Imp Site logo - imps4ever!
  Type or code no a - standard Imp
b - Imp Sport
  Material High grade alloyed cast iron
  Drive Single roller chain from crankshaft
  Position Overhead
  End float 0.002 in./ 0.007 in.
  Valve timing checking clearance   Single roller chain from crankshaft
a - 0.010
b - 0.014
a - 0.010
b - 0.014
  Cam timing
a - 6°   46°   46°   6°  
b - 23°   53°   61°   15°  

The Imp Site


If your camshaft is not the right one for your engine, then you won't get the performance and horsepower you want. Not even if you have a trick flowbench ported cylinder head and a rock-strong bottom end.
The design of a camshaft is based on

The camshaft is cast in a high grade cast iron alloy and is supported in three white metal bearings. It is positioned over the valves in a camshaft carrier, which also houses the cam followers (sometimes called 'tappet blocks').
Valve clearances are catered for by means of steel shims positioned between the cam follower and the end of the valve. The thickness of these shims varies in increments of one thousandth of an inch from 0.075 in. to 0.111 in.

Cam is driven by a single-row chain, which runs between the camshaft sprocket and an equivalent sprocket on the nose of the crankshaft. This drive is situated at the front of the engine (this is the end of the engine which faces rearwards when the engine is installed in an Imp). The cam drive chain is tensioned by means of a spring-loaded, flat strip tensioner which is faced with a synthetic rubber compound.
Single carb engines use a camshaft which has both less lift and less timing than the twin carburetter Sport engines. The camshaft used in the single carb engine has a timing of 6-46-46-6 with a lift of 250 thou, whereas the Sport cam has a timing of 23-53-61-15 with a lift of 310 thou. The timing figures for the single carb engines are measured with 10 thou tappet clearance, but timing figures for the Sport are measured with 14 thou clearance. These clearances are measuring clearances only and normal running clearances are different in each case.

Camshaft profiles

Type Lift Theoretical
valve timing
valve lift
at TDC
Inlet valve
opening at TDC
(Rootes Data;
± 0.002)
Inlet valve
(Rootes Data)
Valve / tappet
clearance ( 0.001)
Uses Power band
in ex in ex inlet exhaust
STD mk2 (std valves) 0.247" 646466
--     0.005 0.011 Road  
Sunbeam B1 0.250" 2761559
--         Road  
Imp Sport 0.312" or 0.310"? 23536115 *)
0.040"     0.007 0.014 Road 2000 - 6500
Rootes 310 0.310" 12565612
?     n.a. n.a. Road  
R15 0.360" 29595929
0.105" 0.108" 0.007 0.007 0.013 Road & Rally  
R17 0.360" 29595929
0.107" 0.100" 0.007 0.008 0.013 Fast Road & Rally 3000 - 7200
R20 0.360" 30707030
0.115" 0.113" 0.007 0.008 0.013 Rally/ Rallycross 3200 - 7800
R21 0.360" 39696939
n.a. 0.130" 0.007 0.008 0.013 Comp. race 3500 - 8200
R22 0.360" 40706535
0.162" 0.155" 0.007 0.008 0.013 Stage, Rally & Race 4000 - 8200
R23 0.360" 48767648
0.178" 0.175" 0.007 0.008 0.013 Full race 5500 - 9200+
R23 (other source) 0.360" 50909050
    0.007 0.008 0.013    
Hartwell R.P.3 0.360" 50 80 80 50                 Race  
GE1 0.360" 35755535
0.150 - 0.155     0.007 0.013 Comp. race 3800 - 8200
GE2 0.312" 28586620
0.110"     n.a. n.a.    
GE3 0.385" 40808040
n.a.      n.a. n.a. Race 5200 - 9000+
GE4 0.290"     
        n.a. Trials; Road 2000 - 6000
Piper kC 1S2Y 8mm ?   
?     ? ?    
Piper 1.R.G. 0.292" 24646424
0.059     0.006 0.006 Road  
Piper 1.R.Y. 0.331" 30606822
0.068     0.008 0.008 Road / Fast Road 2200 - 7000
Piper 1.R.6 0.354" 45858545
0.110      0.010 0.010 Race  
Piper 1.R.9 0.382" exh: 0.356 58828446
0.169      0.011 0.010 Race   
KC122Y (regrind)      
Piper 1RY     Road/ Fast Road? 
Jeff Howe H.60 0.320" 25 65 65 25      8-1010-12Fast Road, Rally; excellent all uses 
Jeff Howe H.700.350" 30 70 70 30       10-1212-15MkII: comp. or fast rally;
MkI: sports or comp.
Jeff Howe H.710.390"40  80 80 40         Race  
Fraser B 0.360" 32727232
n.a.     n.a. n.a. Comp. race  
Fraser D 0.360" 48767648
0.178"     0.007 or 8 0.013 Full race  
BP270 0.300" 28666628
7.62mm     n.a. n.a. Mild road 2000 - 6500
BP285 0.340" 33637125
8.64mm     n.a. n.a. Fast road / Comp. 2500 - 7000
BP300 0.350" 41737341
8.89mm     n.a. n.a. Fast road / Comp. 3200 - 7800
BP320 0.350" 57838753
8.89mm     n.a. n.a. Race 4500 - 8200
BP320 (other source) 0.350" 57838357
8.89mm     n.a. n.a. Race 4500 - 8200
BP320 (source) 0.355" 47737347
0.152"     0.008 0.010 Race 4500 - 8200
BP330 0.360" 57838559
9.14mm     n.a. n.a. Race 5200 - 9000+
Type Lift in ex in ex Cam
timing/ Full lift at
valve lift
at TDC
Inlet valve
opening at TDC
(Rootes Data;
± 0.002)
Inlet valve
(Rootes data)
inlet exhaust Uses Power band
valve timing
Duration Valve / tappet
clearance ( 0.001)

