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MG MGB Technical - Tappet Clearance

Greetings all,

In a thread from last week ("Rough lumpy idle," below) some of the discussion was about the way tappet clearance affects performance. Because, like a good Prussian grenadier, I have always marched by the book, and set my 77 B's tappets to .013; I was intrigued by Iain MacKintosh's experiments with different settings. So much so that I readjusted mine to .015 as Iain suggests. The result is that idle speed increased by 200 rpm and acceleration seems quicker, but the clatter also increased.

So, my questions are: 1) are there any long-term, bad affects from running looser tappets, and 2) how do you find the right tappet clearance if the "book-setting" is a compromise between performance and noise level, trial and error? I would appreciate any thoughts y'all might have on this subject.

Doug
D. Cook

Doug,
This is very interesting that you have set your clearances to .013" I am aware that there is reference somewhere to a .013 clearance but can never find it. Perhaps then your engine group is the one exception as you may have the original handbook.

Maybe I should also have said that the rocker arm clearance is part of the overall design of the camshaft and valve gear and if yours says .013" then so be it.

The clearance in the B is normally 15 thou hot or cold and I find that this is optimum and that there are no benefits in reducing this. However in order to achieve accurate clearances the rockers need to be a good fit on the shaft and any indentation on the pads has to be removed.

Now you've come from the opposite direction and I think we need to wait comments from the like of Chris Betson to conclude this one.
Iain
Iain MacKintosh

Doug,

At a seminar I attended, John Twist, the leading authority here in the states, suggested all motors, 62-80 be set at .015 hot. Mine is set at .015 with exhaust 2 & 3 set a little looser--.016. It has the sewing machine sound as a British car of its era should. Don't try to quiet it to modern standards.

In addition, other authorities I have talked with tell be there were Leyland/BMC "Confidential Service Bullitins" that superceede the manuals, stating the .015 hot. I haven't seen for myself but with that I decided to correct my book in pen with a "per JT" notation because I like you must follow zee book!

Cheers

Paul
Paul Hanley

I'm no Chris Betson, but I do know that camshaft lobes are designed so that the transition from no load to spring load is done on a shallow ramp. Then once the slack in the valve gear is taken up, the rest of the lobe lifts much more quickly. This saves wear on the entire valve train, camshaft included. If you increase your clearance, your tappet is loading the lobe at a later point, and could be on the fast-lift part of the lobe instead of the shallow part of the lobe. This of course, will cause more wear on the valve train.

Similarly, there's a shallow ramp on the opposite side of the lobe which closes the valve a little more gently in order to save wear on the valve and seat. Your larger clearance is going to drop the valve at an earlier point in the cycle, also possibly before the shallow part of the lobe. You may be slamming your valve shut instead of closing it gently.

That's why (in my opinion) it's best to use the clearance suggested by the manufacturer. In addition to performance, they've taken these other issues into consideration.

Matt Kulka

That's exactly the point I was making Matt. And if Doug's cam is one of the ones designed for a .013" clearance then that't the way it must remain.
Iain MacKintosh

Iain, Paul, & Matt,

These are great comments so far. The book I refer to is the Bentley shop manual for the 75-80 cars. Also, my cam is the aftermarket Delta 270 which is fairly mild. It didn't come with any instructions and Bob Ford at Brit-tek suggested setting to the manual clearance of .013 (after a .015 break-in period). I forgot to mention earlier that in addition to the 200 rpm idle speed increase the engine is also running leaner at low speeds.

