AC-6 - the website for all owners and enthusiasts of the AC two litre, six cylinder engine




What is an AC Engine?

Engine  Rebuilds

Blocks, Studs & Water Jackets

Spare Parts List One - Attaching Parts, Seals & Gaskets

Spare Parts List Two - Crankshaft & Cross-shaft Assemblies and Chains

Spare Parts List Three - Pistons & Liners

Spare Parts List Four - Sump Assembly

Spare Parts List Five - Cylinder Head Assembly

Spare Parts List Six - Ignition, 

Spare Parts List Seven - Water Pumps

Engines for Sale and Used Parts

Parts List

Spare Parts List Five - Cylinder Head Assembly......

  * See New Rockers at bottom of page *

A complete and newly restored and assembled Vintage cylinder head assembly, incorporating new guides, valves, springs, rocker shaft, collets, caps, mousetrap springs, pedestal studs and reprofiled camshaft.   


Getting the most from a brilliant design

The cylinder head assembly is a vital part of any engine, especially with the AC as the overhead camshaft design means that all the valves, rockers and the driven timing gear are carried in the head.  This not only makes for an efficient engine but also simplifies maintenance as access is so easy.  There are quite a few moving parts however, - although rather less than in some overhead cam layouts - and they do wear.  Replacing worn parts and taking care in their assembly can pay dividends in engine performance and reliability.  And we can supply the parts you will need.

Different types of head

Successive generations of AC cylinder head have less difference between them than the cylinder blocks have and are, to a large degree, interchangeable.  The distinguishing features are:

  • Vintage:   Water pump mounted on the front of the head on three studs at rectangular water passage in head and driven by the camshaft.  Longitudinal recesses on lower face.  Narrow rollers; inverted tooth timing wheel.

  • PVT:  Early with cam driven water pump as above; later by belt with pump mounted on four unequally spaced studs.  Narrow rollers, inverted tooth timing wheel later changed to Duplex type.

  • Post-war early UMB:  Thermostat housing mounted on front of head on two studs at circular water passage.  Narrow rollers.  Water pump now mounted on side of block and not on head.

  • Post-war late UMB/UMC:  As above.  Wide rollers.

  • CL:  As above but lacking the narrow hot-spot water passage between and beneath the centre pair of Carburettor studs.  Equal length head studs.

Cylinder head components

Inlet and Exhaust Valves

Valves are the most often replaced items in the cylinder head and we keep good stocks on the shelf. 

Our valves are made from best quality materials and are Stellite coated for use with hardened seats.  Please note that at engine or cylinder head overhaul it is advisable to fit hardened seat inserts for use with new valves.

Special valves with semi-tulip heads, large diameter heads, waisted stems etc are all available to order.


Many of the camshafts that we see in AC engines are in very poor condition, whether rust pitted, worn on the cam lobes or bearing journals or even bent!  While it is sometimes possible to find a replacement used shaft in good condition, the supply is finite. Our new and reprofiled camshafts fill the growing gap in supply and provide enhanced performance.

Fast Road Camshaft

Designed to our specification by a leading expert in the field, our fast road camshaft not only provides much better performance but actually makes the engine feel smoother and allows it to go on revving when it would normally run out of breath.  This camshaft has been raced on the Continent where one owner said, 'It made the difference between night and day!'

Sports Camshaft

For those seeking even higher performance we can also supply a sports camshaft. Early ordering for these items is recommended.

Racing camshafts

Camshafts for racing may be supplied to bespoke profiles according to customer requirements.

Reprofiled Camshafts

A cost-effective alternative to a new camshaft is an exchange, re-profiled one.  Reprofiling is a good way of recycling well-used camshafts for further active life and, at the same time, providing some extra performance.  We keep several in stock to various configurations or, if you prefer, we can re-profile yours to your own requirement (within its wear characteristics).

Worn Camshaft Journals and Bearings

When significant wear has taken place in either or both the camshaft journals and the bearings in the cylinder head the situation can often be retrieved by machining both bearings and camshaft and fitting shell bearings., as illustrated below.


Valve Guides

Valve guides can suffer from serious wear and replacing them is an essential part of any overhaul.  Our guides are precision machined from cast iron and the bores finely honed for smooth valve operation.


Valve Springs

Valve springs inevitably become tired after long use and can no longer perform their task of closing the valve quickly and of maintaining the required seal between valve and head.  We always keep plenty of inner and outer spring pairs in stock.

Sports Springs

For use with uprated, higher performance camshafts we also stock springs with higher closing pressures and fewer coils to prevent coil binding that could occur with cam profiles providing increased valve lift.