*) Split timing 4° advanced

   timing of camshaft


Cam timing
The position of the camshaft in relation to the crankshaft.
This is expressed as the number of degrees that full lift occurs after top dead centre (TDC) In the case of the inlet and before TDC for the exhaust. To calculate Cam Timing, take the duration figure and divide by 2.
Example R20: With an inlet cam of 30 / 70, the duration is the addition of these two numbers, plus 180, equals 280. Then divide by 2 resulting in 140. Deduct the number of degrees before TDC that the valve has started to open, ie 30 degrees - the result is 110. The valve is correctly timed with full lift 110 degrees after TDC.
Overlap between the inlet opening and exhaust closing. The number of degrees at the crankshaft, that the inlet and exhaust valves are open at the same time.
To calculate the overlap, add the opening number of the inlet cam to the closing number of the exhaust cam, ie the first and last numbers of the cam timing.
Using an R20 as example of 30 / 70 inlet and 70 / 30 exhaust (for an R20 referred to as 30 / 70 - 70 / 30), add together the first and last numbers (30 and 30) and the total is the overlap. In general terms the larger the number or the greater the overlap the hotter the cam.
Advancing the exhaust cam and retarding the inlet cam moves the peak torque down the rpm range.
With a single camshaft, the only way to do this is by having a specially ground camshaft.
This can be cam lift or valve lift. Valve lift being the cam lift multiplied by the rocker ratio.
Lift (within limits) has a lesser effect, but short-duration cams usually imply a lower lift, because of design considerations.
The only reason for high lift is that you can use the lift to give a longer duration of actual valve open time.
The duration for which the valves are open. The number of the camshaft degrees that the valve is off its seat. In the table below, you see this figure as well as the timing figures. To calculate the duration, add the inlet timing numbers together and add 180. Example R20 cam: with timing of 30 - 70 added, totals 100 plus 180, gives 280 degree duration
Short duration cams have the peak torque and the power lower down the rpm range. But this effect (with mild road cams) is not as pronounced as overlap.
Valve timing
The opening and closing position of the inlet and exhaust valves relative to the crankshaft. As figures before and after TDC and BDC.
Phase angle
Phase angle gives the ability to expell all or most of the unburnt gas and at the same time let you try to put a quart in a pint pot.
A cam phaser is used to alter the time at which the valves open and close relative to engine crankshaft rotation.
    Rootes parts - camshafts
From a 1966 Rootes advert to use Rootes parts

