VALVE GAPS AND EXHAUSTS AND INLETS!

[neil4030]

I have that classic rattly sound when the engine is under a bit of load i.e. going up a hill. I adjusted the valve gaps to 12 thou as per the hand book and that made things a bit quieter. What I mean is that I used the one in nine rule and adjusted the 8 gaps accordingly, noticing that some had no gaps at all when the valve was fully rocked; hence (I thought) the death rattle.
I read on a another recent 'tappets' post (Roy)that there are in fact exhaust and inlet gaps that should be adjusted to differing clearances. Does that mean there are four of each? Bit confused. I do still have the rattle but not as bad and I do not have an unleaded head on my 1098; so run on additive.

[Matt]

Book says 12 thou on all, Roy says 15 thou on the exhausts only

But I normally do mine to 12.

[MorrisMinor-65-1000]

I think the idea is that it's always better to air of the safe side with a standard head and unleaded fuel. Exhaust valves always expand more than inlets owing to the thermal expansion causing by the very very hot (600 deg!) exhaust gases bathing them when exiting the combustion chambers. But unleaded fuel combustion is notably hotter (and slower) than a comparative leaded fuel grade. As such, the valves can theoretically expand a touch further, and if they don't close properly (because the clearance is too small) they can overheat and lose their ability to maintain a "leak free" seal on the seats, thus accelerating seat degradation. As such they become "burnt out". Going fractionally over on the clearances on the exhaust is the easiest (although rattliest!) way to slow that process down so that major cylinder head overhaul intervals can be stretched to tens of thousands of miles at a time, although I've read that many on here have managed hundreds of thousands without issue.

Aside from additional rattling, widening the exhaust valve gaps also has the adverse effect of reducing the relative valve lift and event duration, but the difference is so miniscule, you're really not going to spot the difference on an A-series. In fact, a remapped distributor with a more meaningful unleaded advance curve is going to generate much more of a discernible positive difference.

In terms of setting your valves, the valve arrangement on your cylinder head is (from bulk head to rad, although it's the same in reverse anyway):
Exh
Inl
Inl
Exh
Exh
Inl
Inl
Exh

Michael

[neil4030]

Thanks for your help Michael/Matt.
I think I will have another tinker and increase the gaps on the exhaust outlets cause the science behind what you say is obviously sound and I cannot afford major engine work at the moment! Is the uphill rattling a sign/symptom of something to worry about generally or can I just happily ignore it?

[JOWETTJAVELIN]

Can you clarify a few things, please. Firstly when you mention the clearances when the valve is 'rocking', the rocking valve, being at the end of its travel and fully open, is like this so you can adjust whichever is in conjunction with that number. The feeler gauge should be a sliding fit with a degree of resistance when the adjusters have been tightened.

Secondly, Roy advises 12 thou for inlets (factory spec.) and 15 thou for exhausts to give that little extra time for valve cooling by staying open a fraction longer. In theory this is brilliant; in practice it may be more of a mental comfort - I don't know.

The subject of additives, unleaded heads &c. has been discussed to death. My own view is that valve seat recession is no big deal on a head which may still be easily and cheaply replaced. Then there is the question of the type of driving you do (frequent high speed or pottering about), and your annual mileage. Most LRP additives are quack remedies. The cylinder head has a lead memory thanks to the good old 2, 3 and 4 star grades it ran on for decades.

I am testing these 12/15 valve gaps myself on my guinea pig Allegro 1300 (the Minor has an unleaded head fitted by the last owner and doesn't do the same mileage), and the only additive I use is a mix of 2-stroke oil in with the Shell V-Power. As I do circa 10,000 miles a year in this car, much of these being hammered on motorways, I will be reporting the results, but I have nothing so far apart from clean, sweet running.

[smithskids]

I think you will find that larger valve clearance will give a shorter valve opening but the valve head will be in contact with the valve seat longer so in theory it will run cooler.

[JOWETTJAVELIN]

I was, of course, just testing you all on that one.