Doug
D. Cook

I've read this thread with interest. I would like to adjust or at least check the clearence of the tappets in my MG. Being a novice mechanic, I was wondering if anyone can supply a link that shows and explains how to adjust the tappets and what equipment is needed. Thanks in advance.
Jim

I wrote the original thread on rough idle etc etc...setting the tappets to 13ish greatly improved idle on my car...I'd been too tight at 11 thou...BUT I cannot stand the valve clatter you get at wider clearances. The label under the bonnet says 13thou hot. Seems to be a trade off between idle smoothness vs noise. Also...these clearances are NOT easily set...I found I needed several attempts/crank rotations to get them all correct. Next year I'm going to replace the rocker shaft and related gear and see what's what then. In the meantime, I'll put up with some clatter. Maybe a lead valve cover wouldn't be a bad idea....LOL
P J KELLY

Doug,
I have the original BMC manual for cars up to about 1968 but this refers only to .015, so does the Haynes manual that I have. I would respect John Twist's comments but then that's for an engine with a standard cam. The final setting MUST be the one to which the cam was designed and you don't have that info and I can't find it on the web. Would it be worth having another word with Brit-tek ??

What did Bob mean by "manual" clearance. Was it the cam manual or the car manual in which case I would plump for .015" in the meantime. If you have the "bottle" I would suggest you set to this clearance with the engine hot and running, that way you'll get the most accurate setting consistent with least noise.

Jim, have a look at the car handbook, Haynes or Bentley's manuals and you'll get a full description as it stretches further than this BBS permits !!

Iain
Iain MacKintosh

Find the cam mfr, and get the specs for that cam, there is no reason to suppose that it has any relationship to the OEM spec. All aftermarket cams I've encountered use at least the original spec, and most use more clearance. The .013 seems to be for the cam used in the single carb emissions engine. At the time, they were using overlap to bleed off some compression at low speeds in an effort to reduce NOx. While Matt's thoughts are reasonable, increasing clearance to .015 is not enough to cause trouble, and in view of the "inside" info/rumors, it is likely that the original cam was in fact designed for .015 anyhow. In general, most engines like a quick opening valve to more effectively utilize inertia effects to charge the cylinders; slow openings via tight clearances generally cause a drop in performance. Frequently, a couple of thou more clearance results in better throttle response.
FRM
http://www.usachoice.net/gofanu
FR Millmore

Jim - A Haynes manual or even IIRC the owner's manual gives a good description. Check the archives, which will give you more info than you really need.

The easiest way for me is while the engine is "off" (some folks can do this while the engine is idling - amazing), turning the crank by using my thumb on the fanbelt (for grip). I then use the "rule of nines" - valve #8 open, valve #1 closed 8+1=9. Then it's 2/7, 3/6, 4/5, etc. Adjust the closed one, of course.

All you need is a wrench for the lock nut (7/16's? or adjustable), a flat bladed screwdriver and a set of feeler gauges. A new valve cover gasket is the only new part, and you're in business.
John Z

Just out of curiosity I looked at the spec for my Kent 715 Fast Road cam and the quoted numbers from the Kent spec sheet for the B series 16/1800 are .016" (.40mm) which is the same for the 714 Mild Road cam.
However, the 716 Rally cam runs .018"in and .02"ex. The 717 Fast Road high lift cams are quoted .022"in and .024"ex.

Hope this helps.

Martin.
Martin ZT

The under hood Original BL label on my 74 model year mgb roadster states the valves should be set to .013" hot. My cam has been replaced with a Holman Automotive grind and it came with all instructions needed to degree the cam and set the valve clearance.

I don't understand why anyone sells a cam and fails to provide a cam card. As Fletcher said that is vital information that you or your engine builder needs to set the cam timing and valve clearance for best performance.

