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Rocker gear - General compendium |
28 August 2002 |
Whether you’re building a solid road performer or
fire-breathing monster, the main goal is to improve air/fuel flow into the
engine. The more you can get in, the more power you can get out. Cheapest chunk
of power improvement comes from sorting the asthmatic manifolding
and exhaust by applying a stage one kit. Then what?
Considering the ‘get more in to get more out’ theme, the
main restriction - all else being equal - is the valves and their behaviour.
How big they are, how efficient they are and how long they’re open for
determines how much gets in with each gulp each bore/piston makes. Modified
cylinder heads are popularly next, although it has to be said that although a
well modified cylinder head will improve performance, the gain against cost is
nowhere near that given by the ‘stage one’ kit application. A good stage one
kit’s improvement verses cost is around £12.50 per 1 hp, a decent modified head
typically £30 per 1hp - dependent on application. Hmm.
A change in camshaft will aid volume of consumption by
holding the valves open longer, and/or pushing them further open. Performance
gain here against cost is better than with most cylinder head or stage one kit
changes - typically around £7.50 per 1hp. BUT a right pain to do properly. I
stress ‘properly’ here, as doing a half-arsed job will eradicate the
improvements promised by its fitment. Problem is the engine has got to come
out. Not something many of you are busting to do. Particularly on the later
model Minis with their ‘spaghetti junction’ pipe work and wiring.
The only way of improving valve behaviour then is the
rocker gear. Recognised at an early stage by A-series engine developers as
having potential, a number of companies have developed - others copied - them
over time so several varieties now exist. Their power increase compared to cost
is between around £18 and £35 per 1hp - depending on how you decided to skin
this cat, but they are a cinch to fit! The eternal question ‘which ones are
best?’ is constantly put to me. The answer sometimes makes folk smile in
smugness, other times it’s ‘pass me a large brandy’! To assist in this dilemma,
I’ve studied those generally currently readily available - considering
everything from appearance to performance. First off - what are rockers all
about then?
Rock On
Appliance of science to the rocker gear can change
this significantly by altering the leverage ratio - part of the ‘tech-speak’
banded about when rocker gear is under discussion (1.3 rockers, 1.5 rockers,
etc.). The measurement is the leverage ratio. So 1.5 rockers have a 1.5 to 1
leverage ratio. When applied to cam lift, it’ll multiply whatever this is by
1.5. Theoretically - but I’ll leave it at that for now.
There have been several types of rocker gear fitted as
standard over the years. Pressed steel versions were the first used. The ‘S’
derivatives had a forged steel one. The early 1275GTs used the pressed steel
item from the
The rocker ratio is more influential than most
believe. The main benefit of higher ratio rocker gear is usually conceived as
giving more maximum valve lift. Although this is true, its biggest asset is
having the valve open more at any given
point of the cam lobe. This also means the valve is accelerated faster to
be there, so has a perceptible influence on the ‘as fast’ criteria. For
comparison, a 1.5 ratio set up will typically have the valve open around 20% more than the standard quoted 1.25. Consequently the engine draws
in more air/fuel, and more power hopefully the result. On the face of it, the
higher ratio you cram on, the better. Or is it? Having the valve open too far,
too fast, can be detrimental to power - there may not be enough gas flow to
keep up. So there are limits.
Instrument breakdown
There is also more to the rocker gear than just
opening and closing the valves. The right materials need to be used in the
right places. Controlled tolerances need
to be observed. Functional geometry needs to be maintained. So let’s put each
component under the microscope.
The shaft.
Increasing the ratio not only multiplies valve lift, it multiplies valve spring
rate too. The increased breathing capability afforded means higher rpm can be
used. Increased wear a slight problem, breakage a bigger one. The standard
shaft is adequate when standard cams are used. When modified cams are used, an
up-rated thick wall shaft is strongly advised - a necessity on all race
orientated engines. A worn shaft will contribute to excessive valve train
noise, make setting accurate valve/lash clearances difficult and cause
premature wear to the bushes. Fit a new one when engine re-builds are done,
they’re cheap. Contrary to popular belief though, Tuftriding the shaft is a
waste of time. It’ll wear the rocker bushes out faster than the shaft for no
gain in strength. It’s far cheaper
and easier to replace a shaft than the bushes.
Posts. Early
standard posts were aluminium. Four of the nuts that hold the head down tight
are situated on each post. Being aluminium, they were either collapsing on
torque down or spreading under load after a period of time. As a result head
gaskets were giving up the ghost. Excessive squash also distorted the shaft so
that it broke after relatively low mileage. Consequently they were substituted
very quickly with steel items, an infinitely superior material for the
application and have been so ever since. A fair illustration why using
aluminium for the posts is bad news. The posts were standardised for the whole
range of standard rockers. Any deviation away from the standard rocker ratio
needs posts specific for the rocker type. That includes when using offset
bushes. Don’t be told otherwise.
Bushes.
