Problem with the Head

(Lou) #1



I’m sorry to report that something has gone wrong.

As discussed in earlier posts, I wanted to move the camshafts back to TDC after rotating them slightly forward in order to access the setscrews on the camshaft sprockets. I could have done that by moving them forward, all the way back to TDC, a tedious job given that I can only move them forward a slight bit at a time, by rotating the crankshaft. So I decided to rotate the crankshaft backwards.

Using a 1 5/16” box wrench (about 2 feet long and 5 pounds) I rotated the crankshaft backwards. And, as we discussed, I turned the crankshaft, and therefore the cam shafts backwards, slightly beyond TDC so that I could then turn the crankshaft and camshafts forwards to TDC, picking up any slack in the various mechanicals.

So again, I got the camshafts slightly beyond TDC, and then attempted to turn them forward to TDC. However, turning the crankshaft forward, the camshafts did not move. I observed that the chains attached to the sprockets were reacting to the turn, but did not move. I don’t know what happened, but I would guess that chains have disconnected from whatever mechanism causes them to move.

I would appreciate any thoughts on how to diagnose and fix whatever happened.



(Peter Crespin) #2

The chains cannot come off. There could be a lot of slack because you compress the spring-loaded tensioner when rotating backwards. Keep turning forwards and only turn forwards (clockwise looking at the pulley)

(Lou) #3

Peter: Happily you were right. I am almost home with the reinstallation of the revised camshaft rotations


(tony) #4

I think it is better to rotate the engine forward, no matter how tedious,
as the timing chain slack after backward rotation can introduce an inaccuracy, especially in the exhaust side

a 33mm socket can be bought in 1/2", worth owning if you do not posses 3/4" drive gear

(Paul Breen pay palled it) #5

Can also buy a 1/2" to 3/4" adaptor.

(tony) #6

its probably too tight a fit, 2 of my 3/4" drive ratchet just fits in, another one not

I also have a few 3/4" Whitworth spanners (and sockets)

the 3/4" ratchet you only get a small travel, as the handle is long

hence my suggestion of 33mm x 1/2"

(Frank Andersen) #7

Unless the camshafts are firmly connected to the sprockets, chains fitted and the cam timing verified, Tony - no large turning of the engine should be attempted…

Small crankshaft movements around TDC, either way, with the cam timing roughly set with the timing tool will cause no problem - there is adequate clearance to avoid valve interference…

The precision of the cam timing, verified by the setting tool, must in any case be rechecked after the chains are tightened - and indeed TDC set with a ‘forward’ motion to take out the minimal chain slack…

Two different concerns are at play; to avoid independent movement of cams/engine before chains are firmly and correctly attached - and the precision of cam timing after proper connection. The first to avoid valve damage - the latter to get the best out of the engine…

xj6 85 Sov Europe (UK/NZ)

(Lou) #8

You will recall that I was turning over th engine with a huge 1 5/16” box wrench in order to counteract turning over the engine backwards. It’s very tedious.

Today, I was able to rig up a huge 1 5/16” socket and a short extension which allowed that setup to rest on the bottom of the engine fan housing. As a result, I was able to use a relatively small plumbing wrench to grabe the outside of the socket and easily turn it.

I problably made five or six complete revolutions of the engine The camshafts are not turning because the chains drign the sockets are not moving. Looking in the area above the front chain, I can see that the top of the lower chain, the long one, is moving with each turn. But that movement is not being transmitted to the camshafts which drive the sprocket. One sprocket has two setscrews in tight and the other has one setscrew in tight. I was attempting to rotate the sprocket in order to put in the remaining setscrew.

Any suggestions?


What Went Wrong with the Camshaft Sprockets
(Robert Wilkinson) #9

Oh no, I hope I am misinterpreting what you are saying, Lou. I’ll let the experts comment.

(Peter Crespin) #10

Have you fitted too long a chain so that it is hanging below the driving teeth on the double sprocket? Have the cams ever turned? How did you move the two-bolts-fitted cam?

What you describe ought to be impossible.

(Paul M. Novak) #11

The Jaguar XK engine is an interference engine, meaning that the intake and exhaust valves extend below the level of the head when open and they can impact the pistons if not synchronized properly via the timing chain. It is quite possible that by rotating the crankshaft you bent some your intake or exhaust valves because they were open and they contacted one or more of the pistons.


(tony) #12

That is rather concerning, if I understand your post correctly

did you at any point feel any points of resistance whilst rotating the engine?..with plugs out?

if so, that is bad

As I mentioned, it should be possible (although it may not seem so) to get the camshaft setscrews out, without rotating the engine.

I know this due to dismantling ~6 seized XJ engines, which could not be rotated,
maybe I was just lucky, a special shaped spanner was needed

it may be possible you have not damaged anything, however, imo, the conservative & least risky approach would require you to remove the cylinder head, and see if any valves are leaking and/or bent

other opinions should be sought & considered imo, prior to proceeding any further

  • if you have means to inject air into the cylinders, you can completely back off the camshaft tension, preferably remove them, all the valves should be closed, leak down should be low,
    it wont be low if any valves are bent
(David Jauch) #13

Given that they have moved;

I have absolutely no idea how to interpret the failure mode. I fear that others worrying about valve interference must be right, but short of the upper chain not transmitting because it is too long (even though it moved before) or broken (yes, how), I can only think of the intermediate sprocket broken (yes. How).

I would investigate the situation as good as possible and then I fear the head must be removed. Very sorry.

(Lou) #14


The chains are those that came with the car (1971). I have not changed anything.


(Frank Andersen) #15

If you have rotated the engine 6 complete turns (in any direction) without the camshafts turning, Lou; it’s the(!) cardinal sin - you never(!) ever(!) turn the engine if the cams cannot turn.

And the ‘because’ is immaterial - though you must verify why - to avoid a repeat…

You have bent the valves - and you have to remove the head to assess the damage. And change all the bent valves…

xj6 85 Sov Europe (UK/NZ)