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| Joined: | 05-01-2005 |
| Location: | Minneapolis, Minnesota USA |
| Posts: | 572 |
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Removing and replacing the cams is pretty easy once the cam carrier is off the engine.
Remove the rear cover. Remove the bolt in the back of the cam and the retainer/washer.
The only resistance to sliding the cam out the front of the carrier is the front seal. You'll need to apply enough force to pop the seal out. You could use a seal picker to remove it first, but there's some risk of nicking the journal.
Once the seal is out the cam is free to slide out the front. Just go slow and take care not to ding the bearing journals with the cam lobes.
Install the new cam, using assembly lube on the bearing journals. Replace the retainer/washer and the bolt. Replace the rear cover using a new O-ring and Hylomar sealant on the flange.
Check the cam's end float. If it's out of spec, the rear retainer/washer is also a shim and it's available in different thicknesses.
Install a new front lip-seal.
Put a key in the keyway in the front of the cam and install the timing belt pulley. The pulley has to go on the right way. Flip it around and the timing will be way off. Some pulleys have multiple sets of timing dots for different MOP's, so you really have to pay attention. If you're installing 104 pulleys, they only have one set of dots. The IN mark next to the timing dot faces forward on the intake cam. The EX mark next to the timing dot faces forward on the exhaust cam.
The cams are gun-drilled from the front for an oil gallery. There should be an Allen drive grub screw down in the threaded pulley bolt hole plugging the bore. Probe down in the hole to see if it's plugged. If not, order up a grub screw and put it in since the pulley bolt alone will not provide a leak free seal. After the grub screw is in and seated, apply a light smear of silicone sealant to both the internal threads and the pulley bolt threads.
All the valve clearances go out the window when you change cams. You'll basically have to start over from scratch with the shims. But you can get a leg up on it by accurately measuring the cam lobes' base circle diameters and comparing cam to cam, lobe to lobe. If the new lobe's base circle is .002" smaller than the old one, start with a .002" thicker shim. The first step was to measure the valve clearances. No add in any shim adjustment required to bring the clearanced to the top end of the range... .005-.007 inch for the intakes, and .010-.012 inch for exhausts.
Early 907's used a paper gasket between the cam carrier and the head. It rarely sealed well and was usually the source of a chronic, slow oil leak. Later engines eliminated the gasket, added O-rings in counterbores around the oil passages at the front of the joint, and sealed the joint with Loctite 504. If your engine still has paper gaskets, this would be a good time to upgrade. 504 is old technology and these days the sealants of choice are Permabond A-136, Loctite 518 or Permatex Anaerobic Gasket Maker 518. Eliminating the gasket will force a further adjustment to the valve shims... thinner by an amount equal to the gasket thickness.
During trial runs for shimming the valves, install the cam carrier to the head without a sealant. When it's right and you do the final installation with a sealant, the sealant layer will lift the cam carrier off the head a small amount, opening the valve clearances by a similar amount. The trick is to shim short by the anticipated film thickness and count on the sealant to take it the rest of the way.
Loctite 504 = 0.0015", Permatex Aviation Form-a-Gasket - 0.0010", A-136, Loctite 515 and the 518 twins = 0.0005". Half a thousandth is negligible and less than you can measure accurately with feeler gages... you can ignore it. For the other sealants, consider the film thickness in your shim calculations.
I have an Excel worksheet that calculates the required shim thicknesses for you during a routine valve shim job. No it won't calculate the differences when you install a new cam, or eliminate the paper gasket (okay, you could enter the gasket thickness as a negative number in the film thickness cell... +0.0015 film -0.0100 gasket = -0.0085). Then enter the measured clearance, shim thickness and sealant film thickness you wish to use, and it will tell you what shim size is required to produce a valve clearance at the top of the spec range... .007" for intake and .012" for exhaust.
Is there a place on this forum for posting such a file? If not, it's in the "Files" section of several Lotus mailing lists on YahooGroups... turboesprit, S1S2S3Owners, Lotus-Cars and Lotus4Seaters.
Tim Engel