Fitted into all the standard A-series rockers, but not the latest sintered
ones. Proper finish reamed bore size is 0.563-0.5635”. 0.0005” over this is too
big. As with the shaft, excessive wear (or over-sized reamed finish) will
contribute to valve train noise and setting accurate valve clearances will be
impossible. If they’re worn, replace them and get them finish reamed to the
correct size. If the bushes are reamed bigger than this, there will be too much
clearance on the shaft and it’s back to square one. Service the shaft regularly
as intimated before - far cheaper and easier than doing the bushes. Once fitted
they need reaming out to correct size with specialist tools. Not your average
DIY job. Offset bushes are available to
increase lift, but you must use the
right posts. Bearing in mind the grief fitting bushes is (and these are far
harder) and the cost of the whole job - buy some decent higher ratio rockers.
Spacers and
spacer springs. The standard rocker gear has used springs since the
dawn of time. Aside from keeping the rockers apart, they do a good job of
tensioning them too. This helps damp out noise caused by worn bushes/shafts and
odd valve train harmonics. Makes lining the rockers/valve stems up supremely
easy, and ‘self centres’ where head stud location inconsistency due to
manufacturing tolerances is pretty-much standard. All sorts of hogwash is
talked about what changing them for spacers can do. The biggest claim is the
reduction of friction. This only
occurs when the spacers are made the exact
dimensions for the head that they’re being fitted to, and in my experience this
is super rare. They are usually too tight causing them to bind, or way too
loose allowing the rockers to float about. Both are definitely not good, and
possibly disastrous. If you must use spacers - and they are fitted to a number
of complete ‘ready to fit off the shelf’ rocker gear kits - once the rocker
gear is fitted and head nuts torqued down, make sure the spacers revolve easily
using thumb and forefinger. If not, take it all apart and do something about
it. Can be a pain to sort, but essential for longevity. The friction produced
by the spring is almost immeasurable, any possible power loss through drag
certainly not measurable on an engine dyno.
Rockers. We
already know what the standard ones are about, but if looking to replace the
standard set up, here are a few things to consider. Forged steel rockers are
very strong, and are cheaper than most other types on the replacement parts
market. Alternatively Mini Spares has produced a specific casting method that
gives good integral strength to cast rockers. Overall weight isn’t really
important, as most designs are well shaped. A reasonable pad area means less
hassle lining them up with the valve stem tip. A well- developed pad contour is
essential to reduce side loading on valves and guides to a minimum. On lifts up
to around 0.420” this is negligible.
Aluminium rockers abound in varying designs. They’re
plenty strong enough for the application, particularly as there is a great deal
more material in cross section than conventional steel types. They all have roller
tips, virtually eradicating side loading on valves/guides, but make sure they
do. Roll that is. Pointless having them otherwise. Some roller tip pins are ‘C’
clipped in place. Others are splined/pressed in to
maintain position (ally expands more/faster than steel). Some have needle
rollers instead of bushes running on the shaft. Complete over-kill. Needle
roller bearings aren’t really suited to reciprocating action. Rocker
instability (sloppiness) another feature. A plain bearing is probably better as
it gives far better support against the shaft. Correctly sized, stability is
maximised and keeps costs down too. Anodised in various colours makes them
pretty to look at. But remember - beauty can be skin deep!
And most importantly, those chosen should give and
maintain the desired ratio throughout the lift cycle. Don’t snigger. Achieving
this would get Pythagoras going!
PICTURE CAPTIONS.
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Click image to enlarge |
Pic 1.
Hours and hours of measuring go in to sorting rocker
gear properly and equally as many - if not more went into carefully
establishing the actual rocker ratios of the sets on test.
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Click image to enlarge |
Pic 2.
Identifying pressed steel rockers. Top shows round pad
of the small-bore one - 1.23:1 actual ratio. Bottom shows oval pad of big bore
version - 1.27:1 actual ratio.
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Click image to enlarge |
Pic 3.
From the top -1275 pressed steel (1.27:1), forged
original ‘S’ (1.22:1), and latest sintered type (1.21:1). Actual ratio drops as
cam lift rises, so ratios are even lower when used with performance cams. So
whatever you do, change them sintered rockers!
Assessment of available after-market rocker gear.
Actual working ratios (AR) measured using typical fast
road engine set up. MD276 cam, head skimmed -0.050”, block skimmed to clean
only, and average length standard push rod. No valve clearance used. As a rule
ratios tend to go down as cam lift goes up, and vice versa.
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Click image to enlarge |
Pic 4.
Avonbar Racing 1.5:1
Rockers. Kit contains rockers, posts, shaft, and a few shims. Standard shaft. Bushes finished standard
size. Steel posts. Good quality forging. Reasonable pad width and contour. AR
reasonably consistent. AR 1.47:1.
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Click image to enlarge |
Pic 5.
Mini Spares Centre Rockers. ‘S’ 1.25:1 rocker (left).
