WORKSHOP

KICKING THE BUCKET

There’s a tapping sound from under the bonnet. Not the usual sound of wide valve clearances or general engine movements, but deeper, hollow almost, increasing as the revs rise. Be afraid. Be very afraid…

WORDS & PHOTOGRAPHY: JIM PATTEN

The aforementioned and alarming noise is almost definitely a loose valve bucket guide, a common problem on later XK engines, more especially on the exhaust side, but also on earlier engines as they age. Inside the valve assembly on the XK engine, is the valve itself, valve seat, valve insert and springs. Between the valve stem and bucket is a shim, used to determine valve clearance size. The camshaft turns on a bucket, which fits neatly over the valve/spring/collet assembly, where to ensure precise and accurate movement of the bucket as the cam activates the valve, is a guide insert, fitted to exacting tolerances.

Over the years, heat and abuse, cause a differential in expansion, resulting in a slight shift and loosening of the bucket guide. Potentially causing havoc, not to mention being an extremely costly job. Movement of the guide can arrest the bucket in the lower position, holding the valve in the open position. The rising piston will hit the valve causing mayhem.

During manufacture, the insert is made to 0.003” oversize and shrunk fitted into the cylinder head and precision bored to accept the bucket. However, according to anecdotal evidence, the wise folk on the production line considered the oversize too small and it was increased without recording the data. In later years, when many of these craftsmen had retired, and with nothing written in the records, the original sizing became the norm and bucket guides loosened after extensive use, especially if an engine had overheated. Various efforts to combat the issue have been used over the years. These include inserting

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This cutaway shows the cylinder head, with camshafts and tappet, sliding inside the guide

The guide was so loose that it came out by hand The insert had lifted enabling the cam lobe to hit the guide. Fortunately, no serious engine damage was done

Set the engine to top dead centre and check using the cam setting tool See how the guide has been damaged and the bucket stuck in place

The top timing chain adjuster is secured on an eccentric shaft with splines and a lock nut. Release the lock nut

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a grub screw through the cylinder head to deter the guide from moving. Or tie down clamps, where external bridge clamps are drilled and tapped into the ‘head to lock down onto the guide and prevent movement. These remedies alone are haphazard with doubtful results. Many engine builders incorporate these systems alongside new bucket guides during the engine reassembly as a precaution. Kits are available from the specialists, including SNG Barratt and Rob Beere Racing.

Scare mongering time - If your engine has not been rebuilt in recent years and this issue has occurred, the chances are that it will require a full overhaul and outside the scope of this feature. Even if not, the solution is not simple as it requires the removal and partial overhaul of the cylinder head. We, of course, would recommend a full overhaul of the ‘head. It happened that Rob Beere was carrying out such an operation in their workshop. Their practice is to increase Jaguar’s original tolerance, following the Jaguar old boy’s routine. In simple terms, the cylinder head is stripped bare of camshafts, valves, springs etc. and heated to 110°C. The old guide inserts are removed by either drawing them out or by machining. After letting the head cool, it is machined to accept the new 0.004 – 0.005-inch oversize inserts. The cylinder head is then re-heated, while the guides are frozen before being driven into place. Then with everything resumed to normal temperatures, the insert is machined to size allowing a 0.001-inch tappet to guide clearance. There is little point in doing a single cam bucket guide. Once the machine is set up, it is just as easy to replace the entire set. Unless very recently done, then it is recommended that valve guides and seats are done at the same time. In short, do the whole job!

There isn’t enough space to detail the cylinder head removal or refitting, but there are a few notes to consider. Tools needed are the top timing chain adjuster, camshaft TDC (top dead »

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The spline plunger needs to be pushed down to allow the shaft to rotate

Slowly release the camshafts (they will be under tension) and remove each camshaft. Note that caps are numbered and should be returned in the correct place Using this special tool, it will fit over the plunger to depress it while pegs locate in the shaft, to slacken the top chain

The state of this bucket tells its own story and should be replaced Either lock tabs or locking wire hold the cam sprockets to the camshafts. Remove the bolts and ease the sprockets off the camshafts. They should slide inwards to allow head removal

With the carburettors and exhaust manifold removed, the cylinder head nuts can be removed, not forgetting the ½-inch nuts at the front of the engine

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centre) plate, valve spring compressor and torque wrench. The factory workshop manual gives sufficient detail about removal and refitting for the experienced enthusiast to manage. Although more expensive, the Cometic gaskets are the best and will keep oil leaks to a minimum. Slightly thicker than standard, they cannot be used on an untouched engine where the cylinder head has not been machined. That small increase in size lifts the cylinder head enough to render fitting the timing chains over the cam sprockets impossible. It is accepted that peripheral parts like carburettors, exhaust manifold, coil and hoses will have been removed, ready to carry out this operation. An engine crane has been used to lift the cylinder head, but it can easily be accomplished by two people.

Note that the images show different engines, but all are indicative of the basic XK unit, fitted to a wide range of models between 1948 and 1992. n

Thanks to: Machine work carried out and parts supplied by Rob Beere Racing. Website: www.rob-beere-racing.co.uk

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Lift the cylinder head. It will snag on parts not yet released or even a recalcitrant gasket

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In the RBR machine shop

The stripped ‘head will be thoroughly degreased and heated to 110° to either press or machine out the cam bucket guide insert

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Here, the 0.005” oversize guide is fitted to a mandrel

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With the ‘head heated and the guide frozen, the guide is driven into place

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The new guide in place will be bored to allow a 0.001” bucket to guide clearance

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A dial gauge measures for the correct bore size, for both the insert and the bucket

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New (long) buckets are fitted, in this case, with a protected surface for longevity Some prefer the belts and braces by using bucket guide insert tie downs, not recommended as a fix without new guide inserts Refitting the cylinder head

Whether using new valves or the cleaned originals, valve grinding is a necessity

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Always use new valve springs

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Bearings fully oiled and camshaft rotated several times, the clearance is taken with the lobe fully upright

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In the JP workshop, test shims of 0.070” are used to gain clearances

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Fit camshafts one at a time, for ease of setting clearances

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Keep a chart noting gaps. Exhaust number 1 has a 0.034 clearance. Add to the 0.070” shim = 0.104. Minus 0.006” correct clearance = 0.098” shim, or ‘N’ in Jaguar’s code

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Using a micrometer, always check the valve shims for size, even if they are new

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Thoroughly clean the engine block and fit a new head gasket. The excellent Cometic gasket will not be suitable for an unskimmed ‘head

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This shows the sequence of tightening the cylinder head, once fitted to the block. Start in the centre and work out

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Do not forget to fit new copper washers when fitting the ½” nuts to the front of the ‘head

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Refitting the cam sprockets to the camshafts may take a few takes, turning the centre to align with the splines Retighten the top chain to allow a small amount of slack. Too little and it will rattle, too much and it will whine and promote wear Recheck the valve clearances and do not be surprised if they have changed. Refer to the manual for the right application as later cars had a wider gap