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MG Midget and Sprite Technical - Tappets

Another shout out for help

My 73 1275 engine was recently rebuilt - just run in after about 1000 miles and carried out a full service

The engine is stock 1275 with 1st rebore however has a Kent 266 cam fitted and I believe metro turbo rockers but not totally sure -


The Tappets are very noisy - can anyone tell me what they should be set to ? Obvs adjust them cold and to rule of 9 but what do I set them too to make them quieter and suitable for the 266 cam ?

Thanks
richard weaver

Hi Richard
What ratio are the rockers (standard 1.3:1 or high lift 1.5:1)? I'm not sure about a Kent 266 specifically but when I had standard rockers with a 270+ cam from Peter May he told me to set tappets at 12 thou as usual. Now I have high lift rockers I add 3 thou and set at 15 thou.
Chris Hasluck

I have no idea what the rockets are or how to tell the difference - I'll get a picture of them
- maybe someone can tell me
richard weaver

Hi Richard - whereabouts in the West Midlands are you? I'm in Wolverhampton. Who rebuilt the engine - they should know.
Chris Hasluck

I'll drop them a call Tomoz - Coventry classic engines - according to the rolling road they did a cracking job on the build , they build a lot of high end engines for a friends company - etypes etc - they also build the engines for jigsaw racing - the triumph spitfire people - I'm in Redditch
richard weaver

Careful measurement and some arithmetic should tell you the ratio of your rockers Richard. Using a vernier you can measure the rocker shaft diameter then halve it. then measure between either side of the shaft and (a) the valve stem centre and (b) the adjuster centre. A bit of arithmetic will reveal the ratio between the the two distances and Voila - there's your lift ratio.

Are you sure it's tappet noise though? As you say your engine has been rebuilt, but were the rockers rebushed? The 1275 has bushes in the rockers which wear in an oval because they're reciprocating, not rotating. So noise can come from the rockers themselves, not just the tappets.

This can be very hard to identify. On my engine, which is a 1500 so nothing like yours in fairness, I spent ages trying to calm the tappet noise down to no avail. Eventually it took my very sensitive electronic stethoscope to prove the noise was coming from my worn rockers with too much clearance against the shaft. When it's cold it sounds like a tumble drier full of spoons!

Not a fair comparison as your engine is the much better designed A series, but rocker bush wear is a possibility if it wasn't addressed during the rebuild.

Regards;

Rod
RS Hughes

Just had a Google and I think my rockers are a+ rockers which I believe are 1.5 ? That's what I found on Google anyway
richard weaver

Standard A+ sintered rockers should be 1.25:1

Kent quote lift figures for 1.3 or 1.5, but standard 1275 rockers - be they forged, pressed or sintered - are 1.25
Dave O'Neill 2

Hi Dave

Nit picking, but the later (sintered) ones are 1.25, the earlier ones are 1.28 (pressed and forged). We nearly fell foul of this with Drayton Manor Metro racing. The Piper STR930 cam gives exact lift as used in Mini Miglia racing at the time and drawn up by our amigo Mike Garton. The earlier rockers put you over the max valve lift allowed so we had to use the later sintered ones which are less reliable!

Peter

Peter Burgess Tuning

Peter

I have an engineering drawing for the 1275 pressed rocker. If you work out the ratio from the dimensions on the drawing, it is exactly 1.25:1 - theoretically.
Dave O'Neill 2

Just a side note....

If the rockers are roller tipped, you want to use the feelers to measure from the side , if the rockers are solid, the you measure directly from the head on

If you measure head on with roller rockers then the roller will roll up and give a false reading

Im not a fan of measuring with a cold engine other then to use as a starting point, I perfer a hot engine as thats the normal running temp.

Its a bit of a mess but ive fun the engine close to spec, then found the noisy lifter and gently adjusted it to quite... but do this with lots of reservation

Tervor jessie posted my favorite quote..."noisey valves are happy valves" so true

Prop
Prop and the Blackhole Midget

Clearance should be as tight as possible without damaging things, burning valves or compromising performance. Personally, I think all the other factors which can influence tappet clearances (wear, rocker shafts, cam shaft etc) are too numerous to allow a definitive setting - especially if modified with a cam upgrade and non-standard rockers.

In the event of not finding a recommended clearance for the Kent cam, I would start at 16 thou and work from there - exhaust vvs being the most critical - and observe the effects of taking them down to (say) 12, and adjusting to suit.

