MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Old chestnut - hot or cold tappet figures

In know the question of whether there is any measurable difference in the gap having the engine hot or cold has been kicked around in past threads. Book figures for the standard cam are often quoted at 0.013hot/0.015 cold and as I absent-mindedly started setting clearances with the engine cold yesterday, I thought I'd take a few minutes to repeat the job this morning with the engine properly hot.

The Piper cam in the car is specced at 0.014"inlet/0.016"exhaust (hot), but the pushrods and rockers (where the expansion would occur) are standard 18V and are all new.

I found the gap needed opening up somewhere between 1-1.5 thou when hot. This was on a pleasantly warm Autumn afternoon so to answer the question I guess the 2 thou difference is about right for mid-winter.
Steve Postins

They'll still rattle.

Mike
Mike Howlett

Sorry, you'll have to speak up a bit.
Steve Postins

"They'll still rattle"...yup unless you set the sodding things to under 12thou...then the idle will be lumpy and the valves will burn out. It's my biggest peeve with my 78 MGB...and growing up in Britain as I did in the 70s I just don't remember all those B Series powered cars making constant tappet noise. Have I just forgotten?
P J KELLY

Mike and PJ.

Is that correct? I have set the valves on my B twice now and still get a little rattle. Having set the valves numerous times on my TD without rattle, I was starting to worry I was doing something wrong.
Bruce Cunha

I replaced all my valve gear/cam/followers/timing gear etc...the engine runs great and returns 27-28mpg US, and I have set and rechecked the clearances till I could do it in my sleep with no lights on....they still rattle!
P J KELLY

Steve, I just rechecked mine. They are about 2 thou smaller gap when cold. Almost 3 with aluminum roller rockers.
Jeff
Jeff Schlemmer

With an alloy head and alloy rockers some suggest 7 - 8 thou less clearance when adjusting the valves cold.
Barry
Barry Parkinson

Barry, Jeff,
Thanks for that info. Just to clarify, are you measuring a smaller gap/less clearance when cold or a bigger gap/more clearance?
Steve Postins

Smaller gap when cold. I adjusted mine hot the other day and rechecked cold.

Barry, I suspect my engine was cooling by the time I finished my valve adjustment, but 3 thou was about the difference I saw, outside temp at 75 degrees. I imagine at 30 degrees, 7 is quite likely.
Jeff
Jeff Schlemmer

my experience is 2-3 thou difference between hot and cold...15 thou hot closes up o 13 thou cold. And they still rattle hot or cold!
P J KELLY

The factory specs state 13 thou hot and 15 thou cold, i.e. it closes up when it gets hot. The 4-cylinder engine is always mechanically noisy, whether it is tappets or not is down to individual interpretation. A gave a ride in my V8 to someone who only had experience of 4-cylinders and he was amazed at its quietness.
Paul Hunt 2

PJ, I still hear some MG's (maybe I should say I don't hear them) with quiet tappets. I replaced my 74 MGB cam with a regrind and the tappets are noisier than with the old cam. I think the problem may be due to the base circle on a reground cam being smaller than a stock cam plus most regrinds have a little more lift than stock. Faster opening rates = more noise. David Vizard mentions the noise issue in his book, "How to Build & Modify Chevrolet Small-Block V-8 Camshafts & Valve-trains"

Clifton
Clifton Gordon

Now I'm confused. I found the gap, on every valve, had closed by 1-1.5 thou when hot, much as the factory spec. That is to say, having set the gaps cold, I could not get the correct feelers in when it was hot. Surely it's got to be that way around, with the most expansion occuring in the long pushrods?
Steve Postins

Clifton...you have a good point there...my new cam is made by Delta....got it from Brittek and it has a little more lift...reading up in the archives, this seems to be part of the issue. Now if only the factory cams hadn't been made out of putty...we wouldn't all be having to replace the damn things. My B was only 38000 miles old...I'm sure the PO had done too many short runs...but the cam was hardly a cam at all...more of an idler shaft that happened to hit a few pushrods every once in a while!!
P J KELLY

The only clearance that really matters is the hot one and on the basis that for most of the time the engine should run at operating temperature. However I would be reluctant to adjust the hot setting until the engine had run on the road for about 20 minutes to ensure thourough warm up. Even then I adjust the clearances to .015" with the engine running at a slow idle. It's not as difficult as it sounds. My GT is virtually inaudible at idel when set this way but my roadster is fairly noisy and I don't really know why. I do partly however suspect the cam which is not worn neither are any of the other components. The GT has the standard cast header which I thing acts as an effective dampening mass whilst the roadster has a fabricated steel LCB header and I suspect this is not helping. Can anyone add to this ??