FWIW, Clifton
Clifton Gordon

I'm trying to figure the hot/cold difference as the Piper cam I have quotes gaps hot; awkward to achieve on a cold Autumn day, especially on the last two or three. In the archives I see: ".015 cold equals .013 warm for all MGB engines" (standard cam). Where does the expansion
come from, and is the 0.002 difference hot/cold going to be constant regardless of cam grind?
Steve Postins

My understanding was that the clearance on the B series engine had to be set to .015"hot but if you go to the archives etc others have said that the expansion of the various parts seems to be so consistent that it doesn't really matter when they are set. For me, I believe that our engines spend or should spend most of their time hot so it is the hot setting that is important. I would therefore suggest that this .002" difference may not exist and I would also suggest that the cam would least affect the expansion chain.
Incidentally an engine cannot be considered hot/warm when it has been started from cold and run for say 10 mins or three or four miles. At that point expansion will be all over the place as the motor will not have warmed through to proper working temp. A hot engine, which is when the tappets should be set, is one which has been running say 30 mins and may have travelled the best part of 10 miles to achieve uniform heat.
Iain MacKintosh

The first guy I worked for was a BMC service manager. He pointed out to the factory tech rep that they had variously specified "cold", "hot", cold running (!)", all at the same clearances, on the same engines (A, B, C series). They measured very carefully, and concluded that it did not matter; this matches my experience. Note that this would not apply to engines with alloy parts, like aftermarket cylinder heads, or to such things like esoteric pushrods of alloy or carbon fiber.
FRM
http://www.usachoice.net/gofanu
FR Millmore

FRM, thank you for that excellent information. Glad to hear, because as Steve notes, it's hard to keep the engine at operating temperature for long.

Jim in Philly, the Haynes manual does explain the process of setting the valve clearances. The one thing manuals never explain is the feel of the feeler guage when the setting is correct. I finally read this perfect description on someone else's web page a few years ago: When the setting is correct, pushing the feeler into the gap should feel like "pushing a cold knife through a stick of butter." (As a double check, you should not be able to stuff the next larger feeler guage into the gap unless you use a lot of muscle.)
Matt Kulka

When setting mine I always use two sizes one thou either side of the given figure. The difference between the two is obvious when the gap is correct. If you then use one of the exact size you can easily learn what it feels like.
Paul Hunt

From all of which you can take it that the difference in clearance between cold and a thoroughly warmed up engine is 2 thou (and yes I am not at all surprised if a service manager does not think the difference is significant).

In days gone by, garages used to ask us to bring in our cars the day before the service - so they could be worked on when cold in the morning. When folks brought their cars in for service on the same day - they had to be adjusted hot - so the hot clearance was included in the manual.


Most of the engines are getting on a bit now and the rocker gear is worn - so they sound a bit more tappety then they used - 2 thou can make quite a lot of noise. It's worth renewing the overhead gear - makes adjustment a lot easier. If you have a heavier tap from one of the followers - chances are it has become dished - worth replacing at the same time.

Boringly reliable engine until the service department gets at it - change the plugs and oil regularly - electrics every 50,000, overhead gear about 100K and most will give you 200,000 miles.

FWIW

Roger
RMW

If you're really concerned about "getting it exact", use a dial indicator set on the rocker arm directly over the valve stem. Then all the worn areas will touch as they would if running. Unless you have a squirter, you should be able to run the engine at idle to warm it back up between valves if necessary. Once done, let the engine cool overnite or longer, and, as long as the ambient doesn't drop below 40 Deg F or so, measure clearances again and record for each valve - then you can do it cold. PITA.

Wayne
Wayne Pearson

RMW-
Your logic is appalling and incomprehensible.
"A" service manager is one thing; the service manager I mentioned was on of a scant handful of people I have met who I would consider to be an actual "Mechanic", and the Factory tech man was THE head factory man for BMC in the US at the time. It was NOT that they considered two thou to be insignificant, but that they could not find a reliable difference between hot and cold.
"Cars" covers a lot, and I was speaking of BMC only. Many cars and many operations require certain temperature conditions; for this operation, on these cars, it doesn't matter. Since you are so wise, perhaps you might tell us WHAT exactly is "cold" and "Hot"? For "cold" how about -20F, 0F, 0C, 35C? I have personally had cars at "running temp" anywhere from about 90F through 250F water temp, depending on various factors.
There are many reasons why garages ask people to bring cars in at certain times, and many reasons given for why that is necessary. Sometimes it really is necessary; but, one is to ensure the efficient operation of the shop, for which it is often convenient to have a PITA smartass customer out of the way - any excuse will do, especially if it offends no one.
FRM
http://www.usachoice.net/gofanu
FR Millmore

How can you tell if you have the valve "Pefectly" closed? I have noticed that often a valve will apear to be closed, then upon minor turning, you may notice it wasn't "perfectly" in the closed posdition. It wouldn't take much being off to make the adjustment incorrect.