Do err on the slightly loose/noisy side rather than the tight side though......especially exhaust vvs.
Mark O

If you can't find the recommended clearance and have the time and inclination, do some measurements and find where the opening ramp ends. Running clearance should be a couple of thou shy of that for street, or just on or marginally larger for competition.

To measure the ramp, measure the lift off base circle every 5 deg (ie measuring at the pushrod). For most cams, between 6 and 15 thou off the base circle you'll find a bit where the 5 degree intervals give similar increases in lift (between 1.5 and 2.5 thou per 5 degrees of crank - it will vary a bit due to measurment resolution limits). This identifies a period of constant and modest lift rate known as the opening ramp.
When the opening ramp ends, the lift rate accelerates rapidly to higher values. Plotting the lift uyou measure against crank degrees on a graph makes for a more accurate identification of the end of the ramp.

Whe running clearance is set on the ramp and all valve gear is in good condition (rocker bushes & shaft good, cam followers not pitted and slightly convex) then the valve gear will be quiet in operation. Setting running clearance off the end of the ramp can make for crisper performance characteristics, but it will also shorten the life of your valve gear and make it noisy.

A common problem for clearance setting is wear in the end of the rockers where they contact the end of the valve stem. This results in an indentation the size of the end of the valve which most feeler guages don't fit into and bridge over them instead - giving an inaccurate clearance measurement.
Paul Walbran


"A common problem for clearance setting is wear in the end of the rockers where they contact the end of the valve stem. This results in an indentation the size of the end of the valve which most feeler guages don't fit into and bridge over them instead - giving an inaccurate clearance measurement".



Indeed so, and it is the problem I face - which is why I don't set too much store by what the recommended clearance should be.

That said, it seems to run OK at the engine builder's recommended 16 thou for me, so that it where it is set. I am a little fearful of damaging exhaust valves if tighting up further, and when I tried setting at 12 thou - which due the wear above may actually be more like 14 thou - it did not seem to quieten down that much.
Mark O

Dave

The figures were quoted by Piper and Mike Garton when we queried the cam lift when the engines were stripped by the RAC, we looked after three Metro engines at the time and they all used pressed rockers and the lifts were a gnats wotsit too much. The lift came to cam specs when using Metro rockers which were the sintered ones. We were let off with the CR too high and the lift cos the second place in the class had a 1380, carillo rods and carbon fibre pushrods!
Maybe they didn't read the drawings when they made them :)

Piper quote rocker ratio as 1.28:1 on their cam sheets for A series.

Peter
Peter Burgess Tuning

Peter,

If you wanted to use the pressed steel rockers, could you install the offset bushings that are made to increase the rocker ratio, but install them facing the other way? Or, buy small ID bushes and offset ream them yourself?

Charley
C R Huff

Paul Walbran,

I need some clarification on the method you described for determining appropriate valve lash. When looking for the end of the ramp, do I have to start by setting zero clearance on the base circle so that I will know where the ramp begins? And, then this beginning is the point where I should start plotting?

Then, are you saying that if (for example) the valve has been lifted 0.012 when the ramp ends, then 0.012 is the desired valve lash?

Then, I am not getting the meaning or distinction of “on” and “off” in these lines:
“When running clearance is set on the ramp…” and
“Setting running clearance off the end of the ramp…”

Thanks,
Charley
C R Huff

Why would you want to reduce the ratio Charley?

I cannot see why you could not use say a Bridgeport to mill out the bushes slightly offset.

What were you thinking of Charley?

Peter
Peter Burgess Tuning

Charley
Sorry, my post suffered a bit from being doen at midnight ...

I usually measure the ramp by putting the dial indicator straight onto the top of the pushrod, which measures cam form directly, whilst installing the camshaft and then apply the rocker ratio to obtain at the valve figures.

However, if measuring once the engine is already assembled it is much more convenient to do so at the valve, in which case yes, set the clearance to zero for the measurement process.

Using your .012 example, I would set the lash 1-2 thou less than that for street use to allow for a bit of wear between service intervals when the clearances are checked, so in your example I'd set to .010 - .011. This is how the factory A series settings were: the BMC ramp ends between .013 and .014 from base circle, and the book clearance is .012".

