Tight tappet clearances, what happens?

[stuey]

Hi chaps. I have just been checking the tappet clearances on my pretty new (2hrs running) ASP 61, and found that there was no clearance. I set the piston to TDC and the dot on the cam to line up with the pushrods, and there was no clearance at all. I have now adjusted it to the 0.1mm specified (awkward job, tiny bit of crank movement changes them slightly).

Anyway, was wondering what the effects of this could be, as I have never been entirely happy with the running so far (lumpy idle, inconsistent needle settings to some degree). Any good input appreciated

[ken anderson.]

hello stuey.....at the worst the head gear would be damaged as there been no clearance for the metals to expand and no play for the valve's..but it sounds ok.....do the instructions say to set the clearance when the engine is cold or hot?

 

ken anderson ne..1 ... 4ST dept.

Edited By ken anderson. on 07/08/2011 17:30:43

[stuey]

Pretty sure the clearances are for cold, which is how I have checked it and adjusted.

[Myron Beaumont]

I would re -check the clearances again after a while of running as any carbon build up on the valve seat caused by the leakage will have been "hammered " away thus reducing the valve clearance once again .

Myron

[PatMc]

 

Too little clearance can lead to damage to the valves & the cylinder head.

 

Usually the min & max clearances are specified. It's best to set the clearance near the max as the wear with normal running will reduce it.

If the tolerance is too low, when the engine heats up the valves won't seat properly when they should be closed. There's a danger that hot gas escaping during the compression & firing stroke will burn the valve seats & faces. The exhaust valve is the most likely to suffer first as the intake valve is being cooled by the mixture each induction cycle.

[Myron Beaumont]

PatMc

Couldn't agree more. Brasso or T'cut can be used for re seating such small valves if necessary using a hand drill (gently) on the protruding valve stem to revolve it a few turns at a time releasing pressure every few turns until you have an even matt surface on the seat .Clean all parts well with fuel afterwards of course.

[PatMc]

Myron, if too small clearance is combined with a lean run or two there might not be enough valve face left to re-seat, meaning a new valve required. Or worse the the seat could get too badly scorched & pitted - that's a complete new head needed.

[Chris Channon]

Hi all, i thought i read somewhere that the gap doesn't alter whether the engine is hot or cold, ie the gap will stay the same as the whole engine expands not just the end of the rocker arm?

Regards

Chris.

 

[Doug Ireland]

Not so Chris, that's why the setting we are given in the instructions for a new engine say to check when the engine is cold. Tappets that are touching the valves when cold may not seal properly when the engine is hot and can lead to serious damage. Incidentally, it's the length of the valve stem that lengthens through heat, not the rocker arms.

[Martin Harris - Moderator]

I've always understood that the correct clearance is virtually none at running temperature (just enough to maintain a good oil film and ensure proper closure) but that as it is tricky to set them at that due to not being able to gauge how much an engine has cooled since the engine was stopped, that the cold setting is given as it will be within a smaller heat range - probably at 20 degrees C plus or minus 10.

[Peter Beeney]

I’ve always thought that the screw tappet adjustment, relating to overhead valve engines anyway, was to periodically take up the wear that must very gradually occur on the cams, cam followers, push rods, rocker arms and the valve stems. I’ve always assumed this wear will only ever increase the gap between the rocker arm and the valve; the only way that I can think that the gap could become less is if the valve seat itself wears appreciably and allows the valve stem to move closer to the rocker arm. I’ve personally never known that to happen. Or if an exhaust valve started to burn it might then possibly happen, exacerbating the problem, but by then the performance would really seriously be dropping off anyway. A burnt exhaust valve does happen, but probably not as often as it used to! Valves are, I’m sure, nowadays made of sterner stuff!