In the 50's and 60' you could tell what car was approaching from behind by the tappet rattle. It was either a Austin Cambridge or Morris Oxford or the Riley & MG derivatives. Strange thing is that the C series engine never suffered in this way.
Iain MacKintosh

In an effort to improve insulation for my A/C, I used header wrap material to wrap the downpipes from my exhaust manifold to the muffler. ' Haven't tried to wrap the manifold. Of course now the weather here has suddenly turned cooler so I may not know about the thermal insulating properties until next summer, but I can tell you that this reduced tappet noise heard inside the cabin considerably.

These engines have noisy tappets. There may be a few quiet examples among us, but for most of us, the only way to quiet 'em down is to close the gaps. As for me, I'll take noisy tappets over burnt valves any day.

Some of you are talking about rechecking your clearances when hot. I find they're off if I recheck them immediately! Maybe I'm not careful enough, but I usually have to go through them all at least three times before my readings stabilize. I can't explain this, but it always happens. As a practical matter, I always set them to .015 and I do it cold. Am I maximizing performance? Probably not. Am I minimizing noise? Again, probably not. Am I burning any valves? Definitely not.

FWIW,
Allen
Allen Bachelder

The point that I was making was that the cold clearance is relatively unimportant and only serves as a starting point after rebuild etc. An engine never runs in a continuously cold state so it is important to get the hot clearances correct. .015" is the setting and to eliminate wear and steps on rocker pads etc setting at idle is the best route. When a tappet is ticking you also immediately hear it stop when you insert the feeler blade so it is easy to pinpoint the noisy ones. This method will reduce noise to a minimum whilst ensuring clearances are not to tight so as to reduce performance and reduce valve life.
Iain MacKintosh

Iain,
Are you able to adjust the tappets with the engine idling too? I know some people manage it but I always imagine it to be like trying to assemble a Swiss watch whilst holding a jackhammer. Is there a technique?
Jeff, Barry, PJ,
I'm still puzzled that you are measuring the gap closing up when the engine is hot. As I picture it, any expansion of the metal has to reduce gaps between the metal parts? Any thoughts?
Steve Postins

Steve, it's not too bad a job and really very quick. Take most of the "nip" off the locknuts before you start. Cover the wings with a couple of blankets and start the engine and set the idle to about 500rpm. Slacken off No1 locknut and either with a screwdriver or your fingers (health and safety nightmare)turn the screw until a .015" feeler blade is a fairly easy drag through. Holding the screw with a screwdriver nip up the locknut. The best bit about this method is that you can measure them all before you start and immediately pick out the noisy ones and also get a feel for how the drag through should be. Use an old feeler blade or a piece of shim steel as it soon gets mangled. Do the other 7 then stop the engine and tighten the nuts properly. Start and just recheck quickly before fitting the cover. GUARANTEED it will be quieter. Let us all know.

On the subject of closing gaps I don't think this happens. Remember the block and head height will also expand and open the gaps effectively compensating for expanding valvegear. In my experience the B series is a fairly stable motor and this arguement is supported by the excessive tappet rattle which starts about 5 minutes after start up which is presumably caused by opening gaps due to block and head expansion and only quietens down again after about 15/20 mins when the heat has been transferred to the valve gear.
Iain MacKintosh

Depends on whether the steel of the push-rod has a greater coefficient of expansion than the cast iron of the block and head. A number of web sites indicate that whilst the two materials are relatively close in their coefficients compared to a wide range of other materials, mild steel *does* expand more than cast iron for a given temperature increase. But then again, the cast-iron block and head are much closer to the higher combustion temperatures than the push-rods are. Whatever, the Leyland book figures indicate that it *does* close up as it gets hot, as measured by PJ Kelly and Steve Postins, and this has got to a similar point of usefulness as discussions on how many angels can dance on the head of a pin.
Paul Hunt 2

Hi Paul, I reckon 100. I was intrigued as this has been debated before and Jeff, Barry and PJ all found the opposite to me. Anyway, next time around I'll be trying the dynamic valve setting method (thanks Iain) and assuming I still have 10 fingers I'll post how it went.
Steve Postins

Well I never have tried to set them running...strikes me as guaranteed to spatter oil everywhere and ruin my feeler guages into the bargain...but I'm intrigued by those who have...in particular by finding the noisy ones. With my car regardless of how I set it, my tappets are noisy at 13 thou hot, and much noisier at 15. Any less than 13 though and my idle gets lumpy. Again, given the "technology" of this design and the fact most of these components are either old or very old, or even brand new, but running in old blocks and heads I've come around to just living with the rattle.
P J KELLY

I learned to adjust valves on a six cylinder Chevrolets many years ago, everybody did them with the engine running. It isn't too difficult, however there is less effect on the idle speed of a six cylinder engine than there is on a four cylinder engine when you insert a feeler gauge between the rocker and the valve stem. I don't really see why it would be more accurate than adjusting as the mgb manual reads. Faster, yes.