"On the ramp" means before the end of the ramp, so in std A series cams I mean clearacne less the 13-14 thou.
"Off the ramp" means clearances greater than that.

Interstingly, the B series factory setting of .015" on the same cam form occurs at the end of the ramp, leaving no room for between-service wear. As the clearance increases due to wear, so too does the wear rate due to the hammering action from the greater take-up speed. Running the B series at the A series specified clearance, adjusted to .0135" for the difference in rocker ratio, quietens the engines and prolongs valve gear life significantly.

Mark O, why don't you remove the rockers and get them faced? It's simple to do and makes it soooo much easier to set clearances.
Given that in 1275s it's a major to change worn followers I get really careful about getting the clearances right as followers are big casualties of hammering from excessive clearance.

Paul Walbran

Paul W,

Thanks for the clarification. If I had more sleep last night, I probably would have thought of running the dial indicator directly on the push tube. I will copy and paste your explanation for future reference.


Peter B,

I thought you didn't like the sintered rockers because they were more likely to fail, but you couldn't use the steel ones because they were over the allowed 1.25 ratio for the race class. That's why I thought the problem could be solved with offset bushings to reduce the ratio of the steel rockers from 1.28 down to 1.25.

Charley

C R Huff

A Kent 266 should have tappet clearance of 0.016" with standard rockers and 0.019" with 1.5:1 rockers.

If the '270+' is the Piper camshaft, they run with the same 0.012" clearance as a standard cam (0.014" with 1.5:1 rockers).

We used to run a Kent 276 with 1.5 rockers and the 0.019" inlet clearance as above, but +0.003" for the exhaust, and there was some slight valve train noise.

We now run a Piper 270 (from Peter May), still with 1.5 rockers, using 0.014" inlet and 0.016" exhaust, and the valve train is noticeably quieter. BUT, the engine is more 'cammy', come on the cam around 2,500 rpm - this was not really noticeable with the Kent 276.

I do wonder if this 'camminess' is partly down to cam timing? The Kent was timed at 4° advanced and the Piper is timed as standard?

Richard
Richard Wale

Hi Charley, I woke up this morning and realised what you were implying, a few beers must help the thought processes whilst sleeping :) I suppose it could be done but very time consuming and therefore expensive, mind you at £400 for entry fees....

Hi Richard depending which Piper 270, overlap looks about the same as the kent 276 and from experience of one version of the 270 the power delivery was about the same as the Kent 276, maybe, as you say, not advancing the Piper 4 degrees or so.


Peter
Peter Burgess Tuning

Paul

Mark O, why don't you remove the rockers and get them faced? It's simple to do and makes it soooo much easier to set clearances.
Given that in 1275s it's a major to change worn followers I get really careful about getting the clearances right as followers are big casualties of hammering from excessive clearance

Quite right I ought to! It is more a question of time, other things in life and rebuilding old morbikes that prevents me at present, but I like the thinking, and I should do it....
Mark O

Ha, I'm the last person who could cast aspersions about lack of time ... still waiting to make progress on the WA I started on in 1969! Just getting married, building a house, 4 kids, rebuilt midgets for each of the kids and a whole lot of fun driving have gobbled up the years.
Paul Walbran

These are my rockers - a+ ? Metro turbo ? Any ideas


richard weaver

Richard,

Those look great ... ive not seen anything like that before that im aware of

What material are they made of

Can you comment on the discoloration on the tips of the rockers

Thanks for sharing

Prop
Prop and the Blackhole Midget

Standard A+ sintered rockers.

Possible hardening discolouration Prop.

Peter
Peter Burgess Tuning

Peter is the discolouration anything to worry about ? These with a Kent 266 cam what you reckon for Tappets ?
Thanks
richard weaver

I think the discolouration is where the rocker arms were heat treated/hardened at the factory, no worries.
Tappet settings for Kent 266 are 16 thou " cold.

Peter
Peter Burgess Tuning

Thank you -did it at that but still noisy- I guess I'll have to live with it doh
Thank you
richard weaver

Redo it by seeing where the greatest gap is rather than rule of thumb. With the valve clearance opening keep turning engine and check for loosest gap and reset tappet. To make life easy, with the engine idling use an old feeler gauge of around 12 thou and slip it under each rocker arm and see which the noisy one/s is/are then adjust the noisy one/s.

Peter
Peter Burgess Tuning

This thread was discussed between 21/06/2015 and 24/06/2015

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