What is an AC Engine?
Blocks, Studs & Water Jackets
Spare Parts List One - Attaching Parts, Seals & Gaskets
Spare Parts List Two -
Crankshaft & Cross-shaft Assemblies
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
Parts List Five - Cylinder Head Assembly......
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.
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
of AC cylinder head have less difference between them than the cylinder
blocks have and are, to a large degree, interchangeable. The distinguishing
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.
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.
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.
UMB/UMC: As above. Wide rollers.
above but lacking the narrow hot-spot water passage between and
beneath the centre pair of Carburettor studs. Equal length head
Cylinder head components
and Exhaust Valves
Valves are the most
often replaced items in the cylinder head and we keep good stocks on the
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
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.
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!'
seeking even higher performance we can also supply a sports camshaft.
Early ordering for these items is recommended.
for racing may be supplied to bespoke profiles according to customer
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
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 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 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
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.
spring caps, valve stem caps, adjusters and locknuts and valve collets all
wear but we have good stocks of new replacements.
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).
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.
shafts have been proven in usage on the race track.
note that we no longer supply rocker shafts to the original, shorter AC
design owing to lack of demand.
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.
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.
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.
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.
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
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.
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.
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.
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 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
with the originals.