tappet clearance advise

[M4rk]

Hi,

I've just got my Series 2 on the road after a 2.5 year rebuild, when rebuilding the engine I set the tappets at the 4 & 6 thou as per the manual I was using, I have since found that being a 1970 series 2 they should be at 12 & 14 Thou. I would like to get some miles under my belt before it comes off the road for the winter.

Would there be any damage if I ran the car with these clearances for a while , I am not taking the car over 3000 rpm as it is being run in.

Can anyone give me some advice, thanks in advance for any help.

[Gfhug]

Be careful to check which clearance you should have on yours. If you have the engine number then that can be checked if it is after the change. But setting 4 and 6 on 12-14 will be bad news.

Geoff

[M4rk]

Hi Geoff,

Thanks for the reply, what sort of bad news, it seems to run OK?

Mark

[Gfhug]

The change came at engine numbers 7R 8688 and 7R 38855 in Nov 1969. They are identified by a groove cut in perimeter of end flange.
Damage? I don't claim expert knowledge but if they are supposed to have clearances 3 times what you have set them at then I certainly wouldn't get the engine hot from a run longer than a few minutes. Not a big job to change to correct tappet shims

Geoff

[Hugo]

As long as you have clearance you're ok. Trouble is you won't know that without measuring it when the engine is at max running temp. My guess is that 4 & 6 thou would be fine, but that is just a guess.
If you have rebuilt the engine, however, the gaps might close up as the valves bed in. That is something that has been occupyng my mind a bit lately as I am in the process of completely rebuilding my engine (1971, so later type). I am fitting new valves all round. My instinct is to start with the gaps a couple of thou over then set them properly after a few hundred miles when everything has settled in.

[mgcjag]

Hi this will be of interest on camshafts http://www.georgiajag.com/Documents/Camshafts.html
Just for interest when i first got my 69 E it sounded very tapety.....used it for a while to get to know the car then started a refurb......when checking the gaps there was almost none (should have been 12/14).......however if you have just rebuilt and realised you set them wrong i dont think anyone would advise you to drive with them....just reset them.....still plenty of time befor salt goes on the road..... Steve

[Hugo]

Hi this will be of interest on camshafts http://www.georgiajag.com/Documents/Camshafts.html

Which reads as follows; "To get the most from increased duration cams, increased compression is usually needed."
This phrase suggests that the writer doesn't really understand what is going on. You need to increase the STATED CR with long-duration cams, for the simply reason that the ACTUAL CR will be compromised by the fact that the inlet valve doesn't close till the piston is well on its way up the bore. This effectively reduces the swept volume, and it is necessary to reduce the volume of the combustion chamber by a corresponding amount. That is why some high performance engines have such ridiculously high STATED compression ratios. I have had diesels that run on 14:1 CR, which is about the same as some racing engines today.
The earlier you can close the inlet valve, the more bottom-end torque you will get. Also the lower the STATED CR will need to be otherwise you will get pinging / pinking and other attendant nastiness. The XK engine has one of the lowest STATED CR's I can think of in a high-performance car - same as the Morris Minor at 8:1. But they still suffer from pinking, which indicates that the ACTUAL CR is on the limits of what will work.
I believe the maximum ACTUAL CR on a petrol engine is something like 9.4:1, but don't quote me on that.
I once had a BSA V twin that ran on 4:1 CR!
None of which has anything to do with the original query - sorry!

[mgcjag]

Just take tbe artical for what it is.....it shows the different cams, sprockets, and is a help in identifying what you have and what you gaps should be......do we have to keep going into minute detail on every subject....ok if its needed...Steve

[Hugo]

No, we don't have to, but I thought there might be some other nerds like me watching.

[PeterCrespin]

No, we don't have to, but I thought there might be some other nerds like me watching.

If we are getting into anorak territory, the parabolic cam clearances are 12-14 thou, not 12/14 thou.

I.e. the inlet and exhaust use the same clearance (from 0.012" to 0.014"). The earlier 4/6 cams use the smaller clearance for inlet, larger for exhaust.

[Hugo]

I've been thinking about this - why would they specify a larger gap just because they changed the cam profile? The only reason there is a gap at all is to allow for expansion of the valve stem when running. That isn't affected by fitting a different camshaft.
Also, a bigger gap will affect the valve timing, which I guess is counter-acted by the later cams' profile which presumably opens the valves earlier & closes them later?
I run a few Cummins commercial diesels, which have the injectors operated by the camshaft. They are incredibly sensitive to the clearance setting - back it off just a few thou and they smoke like Hell because the timing is then retarded a bit.

[M4rk]

Hi All,

Just like to thank everyone for the replies, I ended up spending half a day re adjusting the tappets just to be on the safe side, it's not as difficult to do with the engine in the car as I thought.

Cheers

Mark

[chrisfell]

I've been thinking about this - why would they specify a larger gap just because they changed the cam profile? The only reason there is a gap at all is to allow for expansion of the valve stem when running.

Perxactly, there has to be another reason. Perhaps an extension to the service interval between checking valve gaps was the reason, and the parabolic cam profiles were to prevent the wider gaps making the engine sound like a bag of broken spanners. Or am I being sceptical again?

[christopher storey]

I love "perxactly" . Is that fazackerley opposite to "precisely" ?

[PeterCrespin]

[quote="christopher storey] Is that fazackerley opposite to "precisely" ?[/quote]

No, it's opposite Aintree racecourse

[PeterCrespin]

I've been thinking about this

Uh oh!

- why would they specify a larger gap just because they changed the cam profile?

Because the change in profile related to parabolic geometry and inclusion of quietening ramps. By ensuring gradual take-up of clearance and gradual setting down of the valve on its seat, engine refinement is enhanced.

The only reason there is a gap at all is to allow for expansion of the valve stem when running. That isn't affected by fitting a different camshaft.

Incorrect, sorry, in both the reasoning and the measurement.

1) The clearance increases in a hot XK engine
2) The large alloy head expansion is as important or possibly more important than the valve expansion

There are times when 'thinking' is overrated, as our partners will usually confirm....

[Mich7920]

Did you saw "Gunfight at the O.K. Corral" ?
I speak about the movie of course ...

[Hugo]

I've been thinking about this

Uh oh!

Incorrect, sorry, in both the reasoning and the measurement.

1) The clearance increases in a hot XK engine
2) The large alloy head expansion is as important or possibly more important than the valve expansion

There are times when 'thinking' is overrated, as our partners will usually confirm....

Yes indeed both of those factors are relevant, but neither is affected by changing the camshaft. So why did they increase the clearance?

[Gfhug]

Can we, PLEASE, end these long running sagas and just accept that Jaguar changed the clearances.
Unless we can drag someone back from their grave to get their reasoning we will just have to accept it !!!

Mark started this off by asking for advice. He has followed it and reset the clearances. Hopefully he is happy with the result and will have a nice, quiet engine.

Geoff

[chrisfell]

I think the point Hugo was making is that the parabolic camshafts do not appear to need the larger clearances, but that they can work quietly with the larger clearances. In other words, Mark has not done any damage to his cams or followers or valves or valve seats or pistons by fitting shims for the smaller gap.

Now if this assumption is correct, then fitting a parabolic camshaft with tighter clearances will very marginally increase the valve lift, which might give a small performance enhancement, similar to fitting a higher lift camshaft.

If we could find out the reason why Jaguar increased the valve clearances, we could determine whether this assumption is valid.