View single post by Esprit2
 Posted: 02-24-2020 11:06 pm
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Joined: 05-01-2005
Location: Minneapolis, Minnesota USA
Posts: 550
Pricelessjunk wrote:
(Snip)... when I removed my cam carriers, which had what looked like a factory paper gasket, one carrier looks like it has a counter bore and one doesn't.I downloaded your photo and blew it up to full screen. As far as I can tell, neither of the cam carriers has the specified counterbore. One appears to have a slight countersink ('sink', not 'bore'), but that's not what is required.

A countersink is a tapered recess around a hole. The tapered hole accepts the tapered bottom of a flathead screw. Or, it just serves as a 'funnel' to help parts slip into the hole.

A counter bore is cylindrical. It has straight up & down side walls (a cylinder) with a 90 degree corner at the bottom, and a flat floor. It's a counter-square, not a counter-taper.

When you see it on the bottom of a cam carrier, it's a real cut hole, not a bit of a taper leading into a hole. Am I making any sense?

Cutting the counterbore isn't something you should try with a hand-held drill motor. Back in the day, a special service tool was issued that was essentially a 31/64" end mill cutter with a 11/32" pilot drill. Then a collar around the outside of the end mill was set to stop the cutter at a prescribed hole depth... hard to make a mistake.

The drill bit fit into the existing oil hole, dress-cutting the bore slightly for the correct fit with the roll pin that would be fitted, then the end mill cut the counterbore. As the end-mill cut into the cam carrier, the collar would contact the carrier, stopping the cutter at the correct depth. The depth of the c-bore is important, so just winging it on a drill press is not a good idea. If it's too deep, the O-ring will not be compressed addequately to form a good seal. Too shallow, and the O-ring will interfere with the cam carrier fully seating on the head. It needs to be 'right'. I'd recommend that you take the cam carriers to a machine shop and have the work done properly in a vertical mill... unless you happen to have one in the corner.

When properly cut, the counterbore will be:
......... SPEC METRIC.. / Calc'd Inch Decimal
Dia .. 12.30 -12.44mm/ 0.48425" - 0.48976" (service tool was a 31/64" end mill cutter with a 11/32" pilot drill).
Depth 1.20 - 1.30 mm / 0.04724" - 0.51181"

The O-ring is Lotus part number: A907E6202Z. Two are required, one per cam carrier. Sorry, but I don't have the J-H part number at my finger tips.

Sorry, but I don't have a spare cam carrier with the counter-bore laying about (mine are all in engines), so I can't send a photo at this time. However, tomorrow evening I 'may' be going to a friend's shop to help with a Lotus Esprit project, and I'm quite certain he has a few stray cam carriers. I'll take a few photos.

BTW... one of the driving reasons for going gasketless was that the original gaskets would soak up oil, saturate, and essentially become a wick... ie, it promoted the oil leak.

Tim Engel

Last edited on 02-25-2020 12:26 am by Esprit2