Sport camshaft

This number is cast into it along the shaft between exhaust lobe for no.3 and inlet for no.4

An Imp Sport camshaft is the best cam to use on a road-going 875cc motor, Sport or standard. (It will even do for a docile 998.) It gives more power from the mid-range upwards. You may need to work the gears more, but that's where the power is. (An R17 is too top-endy, and hopeless around town in an 875.)

As the Sport has a lift of .31", you don't need the Sport's double valves, if you use it in a standard head. Strong single valve springs will do. Nor do you need the Sport head's oil drain facility, if using single springs. The only modification needed is to machine the cam carrier to clear the Sport cam's lobes. (Or you may be able to find a Sport carrier.) A standard cam timing sprocket is perfectly adequate for this cam.
A Sport cam should not be used with standard 'top-hat' oil seals, because clearance may not be sufficient at full valve lift. Unfortunately the alternatives are expensive. Without inlet valve seals, an oil drain is required. The early pre L4 series have no oil drain tapping and Sport heads are rare.
In the experience of R. Freeman the Sport cam is perfectly ok with Strong single springs and standard inlet valve top-hat seals.
Further points:
If your car is an early model with a tapered boss on its valve collars, then fit the later straight sided type, as they are stronger. Always fit new camshaft followers when fitting a new camshaft.

The L4 head

The L4 series (basic) Imps were fitted with a Sport's cam as standard, tappet block, double valve springs, oil drain and Sport's pistons. They have a standard exhaust system, and a Solex carb. This applies to all models with chassis number prefix L4, L5 or L6 (approx. from 1974 on).
Also the L4 have a 45D4 distributor, which is similar to the earlier 24D4, but the vacuum advance continues right up to 5000rpm in stead of 3500. Any Solex powered, Sport cammed Imp would benefit from the 45D4 conversion. While the L4 head does not have Sport valves, it does have the oil drain tapping. So the L4 cylinder head is suitable for modification and it isn't as rare as a Sport head is.

On fitting a Sport's camshaft to a standard head

Reprofiled Sport camshaft

You might try to have your Imp Sport camshaft reprofiled by one of the specializing companies like 'Kent Cams'. This is said to be cheaper than an R17. Some have found it to be not as lumpy on tickover, as it relies more on overlap than on drift. It is supposed to work well, giving a lot of power at the 'top-end'.

Using a high lift cam on a standard head casting
Controling the oil consumption: The 'top-hat' oil seals should be removed and replaced by exhaust type valve spring collars. Also fit B.L.M.C. Mini valve stem oil seals (O ring type, Leyland part 2A879).

Hairy camshafts


The most extreme camshaft you can get for an Imp is a Hartwell 410. The Works R23 is a couple of steps down from the 410.
The works specification cylinder head used larger inlet (1.4") and exhaust valves combined with a 0.360" lift camshaft using a converted sport cam carrier. This type of engine could produce about 115 BHP.
The standard full race cam, the R23, was a little too wild for the the tighter tracks and was sometimes replaced by an R21 or R22.
Ray Payne of Hartwell also produced the RP3 cam which was somewhere between an R22 and an R23 cam.

Greetham Engineering

Andy Chesman of Greetham Engineering at Edgwick Road, Coventry produced the GE2 cam. It had a 0.3125" lift and timing something like 28-58-66-20 (exact figures of course depend on what you line the cam in at). Torquey: Apparantly the Greetham GE1 camshaft pulls much more clearly lower down than the R17, although its profile is similar to an R22.
Late in the 1970s Chesman developed heads which used smaller inlet valves (1.375") and a higher lift camshaft (0.420") which gave a better spread of torque and more power (123 bhp). These camshafts required very special cam carriers with bronze liners.


