|Type or code no||a - standard Imp
b - Imp Sport
|Material||High grade alloyed cast iron|
|Drive||Single roller chain from crankshaft|
|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
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.
|Contents of this page:
Camshaft timing scheme
Imp Sport camshaft
Andy Dawson advice
Timing of competition camshafts
opening at TDC
|Valve / tappet
clearance ( 0.001)
|STD mk2 (std valves)||0.247"||6||46||46||6||--||0.005||0.011||Road|
|Imp Sport||0.312" or 0.310"?||23||53||61||15 *)||0.040"||0.007||0.014||Road||2000 - 6500|
|R15||0.360"||29||59||59||29||0.105"||0.108"||0.007||0.007||0.013||Road & Rally|
|R17||0.360"||29||59||59||29||0.107"||0.100"||0.007||0.008||0.013||Fast Road & Rally||3000 - 7200|
|R20||0.360"||30||70||70||30||0.115"||0.113"||0.007||0.008||0.013||Rally/ Rallycross||3200 - 7800|
|R21||0.360"||39||69||69||39||n.a.||0.130"||0.007||0.008||0.013||Comp. race||3500 - 8200|
|R22||0.360"||40||70||65||35||0.162"||0.155"||0.007||0.008||0.013||Stage, Rally & Race||4000 - 8200|
|R23||0.360"||48||76||76||48||0.178"||0.175"||0.007||0.008||0.013||Full race||5500 - 9200+|
|R23 (other source)||0.360"||50||90||90||50||0.007||0.008||0.013|
|GE1||0.360"||35||75||55||35||0.150 - 0.155||0.007||0.013||Comp. race||3800 - 8200|
|GE3||0.385"||40||80||80||40||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.Y.||0.331"||30||60||68||22||0.068||0.008||0.008||Road / Fast Road||2200 - 7000|
|Piper 1.R.4||0.360"||0.352"||0.008||0.011||excellent all round race||3500-8000|
|Piper 1.R.9||0.382" exh: 0.356||58||82||84||46||0.169||0.011||0.010||Race|
|KC122Y (regrind)||Piper 1RY||Road/ Fast Road?|
|Jeff Howe H.60||0.320"||25||65||65||25||8-10||10-12||Fast Road, Rally; excellent all uses|
|Jeff Howe H.70||0.350"||30||70||70||30||10-12||12-15||MkII: comp. or fast rally;|
MkI: sports or comp.
|Jeff Howe H.71||0.390"||40||80||80||40||Race|
|Fraser B||0.360"||32||72||72||32||n.a.||n.a.||n.a.||Comp. race|
|Fraser D||0.360"||48||76||76||48||0.178"||0.007 or 8||0.013||Full race|
|BP270||0.300"||28||66||66||28||7.62mm||n.a.||n.a.||Mild road||2000 - 6500|
|BP285||0.340"||33||63||71||25||8.64mm||n.a.||n.a.||Fast road / Comp.||2500 - 7000|
|BP300||0.350"||41||73||73||41||8.89mm||n.a.||n.a.||Fast road / Comp.||3200 - 7800|
|BP320||0.350"||57||83||87||53||8.89mm||n.a.||n.a.||Race||4500 - 8200|
|BP320 (other source)||0.350"||57||83||83||57||8.89mm||n.a.||n.a.||Race||4500 - 8200|
|BP320 (source)||0.355"||47||73||73||47||300||0.152"||0.008||0.010||Race||4500 - 8200|
|BP330||0.360"||57||83||85||59||324||9.14mm||n.a.||n.a.||Race||5200 - 9000+|
timing/ Full lift at
opening at TDC
|Duration||Valve / tappet
clearance ( 0.001)
*) Split timing 4° advanced
From a 1966 Rootes advert to use Rootes parts
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.
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 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
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).
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.
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.
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."
Piper camshafts on the Need for Speed web site.
Repro on BP285 only available on Imp Sport cam.
pipercams.co.uk 2008 cam folder (page 29 of 51) - TALBOT (HILLMAN) IMP
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.
Page 33 from their old catalogue: Talbot (Hillman) Imp
Valve lifts quoted assume a rocker ratio of 1: 1
|Part No.||Application||Power Band||Power Increase||Duration||Valve Lift||Timing||Full Lift||Lift @ TDC|
|Inlet||Exhaust||Inlet||Exhaust||Inl. ATDC||Exh. BTDC||Inlet||Exhaust|
|29 - 67|
67 - 29
|.008" / .20mm|
.010" / .25mm
Repro only available on Imp Sport cam.
|33 - 71|
71 - 33
|.008" / .20mm|
.010" / .25mm
|37 - 69|
69 - 37
|.014" / .30mm|
.016" / .35mm
|47 - 73|
73 - 47
|.008" / .20mm|
.010" / .25mm
|Many other profiles available||old prices|
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
From: Ian Perry (a NZ address)
Date: 2 July 2020
Subj.: feedback - Piper 1R4 cam data
Just looking at some of your cam data and see you are missing the Piper 1R4 cam which was a predecessor of the Rootes R20 and has a very similar cam lobe profile but a wider lobe separation angle. Like the R20, they are excellent all-round race cams, very kind to the valve gear. There are still a few of these floating around, but no one knows what they have.
Compared to your table I'm not sure all your cam data is all recorded at the same measured heights for duration etc. - some of the numbers look like "advertised figures"... not taken off a cam doctor or when installed.
Max Lift 360 thou
Value lift installed 352 thou
Inlet Duration measured at 50 thou lift 252 degrees or 273 degrees at 20 thou lift
Exhaust Duration measured at 50 thou 253 degrees or 274 degrees at 20 thou lift
Installed Overlap 19 degrees
Lobe separation angle 111.5 degrees
Suggested installed lift at TDC 100 thou (Inlet advanced at 107.5 centre and Exhaust 117.5 centre)
Inlet shim 8 thou
Exhaust shim 11 thou
Power Band 3500-8000
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.
"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.
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):
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
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.
with cylinder head fitted and camshaft removed
Refit sparking plugs as soon as possible to avoid foreign bodies entering cylinders.
On refitting the cam followers plus the carrier:
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
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.
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|
Technical bits & pieces
File edition: 4 July 2020