Any advise?
Terry

There has been much talk about the correct clearance and the best position to adjust that clearance. The BMC B is one of the few solid tappet engines that can be adjusted hot or cold. The original books are still with my B and the owners manual says cold and the performance manual says hot, so they couldnt make up there mind even in the sixties. Anyway I worked on MGs when they were new and now I play with them. Adjusting the valves running is good but messy and a lot easier with two ( one with the ring spanner and the clever one with the feeler and screwdriver)when your happy with the feel just say nip. Retighten when finished.
Now I will tell you how I do it. Motor stationary with the plugs out. Turn the fan until a valve is down then turn the motor one full turn. A chalk mark on the crank pulley helps. Adjust the valve and then do the same for the rest. It is more time consuming and I didnt do it this way when doing it for a living, however it is the only way of being sure your on the very back of the cam, and your not doing it that often. As for valve clearance the factory gap only counts for original cams and has no meaning for non factory cams. Each grinder uses his own silincing ramp and that is what governs the clearance which may or may not be the same as standard. Just remember if you dont want to burn valves always error on the loose side but doing it my way helps you not error at all.
All the best Denis
DENIS

I thought I was losing my marbles when I read RMWs contribution, it went completely against everything that had previously been said. Thank goodness FRM and Denis brought the thread firmly back on track.
Iain MacKintosh

Iain - look at FRM's website and it will tell you where he is coming from. If you read Denis' piece very carefully you will see that his well presented advice supplements and supports mine. There are two points; there is a different adjustment hot from cold and secondly if the overhead gear is worn you will find it difficult to achieve a consistent gap.

FWIW

Roger

RMW

RMW - I'm not so sure that there is a "measurably" difference between hot and cold on this engine. On many OHV engines there is but the B series seems to be remarkably consistent when cold and when fully warmed through. I did make the point that a quick ten minute run from cold is no good because it is all over the place at that point but provided it has been well run then there is a considferable time slot where accurate adjustment can be carried out.

Of course you make the point that if there is wear then the whole thing is a waste of time and I couldn't agree more. I would like to think however that those who contribute to this BBS are the real enthusiasts whose cars are in very good condition and are determined to achieve as near perfection as possible. Unfortunately we are never going to get these things running as quietly as the modern motor with hydraulic lifters but we can keep trying.
Iain MacKintosh

Terry - For many years I adjusted my valves using the Rule of Nine method (i.e. when 1 is fully down you adjust 8, when 2 is fully down you adjust 7 etc. i.e. the two valves ad up to nine) but when spinning the engine on the starter and rechecking the measurement some of the valves were always out again and I could never understand why. One year I noticed that although its partner valve was fully down, the gap on the valve I was adjusting wasn't neccessarily at its maximum gap, that could be a little way either side of fully down depending. Since then I use the descending valve as a guide but only check/adjust when the valve I am adjusting is at its biggest gap and since then it has been quieter and consistent both immediately and between services.

Whilst I can accept that worn gear will rattle no matter what, surely the point of putting a feeler gauge in the gap is that it takes account of all wear throughout the mechanical linkage 'tween cam and valve - unless the rocker face is grooved and the gauge is bridging it. Wear on the lobe giving a smaller lift and hence valve opening is again a different matter.
Paul Hunt

This thread was discussed between 18/10/2004 and 21/10/2004

MG MGB Technical index

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