[neil4030]

The car is a daily driver although not massive miles; about a thousand over last couple of months. The previous owner had the valve clearances adjusted to rattle a bit (he said) though I was never that sure why that was. Since I adjusted them a couple of weeks ago to 12 thou, the tappety rattle has diminished but still kicks in going up hills. Otherwise, car runs great on premium unleaded and happily cruises along at 50-55 mph. As I say, I will adjust the exhaust gaps to 15 thou in line with the advice but is that all I need to do relevant to the rattle?

[JOWETTJAVELIN]

Just keep your eye on them at service intervals and if they are the same, be happy! These engines often have a bit of top-end noise even when well set-up.

There is a very slim possibility that the rattle is being caused by worn little-ends or even piston rings.

[bmcecosse]

MG/Rover increased the advised gaps to 15 thou inlet and exhaust. I don't see the need for the increase on the inlets - but I DO see the need on the exhausts. As mentioned above (and by me ad nauseum) it is to make sure the gap never closes up when the hot running ex valve expands.... In fact - I now run mine at 18 thou.. after experiencing some hot running misfire under load - which I thought could be the gaps closing up.... The difference in valve timing/time on the seat etc will be insignificant - it's just to make sure the gap never closes. NO need for costly additives - just check and reset every 3000 miles. Simples!!
The 'rattle' mentioned above could be pinking (retard the timing slightly) or bearings rattling.....what's the oil pressure when running ??

[neil4030]

I think when travelling at 30 ish about 65-70psi & about 30-40 ticking over but I would need to double check to make sure on that.

[bmcecosse]

Well -if that's with HOT oil - likely nothing wrong with the bearings.....so retard the ignition timing slightly and hopefully it will disappear.

[IslipMinor]

after experiencing some hot running misfire under load

Now where have I heard that before I wonder??

Roy, Do you know which cylinder is misfiring? Did anything else happen when it misfired? Smoke from the exhaust for example?

At the risk of igniting a very old thread, what valve guides are fitted and is the by-pass hose fitted??!!

Bronze guides were fitted as part of the unleaded conversion and bronze expands much more than steel for the same temperature difference. The problem, I eventually found, was that under load the difference in the coefficient of thermal expansion between the cast iron head and bronze guide, forced the guide to expand inwards and eventually nipped the valve stem (the sight of yellow marking on the stem of the burnt out No. 1 exhaust valve was the beginning of finding the solution to the problem after 2 years of frustration and unnecessary replacement of the entire fuel system from tank to pump!).

The solution was two-fold - refitting the by-pass hose to reinstate the water flow round the front of the head, to prevent a local hot spot, plus new guides. The advice from Peter Burgess, who fitted the guides, was to open up the exhaust valve clearance by 0.003" to give more 'seat time', thus more cooling to the valve head and less conducted through to the stem and guide, for at least the first 1,000 miles or so. It worked immediately, and have never had a 'hot' misfire in the 4/5 years since. Whether just opening up the valve clearance alone would have worked I have never tried - I'm leaving well alone!

[les]

I'm using bronze guides, installed 13 years ago with no problems, and the engine has worked quite hard. My valve clearances are standard 12 thou but I do keep the bypass hose! I do wonder about the 'valve seat time' a larger clearance must delay the opening by a millionth of a second!

[neil4030]

I can confirm my traveller is/was pinking; everything else OK.

[bmcecosse]

Richard - standard well worn guides and yes, as it happens this engine has a bypass hose. The thermostat would be WIDE open anyway - we had already travelled some 30 miles at good speed. It was a VERY hot day - the car was heavily laden - and was being flogged up a steepish (for a Minor) M way hill on full throttle. Misfiring set in almost at the top with spitting back through the carb. As soon as I was over the top and able to ease off - it ran fine all the way home. Yes - it could have been sticking valves - but I opened up the gaps slightly (to 18 thou) in case it was simply that the gaps had closed up (when checked cold - they had closed a couple of thou). But I rather suspect it may have been lack of fuel - maybe the pump couldn't keep up with the high demand - although I'm not clear why that would spit back through the carb.... I haven't been able to replicate the circs since .