Valve Spring Spacers

Sometimes it is necessary to adjust the fitted height of valve springs to ensure consistent valve closing pressures on all valves.  We are currently developing a range of different thickness spacer shims for this purpose.  At the moment we have 0.020" shims in stock;   these may be used in multiples to achieve consistent spring height.


Valve Gear Fittings

Valve spring caps, valve stem caps, adjusters and locknuts and valve collets all wear but we have good stocks of new replacements.

Uprated Rocker Shaft

A weakness of the original rocker shaft is that it only extends a short way into the bearing pedestal at the front of the cylinder head.  With time and use the restricted bearing area can allow the front of the rocker shaft to fret up down, causing the bearing to wear oval making correct valve adjusting impossible (see above).

Our uprated rocker shaft overcomes this problem as it has been extended at the front end so that it seats more deeply in the front pedestal, thus being more firmly held in place.  Additionally, the shaft is extended at the rear to take advantage of the considerable amount of unused, extra bearing space available in the rear pedestal.

These shafts have been proven in usage on the race track.

Please note that we no longer supply rocker shafts to the original, shorter AC design owing to lack of demand.

Exhaust Manifold Nuts

Our brass exhaust manifold nuts can smarten an engine bay enormously.  They can be used on either the cast types of manifold or the tubular Ace/Aceca type where access can be restricted.

Rocker Rollers

Accurate valve adjustment can often become difficult owing to rocker rollers and pins being very worn.  Our new rollers and pins cure the problem.  We stock the later, wider roller as well as the earlier, narrower type.  In either case the pin should be ground to suit the somewhat variable width of the rocker and peened in place.  

Rocker Shims

The spacing of rockers on their shaft, even on very original engines, can be somewhat haphazard.  With our shims, much more accurate spacing  and alignment of the rockers with the valve stem and the oil supply hole in the rocker shaft can be achieved.

Rocker 'mousetrap' springs

Often ignored, the rocker spring serves a useful purpose in keeping the rocker roller in contact with the camshaft.  When the spring weakens the rocker is allowed to 'chatter' against the cam.  This accelerates wear to the camshaft. 


Camshaft Location Bearing

This particular bearing is another much ignored item whose presence and function is just taken fro granted.  They do wear however and this allows the camshafts to move back and forth.  They also provide the oil passage to the camshaft and rocker gear and if worn, cause oil pressure to drop.

Rocker Gear Pedestal Studs and Nuts

Original pedestal studs often have very poor threads at the top and it can be difficult to tighten the rocker cover nuts, particularly if the nuts have been over-tightened in an attempt to prevent oil leaks and the stud thread strained and those in the nuts stripped.  Our high-tensile studs are made a quarter of an inch longer at the top so that the thread engages much more deeply with the nut, thus allowing more effective tightening down and reducing the risk of oil leaks, especially with fibre washers fitted beneath our new, very high quality stainless steel domed nuts.

Adjustable Timing Sprocket

Tested and tried in competition, our adjustable timing sprockets not only provide a full set of new, unworn teeth but allow much easier assembly and, once installed, very fine adjustment of the valve timing without the tiresome and perhaps risky business of splitting the chain.  

Initial setting up becomes much easier than with a standard sprocket and if you are involved in motor sport, finding the optimum valve timing for different events is made very simple.



Vintage Rocker Boxes

We see a lot of rocker boxes that are cracked or otherwise damaged, the most common cause being over tightening to prevent oil leaks without renewing the rocker box gasket.  With the PVT and post-war engines a replacement rocker box can sometimes be found but with the numerically sparse vintage models this is seldom the case.  Our recently introduced vintage rocker box removes this problem completely.

Camshaft Pedestals

For some reason there are plenty of bare heads around, their fittings having been purloined for use elsewhere.  Quite often such plundered heads have a perfectly repairable fault, perhaps a crack or corrosion but lacking sufficient parts, just gather dust.  Our new pedestal castings now make it possible to bring abandoned heads back into use.  The castings are part machined and pilot bored ready for final matching to the head in question.

Important Note:  We hold a substantial stock of cylinder heads and have begun a programme of restoration to bring them back into use and to help alleviate the known shortage of sound heads for the AC engine.  Further details will be announced in due course.

New rockers......

Our new valve rockers are proving increasingly popular.  Each rocker is a precision machined investment casting that duplicates the geometry of the AC rocker original but which offers approximately twice the strength of the original.  The cam follower is a roller bearing that is slightly larger in diameter than the original to lessen wear on the cam lobe and which also offers a small increase in valve lift.   These rockers are used to best advantage with high lift camshafts, where their increased strength resists the breakages sometimes seen with the originals.  


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