With regard to the actual gap, as Martin says, this must be a bit of a compromise. I’d consider that as the engine warms up it expands; in all directions. One hot? candidate for this inevitability that immediately springs to mind might be the exhaust valve and stem; it’s creeping ever closer to the rocker arm. However the cylinder block is keeping pace, it’s also getting taller, lifting the rocker shaft away from the camshaft. But the pushrods are also getting longer, so the rocker arm is now perhaps again getting closer to the valve stem. Maybe the co-efficient of expansion plays a part here too, different metals expanding by varying amounts? I’ve no idea. In the event I’d say the cylinder block gets to be the biggest and best, the pushrods won’t expand so much, because they don’t get so hot maybe, so there is always a gap between the rocker arm and the valve. Different parts of the engine must be at different local temperatures. I, too, would imagine the lump is designed to run with minimal valve clearance, but there always has to at least a iota of gap so I would always check this after doing any tinkering.
I’d run the engine up to working temperature, stop it, (it’s easier to work on that way!), turn it until one valve is fully depressed and then try and spin the other push rod. Then swap. If there is any ‘weight’ on the rocker arm, i.e. the valve is being slightly held open the pushrod will be difficult to twist. If there is only a tiny perceptible clearance it will be easy. Or the standard feeler gauge procedure might suffice. We also need clearance when it’s cold, a valve held slightly open might make it difficult to start. I must say I don’t think I’ve ever seen a case of a valve with no clearance but then I’ve not had a massive amount of practical engine experience anyway.

I have to say I’m always a bit confused when folks talk about checking the tappet clearance with the piston at TDC. As I see it, this is fine if you happen to be on the power stroke, both valves will be fully closed, and any slight movement either side of TDC will not move the them; however, if you happen to be on the exhaust/induction stroke the valves are rocking, that means their cycles overlap at this point and both valves are a tiny bit open, any slight movement will cause both valves to move. There will be no clearance, and if the tappets are adjusted to the correct clearance at this point then the normal clearance will be excessive. To say the least... And with the greatest respect, that sounds a smidgen like where stuey is now. In his OP he remarked that a tiny crank movement changed them slightly.

Also relating to stuey’s OP, if both valves had no clearance, meaning they were both slightly held open all the time then that might give rise to some very severe starting difficulties, let alone running; and stuey said this is how it appeared to be when he was working on it. But I might be completely wrong, he has probably found his engine now runs purrfectly!

PB

[Bob Cotsford]

The valve seats do wear, and the valve stems stretch, especially in the first few runs. On modern motorcycle engines valve clearances tend to close up with age, so the seats are wearing and stems stretching faster than the drive mechanism is wearing. That's what happens with modern materials and oils. These engines are actually running at higher revs than model 4 strokes, typically 12-16k rpm peak so effects are magnified.

If the clearances are too tight, the engine may start ok but will definitely run hot as combusting gases are not being confined within the cylinder. That burns the valve seat particularly the sharp edges, which just makes matters worse.

[Engine Doctor]

Hello stuey . You shouldn't have any damage if the valves were just tight when cold and motor still had compression.If it had run and burnt the valves/seats then it would have had no compression and likley damaged valve seats . This is unusual in glow engines as the fuel doesn't burn hot enough ,unlike petrol. As a motor heats up the aluminium cases expand more than the steel pushrods so the clearance actually increases when hot , thats why you adjust then when cold ( smallest clearance) .some of the older car engines that were all iron had the clearances adjusted when hot as it worked in the oposite way whereas slightly more modern engines ,alloy blocks and heads with steel pushrods were adjusted when cold . That meant completely cold .Pushrod engines usually wore loose whereas overhead cam engines wore tight. Dont be tempted to reseat the valves on the cheaper engines unless really needed . You can reseat a valve and get a perfect seat/seal ,tested with parafin or similar and as soon as you refit the head it will leak as its usually a distorts as the head is tightened ,re-creating the problem .The valves can regress after a lot of running but normal adjustment should sort it .In general our model engines have so much oil slopping about that valve seat burn is not a problem unless converted to petrol and the associated higher temps involved.

[Peter Beeney]

Bob,

Thanks for your reply and I’m sure you’re right in what you say, it’s just that I’ve got a couple of mates that have spent their lives around engines and I don’t think they’ve ever said much along these lines, so maybe it doesn’t happen all the time. I must admit, I’ve never heard of valve stems stretching significantly, so I didn’t realise that this was happening to such an extent. I’ve used four stroke model engines, right from the time of the first OS 60FS, with it’s exposed rocker gear, and I’ve personally never really had any problems with tight tappets, or tappets actually going tight. But as I’m very much a ‘I don’t fix it if it ain’t broke’ sort of bloke, usually ’cos I’m just lazy, I only fiddle with them when I have to, which means perhaps that I’ve experienced this without even knowing it!