Piper camshafts on the Need for Speed web site.
Repro on BP285 only available on Imp Sport cam. 2008 cam folder (page 29 of 51) - TALBOT (HILLMAN) IMP

David Vizard finds DynoTest No. 2 on the Imp a lot harder going / David Vizard. - Hot Car 1975, May. - p.36-37
the Piper 1RG is suitable for the ordinary Imp
Our cam swop, then, is really a comparison between the Piper 1RG and the Imp Sport. Those of you whose cars were built before the chassis number Imps with a standard Imp camshaft will get 3-4 bhp more with the Piper cam. Fitting the Piper cam to earlier Imps is no problem, as it doesn't affect the oil seals, but fitting the Imp Sport does, so the swop to the Sport cam is much more involved.
I did quite a lengthy piece on fitting Imp cams only a few issues ago, but it did deal more specifically with fitting the cam while the engine was out. the runs was that the mixture end which seats up against the Solex analyser showed that the mixture casting.

RJE on the club's forum: Piper 1R4
onomatopoeia on the club's forum: a Piper 285 in my Chamois and that's perfectly tractable for road use in a 998. a useful increase in go in the top half of the rev range, especially when used with twin Webers and either would make a good upgrade to the sport cam
Adrian on the club's forum: Piper IR6 cam fitted and comes in with usable power at 4500 to 9200Rpm when its dropping off Bhp. Peak was 96 Bhp. Used it in a wet race at Silverstone finished 7th overall so can't be bad, but who knows in the dry could be different story.

Piper say they have no details on any of their old cams. However, if you ask them, they will measure your classic Piper cam for you and tell you what it does and how to time it.

R.J. (Bob) Gayler was the co-founder and Technical Director of Piper Cams from 1966 to 1990
Piper Tuning Manual -> The BG Tuning Manual (BG = Bob Gayler) index to the manual
He says Piper Engine Developments did the the development work on the George Bevan Imps.

Piper Cams

Page 33 from their catalogue: Talbot (Hillman) Imp

Valve lifts quoted assume a rocker ratio of 1: 1

Part No.ApplicationPower BandPower IncreaseDurationValve LiftTimingFull LiftLift @ TDC
with clearance
excl. VAT
    InletExhaustInletExhaust  Inl. ATDCExh. BTDCInletExhaust  
IMPBP270FAST ROAD2000-7000 276°276°0.294"
29 - 67
67 - 29
.008" / .20mm
.010" / .25mm
£120 Repro
£400 Billet
Repro only available on Imp Sport cam.
3000-7500 284°284°0.323"
33 - 71
71 - 33
.008" / .20mm
.010" / .25mm
£120 Repro
£400 Billet
IMPBP300RALLY4000-8000 286°286°0.352"
37 - 69
69 - 37
.014" / .30mm
.016" / .35mm
£120 Repro
£400 Billet
IMPBP320RACE4500-8500 300°300°0.355"
47 - 73
73 - 47
.008" / .20mm
.010" / .25mm
£120 Repro
£400 Billet
Many other profiles available °°         

Kent cams

"Kent cams are computer designed with longer duration and more lift on the exhaust for better scavenging and bottom end power." - according to a company that sells them.

Ben Boult has designed camshafts:
"...the best I achieved was a 1040cc engine running one of my .410 cams through a Hewland MK9 gearbox, that gave 125BHP at the wheels at 7500rpm.
Incidently if anyone needs Cams I keep most profiles in stock."

following advice is taken from Dawson's Dodges, part 1

According to Andy Dawson

Camshafts, their uses and timing are in the adjacent table, but my own preference for almost every use is the R20 as it

On fitting a high lift camshaft (too easy to break):

  1. Stone off the edges of the cam lobes to stop the followers getting scratched.
  2. Use a carrier to give it a bit of extra support (contrary to the workshop manual). Fit the cam into the carrier, which will need relieving with a small rotary cutter to stop it fouling the edges of the follower guides, and
  3. then fit the the carrier assembly to the head.

