1968 S engine rebuild - seeking wisdom

[Richsmith901]

My suspicions are with the cams, I had new cams made, the cams guy had to re-grind them because he put the standard profile on rather than ‘s’. Cam-man said he would simply take some material off the bottom of the cams and that the rockers could be adjusted to accommodate...

[edmayo]

That won't do it.

[Gearby]

Material off the bottom of the cams? I don't understand. Can somebody please explain this some more? Does that mean to grind material off the bottom of the lobes to increase the lift? I'm not a machinist but that would sound like a bad idea.

[66S]

When cams are reground it usually reduces the base circle Check the main bearing thickness, I hear new production bearings are not real uniform. I will only use NOS German Glyco or KS mains in my motors. For std. rod bearings use ACL. Check the IMS gear fit for adequate backlash and check the crank for runout.

[chris_seven]

Reducing the diameter of the base circle on a 911 cam isn't a great idea and is the reason why Web Cam and others used to weld old cams to grind alternative profiles. The difference in the 'core' diameter of the cam and the base circle of the lobe is very small.

There isn't much difference in lift between the S Profile and the T Profile on a '68 engine and the L and 'Standard' Weber engine used very similar lift but the Timing was quite different and this makes it very difficult to re-profile the cam in a satisfactory manner.

I think that to create an S profile from a 'standard' cam you would have to reduce the base circle below the core diameter.

I have to say that I wouldn't have accepted this solution and would have wanted cams that had been correctly made.

Glyco Mains don't seem to bad as they are made in a different factory to the rod bearings which is where the problem seems to exist.

On 2.0 litre engines we have used ACL Race Series Rod Bearings for the last 10 years and they have performed well and we have never had any issues.

It is always worth making basic clearance measurements and checking the torque to rotate the crank during initial assembly as well as checking ring gaps and the oil control ring tension.

Have you measured the lobe centre angles ? It is quite common to find 911 cams with different lobe centre angle Left to Right and I have seen several UK made cams which differ significantly and they all came from the same cam grinder. This can make the engine run unevenly side to side

We now have a simple set up and we measure every cam we buy before it is fitted.

[Richsmith901]

Hi Chris, thanks for the input, engine runs ok, actually sounds really nice, but the concern is that it is too tight rather than uneven. I going to be popping into the workshop tomorrow, as long as there isn’t too much snow on the ground following ‘the beast from east’ .. thanks to everyone who has contributed.. Rich

[304065]

All right I am going to go out on a limb here with my final recommendation-- wrong size intermediate shaft gear and/or insufficient backlash.

[Frank Beck]

Well?

What happened?

Please include pics.

[Richsmith901]

Sorry to keep the suspense, the workshop was closed last week, due to the snow...

[Richsmith901]

Torque on cold engine is 45 nm