Two quick questions. A friend has a 73 series 1 with a cracked head (no 4 cyl). I have proposed to replace his head with a head I pulled from a 71 car. This head is not perfect (has some steam erosion and valve guides a bit worn), but I have cleaned the head, lapped in all the valves and fitted new stem seals. Then I refitted the cams and set the valve clearances and removed the cams.
Q1. If I use my friend’s camshafts, can I expect those clearances to be the same or similar?
Q2. Which way does the head gasket go - large cooling holes on the exhaust or inlet side?
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Irrespective of work done, John - valve clearances must be checked, and reset, when cams are replaced. The specs are the same for all xk engines - and theoretically all cams should give the same clearances, once set, when replaced. but that is sheer theory - better safe than sorry…
The gasket should be marked ‘top’ or similar - and that done; the fit should be obvious? If the gasket does not exactly match both the head and engine bolts and ports; you have the wrong gasket…
Frank
xj6 85 Sov Europe (UK/NZ)
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The small holes to the inlet side, and I expect the clearances will be the same or close if the camshafts are both the same style they changed to in 69/70. After all the cams were ground on a machine from a pattern. But I would still check of course.
David
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Jaguar provided adjusting shims from .085" to .110" so they expected some significant variance, especially after valves and valve seats had seen some wear. I have even seen wear in cam lobes.
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Thanks David, we will use the cams from the cracked head and then check clearances again.
Am hoping that valves and cams are pretty much the same dimensions and that any variance is in the head and valve seat. If this is the case we will be OK.
If after changing the head, lapping the valves and using the old cams it would be an act of God if the valve clearances didn’t change an appreciable amount.
God not needed. This is about changing cams only
Not all XK cams are the same, although all 3/8” lift ones are functionally equal.
Parabolic cams are set to 12-14 thou both cams unlike the earlier 0.004” and 0.006” cams. Since this is a mix ‘n match rebuild it would be safer to distrust and verify the cam types.
Even if they are all four parabolics, which is likely, I would be amazed if all clearances stayed the same. But since parabolics have a 0.003” range, they may stay usable.
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Isn’t those clearances related to the lesser valve clearances for the early engines, Peter - rather than the cams themselves?
Frank
xj6 85 Sov Europe (UK/NZ)
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The plot thickens. The engine in this 73 car has been fiddled with in the past. My friend is 200km from me so I asked him to confirm that the head he has removed is not from a series 3. Turns out it has the 1 7/8 valves. So now I will take my cams down later this week when I go to install the head. I will take some measuring equipment and determine the course of action at the time. I have no idea whether series 3 cams are different or not. I am trying to line everything up with displaying my own car at the “All British Day” at Queenbeyan (near Canberra) this weekend.
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Yes, Frank, the parabolic cams are less loud and have the larger clearances (so there is more room for error as a side effect).
Distrust the engine is the best advice, and have a go.
Maybe you can work something out regarding the cams though? I would probably want to have the parabolics in such a situation.
Cam shafts are stamped with a part number between the lobes 1&2 or 5&6.
Very tiny numbers, very hard to read. Useful info when determining how to proceed.
All parabolics have 4 holes. A few two-hole cams were also parabolic but these are clearly identified by a groove machined around the circumference of the flange.
The only difference is refinement not performance, assuming they are set with the correct clearances. They can all be interchanged except for the various fitments at the rear end for various tach drives or none. Lobe holes came and went. Flange ends are best plugged after cleaning and the rear ends plugged with tach generator or cable drive dogs, or slinger washer and bolt.
1 7/8" valves are sometimes fitted to 1 3/4" heads with new seats.
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But the clearances are to allow for heat expansion, David - which is independent of cam type…
Frank
xj6 85 Sov Europe (UKNZ)
Actually, no, Frank.
Basic expansion does not vary by cam type but the effect of incorrect clearances on heat-shedding does.
The parabolic cams have very gradual ramps where the earlier cams opened and closed the valves over a shorter path. It follows that if a valve opens and closes quickly, the effect of an incorrectly tight clearance will also happen over a few degrees of cam rotation.
With a long, gradual, lifting/dropping ramp the effect of running with too tight clearances will likewise hold the valve off its seat for a considerably longer time, This might sound like a performance enhancer but it isn’t.
Because the ramp is gradual, the extra degrees of inlet lift timing from tight clearances may look impressive, but the lift is so low that no significant extra mixture flows in. Likewise, although the exhaust closes later, the lift is minimal.
Where these extra lifts DO have an effect are on delaying inlet closing (losing some compression ratio) and opening the exhaust earlier (loss of force on the power stroke). Because both of these events happen during the times of higher cylinder pressure, even a few thou of ramp lift due to tight clearances will flow more gas than on the low pressure parts of the cycle and the loss of pressure has a ‘significant’ efficiency effect (also noticeable during the high-vacuum idle situation).
In the case of the exhaust valve, tight adjustment on parabolics would therefore also reduce the time the valve spent on its seat, shedding heat. So clearance issues are not independent of cam type.
A side effect of what was just explained very well is that the cams are quieter and allow for larger clearances with less clatter. So the adjustment is not as critical. I think race engines with the early cams were set up 6/8 instead of 4/6, not sure about the numbers but definitely larger.
As a matter of course, I always set them at 6/8: the increase in noise is minimal, it lessens the frequency of adjustments, and lessens the chance of a burned exhaust valve.
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For sure, Peter - the only cooling the exhaust valves is the contact with the head, which makes clearances critical. Excessive clearances is harmless - too tight is not. And certainly, if the valves do not close completely; high pressure blowby will very quickly burn the valves. Any gain by tight clearances is miniscule - but the danger of burnt valves very real…
In short; adjust to specs. And for the record; has anyone actually measured clearances on a hot engine…
Frank
xj6 85 Sov Europe (UK/NZ)
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More problems,
I am now totally confused as to XK engine camshafts, their identification and valve timing.
Theory is not in accordance with practice.
I have been helping this friend with a cracked head on a series 1 XJ6, Some time back I pulled an engine from a wreck. I offered the head to him after I cleaned it up and lapped the valves in. Unfortunately when removing the cams I did not label them. So I used the internet to find photos of an XK head and found the following.
Clearly the exhaust cam has the notch at top with the no 6 cyl cam to the left and about to exhaust. So using this I identified the exhaust cam as the one with No 6 cam pointing left (when viewed from the back). The problem I see is that the timing is supposed to be set with no 1 at TDC (ignition), yet all drawings in the manual, and photos as shown, have no. 6 at TDC (ignition - or am I missing something?
Same thing. Piston 1 and piston 6 move together, hit TDC at the same time.
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