View single post by Esprit2
 Posted: 06-03-2019 09:38 pm
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Joined: 05-01-2005
Location: Minneapolis, Minnesota USA
Posts: 512
The main point is to stay focused while working, and be aware of where the pistons are before independently rotating the cams. Which includes accidentally rotating them while attempting to loosen or tighten the bolts that retain the pulleys.

If you will be removing any T-belt pulleys, or the crank V-belt pulley, loosen those bolts while the timing belt is still on and tensioned. That way, if you accidently rotate the engine while wrenching on something, everything will still move together like they're supposed to. No valves get bent. Just crack the bolts loose, don't remove them yet.

Then, as far as which one to put where first (crank or cams)... either way works, "IF" the timing belt is still installed and tensioned.

I choose to set the crank to TDC first, because I'm lazy. That one action simultaneously puts both cams in their 'timed' positions, where you want them. Plus, more often than not, I'm just replacing a timing belt, and it's a convenience to have both cams close to their timed positions when you start. In your case, the exhaust cam will be one tooth off, but that's minor. Then, after the timing belt is removed, the FIRST STEP is to turn the crank back to 90°, putting all the pistons half way down their bores.

The alternative is to set the crank to 90° first. Then, after the timing belt is removed, the FIRST STEP is to rotate both cams to their 'timed' positions.

It's six of one, and half a dozen of the other... as long as you're thoughtful, and know where the pistons are before you rotate any cams.

Again, more often than not, I'll just be replacing a timing belt, so having the cam pulleys start out where they need to be is a convenience. Since you'll be doing more disassembly, you'll have to re-position the cams after flipping the pulleys over anyway, so there's no advantage to following my preferred way. Use the method that makes you the most comfortable.

Both TDC and BDC put the valves at risk of being bent. At TDC, pistons #1 & #4 are at the tops of their strokes. At BDC, pistons #2 & #3 are at the tops of their strokes. Setting the crank to 90°, at either BTDC or ATDC, puts all pistons halfway down their bores, far from the valves.

Once the timing belt is off, NEVER rotate the crank 'through' either TDC or BDC. Just go to the nearest 90° position by the most direct route without passing through TDC/ BDC.

With the crank is set to a safe 90° position (BTDC or ATDC), you can spin the cams at will. The angles of the valves and diameters of their heads are such that even if two adjacent valves are fully opened simultaneously, they can't touch each other. So you can go nutz spinning the cams and not have to worry... once the pistons are out of the way.

The wildest Lotus cam's valve lift is well less than 1/2" / 12.7mm, so the pistons don't really have to be half way down their bores in order for the valves to be safe. Don't get paranoid about setting the crank to exactly 90°... just eyeball close is good enough.

1) While the belt is still installed and tensioned, loosen all the bolts you will be removing.

For the bolt retaining the crank's V-belt pulley, have a helper get into the car, select top gear (4th or 5th, whatever you have), release the clutch, and 'stand' on the brakes hard. That should hold the crank still while you apply torque to loosen the bolt.

2) Set the crank to TDC, or to 90° BTDC / ATDC, whichever is the most direct route to 90° without passing through either TDC or BDC. Both TDC and BDC are were valves get bent... stay away from both.

3) Remove the timing belt. A utility knife makes short work of that task.

4a) If the crank is at TDC, move it back to 90° BTDC by the shortest route.
4b) If the crank is at 90°, move both cams to their timed postions.

Now everthing is in a safe place, and you can get to work.

Don't forget that the auxiliary pulley must also be timed. It drives the distributor, and if you just randomly change it's position during all your 'cam' & timing belt work, the ignition timing will be changed by a similar amount. Hold a straight edge between the centers of the aux pulley and crank's V-belt pulley. Place a white paint mark where that straight edge passes the lower rim of the aux pulley. When installing the timing belt, make sure the aux pulley is rotated as required to put that paint mark back were it's supposed to be.

Rotate the tensioner's eccentric hub counter-clockwise to tension the belt. Yes, the eccentric will tension the belt in either direction. But if you rotate it clockwise to tension the belt, it will tend to back off in service. If you rotate it counter-clockwise, it will tend to stay where you put it.

Tim Engel

Attachment: 9XX Timing Belt - Routing & Tensioner Direction - 98kb.jpg (Downloaded 104 times)

Last edited on 06-03-2019 10:35 pm by Esprit2