From my personal capability, which as I said is not great, I’ve adjusted the valves occasionally and on the basis that the engine is hot for the vast percentage of it’s running time I’ve also always re-checked them when hot; as I recall, very generally there seems to be very little difference either way so I’ve long concluded that any increase in overall engine dimensions is maybe very minimal and is also fairly uniform. But maybe that’s just my system of doing things, which may not always be fully approved of anyway.
With regard to modern vehicle engines, compared to only a few years ago, I would be tempted to say they’ve really improved phenomenally. Stories now appear commonplace about the vast milage's covered with very little attention, other than just routine servicing; and I’m not sure that includes adjusting the tappets, ever. So it seems not all engine suffer with components wearing that much, or at least not for a very long time.

It would be interesting to know if in fact stuey has had a result on his engine, I can only think it must be well improved if in fact both valves were being held open. Also I can feel another little experiment coming on, one that turns the tappet screw down on the inlet valve until it’s held open by a few thou and then see how easy, or otherwise, it is to start. I’m always game for a bit of a meddle; and I’ve certainly learnt a few times that things do not always act like wot they’re supposed to!

PB

[Martin Harris - Moderator]

Most engines I've had dealings with have had smaller gaps specified for the inlet valves which suggests to me that valve expansion is significant as this would be the most likely candidate for differential expansion. Maybe a peculiarity of the cam profile characteristics or some other design parameter but it seems a fair assumption to me...

 

ED - I certainly agree with your statement - some of the older car engines that were all iron had the clearances adjusted when hot - but it's got me wondering why they couldn't have specified the cold clearances - perhaps due to the wide manufacturing tolerences when they made cars the way " they don't make cars like that any more!"

 

Edited By Martin Harris on 09/08/2011 22:29:30

[Myron Beaumont]

There's a big difference between the "old" push rods sbout 10" long and our tiny 1"'ish push rods .Although I don't know the coefficients of expansion of the various metals used,and the temps involved ,I'm pretty sure we are in the realms of tenths of a thou'.

Excuse the ref. to "old money".

Edited By Myron Beaumont on 10/08/2011 09:11:57

[stuey]

Thanks for all the feedback. I dont think my engine has sustained any bad problems, it might well be my own lack of experience with 4 strokes and their tuning. Now they (the tappets) are adjusted better, I am hoping to see some improvement on the niggles I have had. At least it shouldnt be any worse!! I will post how its going once I have had a chance to try it again.

[Peter Beeney]

Martin,

Maybe the tappets used to be checked when hot just simply to ensure that the clearance was correct when the engine was hot? For the same reason that I double-check my model engine when hot. If the types of metal used on older engines always ensured that the gap was consistently greater when the engine was cold I guess that would also mean there would be no starting problems. From what little I read about today’s modern transport engines, pushrods and rocker arms may be getting a bit old hat now, with overhead camshafts, variable valve timing and hydraulic tappet adjustment seemingly just standard run of the mill stuff. Indeed, I’m of the opinion that the modern car engine is truly a masterpiece of engineering, in fact the whole vehicle is really! But, to be fair, because they are so popular, they’ve always had the necessary R & D!

With regard to the valves, they operate in very different environments. The inlet being continually cooled by the incoming air and the exhaust heated by the outgoing hot gasses. These temperatures can be considerable, when a 40 kW generator is on a test run, at 110% load, the exhaust manifold would sometimes glow faintly red hot; and if you then start tinkering and playing ‘what if’ sort of games, such as gradually increasing the load still further in small increments the temperature would then rapidly start to climb exponentially, from say 700 degrees C to way beyond; but of course at this point it’s not long before the effort gets too much and it then cries enough is enough a bit sharpish! High performance racing/competition engine temps. can be considerably more, sometimes up to around the 1200 degrees C level, I believe. The manifold is white hot! And the exhaust valve is right in the middle of all this! The two valves are made from different types of metal, and I guess that the difference in the tappet clearance might be explained by the exhaust valve getting much hotter. As a quick aside here, when you see a larger engine, 400 kW, 500 + BHP output, stalled by the electrical load, it makes you wonder at the strength of the magnetic field within the generator, it has to be pretty intense! It almost seems like solid!

