Fast Road camshaft conundrum!

[Simonpfhc]

Hi all,

Looking for some help with setting valve clearances on “fast road” camshafts.

Some background…….

I’m doing some general maintenance work on a friends XK engine (actually in a 120) and checked the valve clearances. The exhausts are all around the 6-8 though range and the inlets are around the 4 thou range, but one is 2 thou and two of them 3 thou. So have removed the inlet cam to set them all to at least 4 thou. However, the owner informed me that the camshafts are “fast road” cams that were already fitted when he bought them. He has no record of where they cam from or who fitted them.
Both cams are of the 4-bolt variety. Normally in this situation, I would just remove them both and set them to factory correct 12 and 14 thou clearances for theses types of cams. However, they are not factory cams!

So the question…..do I set them to the larger 12 and 14 thou??

Both cams are definitely the later parabolic ones (the inlet is stamped C35556). They are also stamped KCJAG2 on the end flange.

Antone know these types of cams?

Are some reprofiled parabolic cams set with smaller valved clearances??

Any advice appreciated.

[mgcjag]

Hi Simon....KC is probably Kent Cams... they do a Jag 2 camshaft....just beware that some fast cams also come with replacement tappets and gaps to suit...give them a call or look on their Web site..Steve

[Simonpfhc]

Hi Steve,

I think you’re spot on! I’ve just had a look on the Kent Cams website for the Jag2 camshaft and they state 8 thou and 10 thou valve clearances. I’ll give them a call in the morning.

Thanks so much.

[christopher storey]

Unfortunately, unless my memory is leading me astray I have experience of these camshafts on a Series 1 4.2 ( not my car) and they were a disaster, digging a terrible hole in the torque curve at around 2250 - 2500 rpm . It was all some years ago so I will make a few calls tomorrow to refresh my memory, but on balance if i were you I would acquire a pair of standard cams and stick to standard clearances . It is very significant that in the whole 44 year life of the XK engine, Jaguar stuck to the 57/15/15/57 cam duration although they did from memory retard the whole timing by 4 degrees towards the end of the life of the engine, probably for emissions purposes

[abowie]

The clearances for your cam will be specified by the manufacturer.

The clearances for the various iterations of the factory cams depend on the lobe profile and yours will be different from this.

For example, I have a "D Type" grind on the cams in my MK2. The manufacturer specified 8 thou clearances for both inlet and exhaust.

[abowie]

https://www.kentcams.com/part/JAG2

So 8 thou inlet and 10 thou exhaust.

[Simonpfhc]

Andrew/Christopher, really appreciate your input, many thanks.

I totally agree, if it were my car, I would revert to correct cams.

Alas……

[Simonpfhc]

This engine keeps giving!!

Just removed the valve buckets and shims from the inlet side and found these:




Has anyone ever seen these before?

[rfs1957]

This banter included some good nuggets that are relevant to this exchange.

viewtopic.php?f=3&t=16637&p=139236&hili ... ts#p139236

I never felt I got enough credit for publishing this technical diagram



which has to be accompanied by the question :

Now, exactly where on this map was your car short on horsepower ?

[mgcjag]

Hi Simon...xk120 should be as our E types...but I did mention above iv heard that some after market cams come with replacement buckets that need fitting...Steve

[Simonpfhc]

Well, I think I’ve found my answer - they are top-hat shims used in racing due to their low mass. They are accompanied with matched racing buckets. Nice.

Thanks for everyones input on this.