Clifton
Clifton Gordon

with a modified cam the idle is less stable and at a higher rpm. Adjusting the valves while running is dificult at higher rpm. You do need to use shim stock as the stock will get pounded thin before long. I'm finding that few parts stores stock shim stock anymore.

One of the problems that has been reported is that the lifters are ground off center. As the lifter rotates the cam clearance changes.

Crane reportedly advises reducing the valve gap setting to 7 to 8 thou when the engine is cold, for 14 to 15 clearance when the alloy head is at full operating temperature.

The cam has an initial ramp of low acceleration. This ramp may vary in slope depending on the manufacturer/grinder. The goal is to match the valve clearance with the initial ramp. The velocity of the ramp is much less than the main part of the lobe lift. The valve clearance should theoretically be equal to the initial ramp, so that all clearance is closed at the end of the initial ramp and then the valve begins to lift.

These ramps vary from cam grinder to cam grinder.

Barry
Barry Parkinson

I can't explain why the gap closes up when cold and expands hot, but its very clearly the case! Maybe its from using an alloy head and alloy rockers?

Barry's comments about setting the gap at the initial ramp are spot on. I've noticed that to be the "low spot" on the cam and the easiest way to get a consistent setting.
Jeff Schlemmer

My 79 mgb has an alloy head and an Iskenderian cam. The spec sheet from Iskenderian states .018 - .019 hot. I wonder if the alloy head comes into play here. I do notice the valve train does quiet down after the engine has warmed up. So the clearance does close a bit after things warm up. But yes, the valve train does rattle. Those who remember the Chrysler slant 6 had to deal with a noisy valve train as not many mechanics would adjust the valves when doing a tune up.

cheers

Gary :>{D
gnhansen

On the subject of clearance. And this may be another of those "A little different for everyone.

I normally loosen the lifter. put the correct feeler gauge in, then tighten the adjuster until the blade slips out with some pull. Normally when done I can slide the gauge in but it takes slight pressure to slide it in.
Bruce Cunha

Gary
Your clearance increases as the alloy head gets hot. The push rods and valve stems warm up and get a little longer which may offset the expansion of a cast iron head. The alloy head expands much more when heated than the iron head resulting in it getting taller by a few thousandths of an inch, and consequently widening the valve gap.

Crane cams specifies a different cold gap for the alloy head for it's cam installations.

I have alloy rocker arm stands and an alloy head, so I set my valves hot at the specified gap. When the engine is cold, the valve gap is about .008" less.

Barry
Barry Parkinson

No wonder some bright spark invented the hydraulic tappet!!!!! Pity we can't make that a retrofit item!!!
P J KELLY

I've set up uncountable BB FE Ford and early Chev small block engines over the years so setting valve lash "on the run" is like second nature. For Chevs and Fords, I have several valve covers with the tops cut out and some strategically placed oil deflectors that don't interfere with the process.
With the B, hardly any oil splatters if you use an open topped valve cover. I glued the gasket to the base, and left the hold down holes in place, cutting out around them close enough to have access to the rocker arms on either side of the pedestal. It's easiest to loosen the lock nuts before starting, and just snug the nuts one at a time until the motor is shut down. I do a quick recheck after final tightening before reinstalling the valve cover. If the engine is susceptible to stumbling due to vacuum loss, simply put a clamp or vise grips on either the tappet chest or PCV hose to squeeze it down until the engine runs okay.
Mark

Barry, if the head expands as it warms the gap would become closer one would think. I'm not clear about your statement. The valve "clatter" does quiet down as the engine reaches operating temp. I am getting ready to readjust the valves soon and am curious to find what the gap is presently compared to when I set them over a year ago. More on that later

cheers

Gary :>{D
79 mgb
gnhansen

I would tend to think the expansion that would take place first would be the valve stems, decreasing the clearance. The valves may be have the most temp change of any internal engine parts.
Tom

With an alloy head, try it and find out. Adjust cold and see what it is hot or vice versa.
Barry
Barry Parkinson

"No wonder some bright spark invented the hydraulic tappet!!!!! Pity we can't make that a retrofit item!!!"

Quite. Beats me why people with V8s go to the bother of fitting adjustable push-rods and solid tappets! Perhaps less a case of "if it ain't broke don't fix it" than one of "it ain't broke so lets improve it".
Paul Hunt 2

This thread was discussed between 27/09/2005 and 06/10/2005

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.