken in for the first 3,500 miles. Now, after 5000 miles, it revs up in a instant and takes forever to rev back down; just like a happy stroker should!


Bolt torques, all engines:
Crank case 5/16" bolts 18 lb-ft.
Crank case 3/8" bolts 29 lb-ft.
Flywheel bolts 22 lb-ft.
Crank pulley 36 lb-ft.

1955 through 1964 engines only:
Head bolts 850cc 36 lb-ft.
Head bolts 750cc, "80" on bolt head 47 lb-ft.
Head bolts 750cc, "100" on bolt head 58 lb-ft.

Note: The head bolts should be re-torqued as soon as the engine has cooled after the first run.  The second tightening at 600 to 1000 miles and the final tightening at 1500 to 2000 miles.  All tightening should be done with the engine cold.

1965 through 1968 engines only, head bolts (all):
  • Torque to 22 lb ft. then turn an additional 90 degrees. 
  • Run the engine through several heat cycles and turn an additional 20 degrees. 
  • After 1000 to 1200 miles, another 20 degrees. 

All tightening should be done with the engine cold.

Note: Another source for Two Stroke parts is Motorsport Sweden, check them out here
 
Bruce Turk, 3/01

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Last modified: February 29, 2016