Supplied in single rockers only. Specially cast to original S dimensions,
except ratio has been sorted to give better lift. Bushes finished standard
size. AR slightly erratic. AR 1.26:1. Hi-lift 1.5:1 forged rockers (right). Kit
contains rockers, posts, shaft, adjuster screws/nuts. Thick wall shaft. Bushes
finished standard size. Steel posts. Good quality forging. Ample pad width and
well formed contour. AR consistent. AR 1.58:1.
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Click image to enlarge |
Pic 6.
Mini Spares Centre 1.5:1 roller tip rockers. Kit
contains rockers, posts, shaft, adjuster screws/nuts. Thick wall shaft. Bushes
finished standard size. Profiled steel posts (gold coloured). Extruded alloy
rocker (anodised purple), profiled, with roller tip. Pin and roller finish
ground for smooth action. Pin splined/pressed in.
Roller/valve alignment good. AR extremely consistent. AR 1.51:1. Also available
in 1.3:1 ratio.
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Click image to enlarge |
Pic 7.
Mini Sport roller tip rockers. Sets come built
complete with rockers, posts, shaft, adjuster nuts/screws, spacers.
Aesthetically very attractive. Standard shaft. Bushes finished some 0.0013”
oversize. Profiled alloy posts (anodised - 1.3 gold, 1.5 black, 1.7 silver/self
colour). Profiled rockers from alloy billets (anodised - 1.3 light blue, 1.5
red, 1.7 gold) with roller tips. Pins and rollers heat-treated steel giving notchy action. Pin held in with circlip. Roller/valve
alignment good. Anodised alloy spacers are short, allowing rockers to wander
excessively. AR fairly consistent. AR 1.27:1 (1.3), 1.53:1 (1.5), 1.67:1 (1.7).
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Click image to enlarge |
Pic 8.
Titan Motorsport 1.5:1 full roller rockers. Sets come
built with rockers, posts, shaft, adjuster screws/nuts, spacers. Special shaft has
bigger diameter and super hard finish to withstand needle rollers. Needle
rollers in rockers, feature seemingly excessive ‘‘play’ on shaft. Steel
profiled posts coloured black. Extruded alloy rockers (typically anodised gold)
with roller tip. Pin and roller finished ground for smooth action. Pin splined/pressed in. Roller/valve alignment good. Steel
spacers fractionally too long, causing binding on rockers when fitted to engine
so need careful attention when fitting. AR reasonably consistent. AR 1.53:1.
Also available in 1.3 and 1.7 ratios.
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Click image to enlarge |
Pic 9.
Which suits what?
1.3 type - Roller/non roller. All small-bore
irrespective of cam, including full race except where every last ounce of power
is sought at the expense of mid-range.
1.5 type - Non roller. All large-bore. Negligible
valve/guide wear when valve lifts barely exceed 0.400”. Use on full race
small-bore only as extra lift/acceleration can out-strip available gas flow
available causing power/torque loss.
1.5 type - Roller tip. All large-bore irrespective of
cam type. Full race small-bore only as above.
1.5 type - Full roller. Instigator of the trend.
Exotic and over the top, but recommended by many engine builders despite cost.
Over the top for road use and noisier than bushed types because of extra 'slop'
on shaft.
1.7 type - Roller tip/ full roller. Should be
considered for carefully developed full race package only as the lift developed
by this set up can be too much, hurting power rather than improving.
Think on this - it’s rarely worth lifting a valve
higher than the cylinder head's peak gas flow point!
Useful part numbers:
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AEG167 |
Adjuster
screw for 12G1221, C-AHT400/405/436/446. |
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C-AEA692 |
Set
of 8 extra-long adjuster screws (approx 8 threads longer) |
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NT605061 |
5/16"UNF
lock-nut for AEG167 and C-AEA692 |
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AEG168 |
Spacer
shim (not for Titan rocker gear kits) |
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6K556 |
Spacer
spring |
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2A18 |
End
spring washer |
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6K555 |
End
flat washer |
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C-AEG392 |
Set
rocker spacers (not for Titan Rocker gear kits) |
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2A21 |
Rocker
bush |
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C-AEG644 |
Offset
rocker bush set (must use C-AEG645 post set) |
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C-AEG645 |
Steel
rocker post set for C-AEG644 |
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12A1950 |
Thick-wall
rocker shaft |
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C-AEG399 |
Super
thick wall rocker shaft |
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CAM289 |
Standard
sintered rocker less adjuster screw/nut |
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12G1221 |
Mini
Spares re-made S rocker less adjuster screw/nut |
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C-AHT436 |
Cast
1.5:1 rocker gear set |
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C-HT435 |
Individual
cast 1.5 rocker less adjuster screw/nut |
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C-AHT446 |
Extruded
billet ally 1.5:1 roller-tip rocker set |
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C-AHT447 |
Individual
rockers for C-AHT436 set. Suffice with 'RIGHT' or 'LEFT' for tip offset. |
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C-AHT400 |
Extruded
billet ally 1.3:1 rocker set for large-bore engines |
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C-AHT405 |
Extruded
billet ally 1.3:1 rocker set for small-bore engines |