Ian Carter, who built a lot of race engines, redrilled the cam carriers to feed more oil to the cam followers, but I have found that either standard followers replaced regularly, or competition followers (CTS 1043) with the edges radiussed off, work perfectly well. With a very hot cam the standard followers can either crack or hollow out, but this always happens during initial running, so a thorough check after about 5 hours running will show up any problems.

When using a camshaft with a non-standard base circle radius, such as the many regrinds that are available, it is necessary to use

Under no circumstances should the cam carrier itself be thinned down, as the followers will tip and lock at full cam lift, causing horrible results.

The only other tweak concerning valve gear is the use of a stop on the timing chain tensioner to prevent the chain from jumping a tooth if the engine is turned over backwards. All I do is to braze a little strip of metal across the tensioner slide, usually about ¼ inch down the slide, although on an engine which has had a lot off the head it might need to be even further down.

There is no magic in the timing of an Imp cam, you just need the right equipment.
With a Sport cam it is possible to set the timing with the standard sprocket and then transfer to a multi holed sprocket if necessary.
With a higher lift cam, you just

  1. set the cam at the required No. 1 inlet valve lift
    (this is in fact the rock point on the cam, when the inlet and exhaust valves are open by the same amount)
  2. bring the engine up to top dead centre, No. 4 firing, and
  3. find the hole on the sprocket which gives the best chain fit.
  4. Then check the valve opening when the engine is again turned onto TDC.
    As long as the valve opening is within 5 thou then don't worry, unless of course you are looking for the last half a bhp.
Make sure that you don't fit the sprocket the wrong way round, as there is no chamfer on the front and the sprocket doesn't sit properly on the end of the cam.
Also make sure that the sprocket bolt is torqued to 20 lb.ft., as the dowel shouldn't have to do any work when turning the cam - it should be driven by the friction between the sprocket and the cam.

Timing competition camshafts
issued by: Competition Centre, Coventry

with cylinder head fitted and camshaft removed

  1. Remove sparking plugs.
  2. Turn engine to top dead centre, number 4 cylinder firing (rotor arm pointing at number 4 plug lead segment)
  3. Turn engine anti-clock (against direction of rotation) until pistons aproximately 1" down bore, to stop valves striking pistons when setting the camshaft timing.
  4. Fit camshaft assembled in carrier. Position a dial gauge with its stylus on number 1 inlet valve tappet.
  5. Revolve camshaft in the direction of rotation until number 1 inlet valve is the correct amount open, with competition tappet clearances as listed.
  6. Turn engine in its direction of rotation carefully back to TDC.
  7. Fit multi-holed timing sprocket in chain and offer to camshaft; if hole does not coincide with dowel, turn sprocket in chain until hole fits on dowel when offered. Tighten retaining set screw to 19 lbs ft torque.
  8. Now re-check timing by turning engine, and, if correct, lock locking tab.

Refit sparking plugs as soon as possible to avoid foreign bodies entering cylinders.

On refitting the cam followers plus the carrier:

Place the followers on the valves and try to juggle the carrier over each one.
Turn the carrier upside-down and fit the followers. Use thick oil to (try and) retain them, while the carrier is rapidly inverted and will hopefully slide over the studs before the followers fall out of position.
To stop the followers from falling out of the carrier, lay a 12" steel ruler (or some such) along the followers; invert the carrier and gently lower it into position.


Vizard, D.
CCC Look-in on the Imp / by David Vizard. - Car and Car Conversions 1972, March. - p. 66-67,69
Camshafts: What follows when you change the camshaft

Vizard, D.
Imp cam craft / David Vizard. - Hot Car
Imp camshafts only give their best if fitted the correct way. David Vizard gives you the low-down on doing it right.

    Andy Dawson

Dawson, A.
Dawson's Dodges / by Andy Dawson. - Cars & Car Conversions 1976, May. - p.67-69,71
How the experts prepare competition Imps, by the man who probably knows more about it than anyone else. This month Andy Dawson looks at the basics of screwing together 875cc and 998cc engines.


The Imp Site
   Imp anatomy
      Technical bits & pieces
© Franka
File edition: 30 Nov. 2013