Sleeve Nuts

Beginning about 1968 engine designs changed.  Up to this time most models had long studs that were used to retain both cylinder and head.  These 10mm bolts were quite long.  Due to expansion rates for the aluminum of the cylinder and head versus the steel stud the torque values on a cold engine were light.  The engine casting was unable to handle much torque and would distort slightly and/or the stud would want to pull out.  This torque value was 11 to 15 lbs. ft.  To have a more secure fastening of the cylinder to the engine, shorter and thinner 8mm studs were fitted and the cylinders were modified to accept a sleeve nut.

This nut (part #3360.161) was sized to fit in the 11mm hole.  To accommodate a tool one end was made to accept a 7mm Allen wrench.  These sleeve nuts also have the torque of 11 to 15 lbs. ft. but they only have to hold the cylinder to the engine cases.  They don’t have any other task.  The cylinder heads may then be torqued to 21 to 25 lbs. ft.  These engines don’t have a head gasket.  The addition torque will more likely resist leaking a bit more than the former through-bolt designs.  Neither design has a tendency to leak if proper assembled.

With the new attachment it was necessary to move the cylinder head fixing studs 45° around from the base studs to. Typically a boost port was inserted between a modified casting on the intake side allowing for a 3-transfer arrangement.  This is found on the 247 and later Cota’s as well as most of the Cappra’s.  Anytime you see the cylinder fixing nuts located front and rear and left and right on the top of the cylinder the engine will have these sleeve nuts.  The 360 Cappra GP’s and DS’s also have these nuts.  They are beneath brass inserts (3660.174).

If you are going to take an engine apart it is necessary to deal with these nuts.  The cylinder will not come off without destroying the cylinder and/or engine cases if you attempt it.  Scratch that.  The exceptions are the 360’s.  The 360 GP’s and DS’s have such strong cases you would have to use explosives to damage them.  O yes, the only likely damage you can inflict on the cylinder is breaking fins.  When you get that cylinder off you will see why.  Heft it.  These things are heavy.

Water can get into the holes and cause corrosion between the cylinder and nut as well as the nut and stud.  This will make them difficult to remove.  This is more prevalent on the 250’s.

• Ratchet Extension Tool
• Torque wrench
• Pencil Magnet
• Dead blow hammer

Procedure for removal:

Remove Cylinder Head:

1. It will typically take a 17mm socket to remove the 10mm nuts.
2. Insert 7mm Allen wrench into hole and engage sleeve nut. 
3. Turn counterclockwise and unscrew from stud.
4. Remove with pencil magnet.
5. Tap cylinder with rubber or dead blow hammer to loosen connection.  Strike against the edge of the fins, not on top or bottom of them.  You can also strike beneath the intake manifold and exhaust port.  Do not use a hard hammer.
6. Slide cylinder up part way.
7. Press rags or paper kitchen towels in crankcase beneath piston.
8. Remove cylinder.

This all seems very simple.  About 1 out of every 2 engines has a severely stuck sleeve nut.  You can soak the nut with liquid wrench or any number of penetrating oils and let it stand for a day or more.  I haven’t the patience.  Even after a week of using penetrant it only works about 1/10th of the time.  What to do, what to do?  Heat?  Doesn’t work.  Maybe the stud will pull before the Allen wrench rounds off the inside of the nut.  Been there, done that.  The sleeve nut will round out.  Once you rounded out the nut it’s time to drill.

Drilling:

1. Get some cutting oil and an electric drill.
2. Select a suitable sharp drill bit.  Make the selection of a size that will only drill the stud out.  Remove all of the sleeve nuts that can be removed.
3. You don’t want to drill into the cylinder casting.
4. Start drilling.  This will take some time but at least you will feel like you are doing something.
5. Remove chips occasionally with the pencil magnet.
6. Measure your progress in some fashion.
7. Resume drilling.  Eventually you will destroy enough of the stud to remove the cylinder.
8. Use locking pliers and remove the remains of the stud.
9. Order another stud and nut.

There are special steel washers at the bottom of those holes for the sleeve nut to press against.  If they fall out; save them.  If they stayed in the hole, leave them there.  Just remember to return the washers to the correct holes on reassembly.  Clean all nuts and loose washers.  Use a 7mm x 1.00 tap and die to clean the threads.

Reassembly: I am assuming that you have managed to get the cylinder over the piston and rings and seated onto a fresh cylinder base gasket.

1. Replace any steel washer that escaped from the sleeve nut hole (s).
2. Apply a small smear of anti-seize to the internal threads of the sleeve nut.  (You don’t want to go through all of this again)
3. Using a 7mm allen wrench turn the nut down and seat against the washer.
4. Use a torque wrench to tighten fasteners to recommended torque.
5. Assemble head to top of cylinder.

The question will come to you about where to get a 7mm allen wrench.  I purchased mine from Sears.  If you don’t have a good hacksaw blade you can get one there also.  You can also get a 7mm socket.  Sear’s has them in ¼” drive.  If you don’t have an adapter then get on of those too.  Sear’s tools are of uniform strength but are not hardened like some tools.  They are strong enough for the job.  A hacksaw blade will cut them.  Cut off the short part of the L.

Assemble this tool in this order:

1. Insert the long segment of the 7mm allen wrench into the sleeve nut.
2. Place the socket on the other end.
3. Install adapter(s).
4. Install torque wench.
5. Torque away.

Note:  The 360 GP/DS models have a bolt inserted over the sleeve nut and therefore hide it from corrosive forces.  You just have to remember the nut is there when removing the cylinder of one of these models.  They won’t fall out.  The inserts above them will retain them.  You will hear them click every time you turn the cylinder over.  Anti-seize will have to be applied to the threads on the stud before installing the cylinder instead of on the inside of the sleeve nut.

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