I don’t think model engines get quite this hot, although four stokes can warm up a bit; some heat is thrown out with the unburnt oil in the exhaust gasses. I mentioned the original OS 60FS before, this didn’t have a silencer, just a curved outlet pipe. I can remember seeing the top of this curved pipe regularly glowing hot in the twilight many many moons ago, the motor was in a friends Flair Puppeteer.

Just another insignificant and trivial ramble from the past…...

PB

[Martin Harris - Moderator]

Peter - I think we're singing from the same songsheet - hence my reference (without explanation) to the differential expansion.

 

I'm fairly sure that the preference for cold setting is the difficulty of defining "hot". Where tolerences are such that a figure can be given to allow for the calculation, but there's no doubt that the correct clearance at running temperature is the ultimate aim.

 

Talking of glowing exhausts, I remember fitting a new set of contact breakers to my S1 Land Rover one lunchtime, watching the exhaust manifold glowing bright red with some amazement and scratching my head until I realised that the heel was located differently to the old ones which was retarding the ignition by several degrees!

Edited By Martin Harris on 13/08/2011 00:09:31

[stuey]

Right, had a chance to fly this engine again today. Started fine, seems to be running ok. Whilst resetting the top end needle (needs to be moved to get cowl off), I seem to have lost about 1000rpm from the most it would give before. With a 12x6 it would top out at about 10100ish, which I backed off about 500rpm for safety. The best I could get today was 9000rpm, and that was a bit leaner than last week. What was also a bit strange is that my SC52fs in my Mustang used to hit about 10000rpm on the same prop, and that also looks a bit down too. Assuming that adjusting the tappets has not caused this, it must be my RPM gauge... right?

The engine ran ok today, the idle if anything may have been slightly better, and throttled fine. Bit confused now!

[stuey]

I have to declare that I'm an eejit. Having consulted my SC instructions rather than the ones supplied with the ASP, I find I had adjusted it wrong. I think I had the cam dot 180deg out, and was adjusting the gaps at the wrong point. When set at the position described in the SC instructions, I had gaps more like 0.6mm than 0.1mm. Probably explains the lack of power, and I bet the gaps were probably about right in the first place. Thought it took a lot of adjusting....

Apologies for wasting everyones time

[William Morrison]

I would suggest looking closely at the cowl to ensure the cooling air exhaust area is about twice the size of the intake area. It sounds to be that you are running hot due to poor cooling. Good air circulation is necessary. Try flying without a cowl and see what happens.

Chip

[Peter Beeney]

stuey,

I’ve just caught up with this thread and with the greatest respect this is not the first time the tappet trick has been done. Or the last, it’s easy to make this little mistake. I was pretty sure this was what you doing in a previous post, PB @ 09/08/2011 07:59:13. When you are ‘rocking the valves’, the small period of valve overlap, you are on the wrong stroke, you need to go down and up again. It sounds as though your marks indicate the start of the cycle, which unfortunately is in exactly the wrong place.

I just simply turn the engine over until one valve is exactly fully open. I then consider that the other one is fully closed; so I can then adjust it. But again you have to be a bit careful here, because of the valve timing it doesn’t take much movement to alter things.

0.1 mm is around 4 thou, thousands of an inch, so 0.6 mm is virtually 24 thou, it’s remarkable it ran at all! Did it rattle a bit? I suspect you’ll find there’s not a lot of difference between the cold and hot clearances. The overall difference in expansion between these components must be minimal.

Talking of heat, again with the greatest respect, but if you feel that it is possibly overheating, another little trick you can do is install some baffle plates in the front of the cowl to force the air through the fins on the engine, rather than going round it. Rather like the cowl on a lawn mower engine, for instance. That’s if you haven’t already done this, of course.

Hope you crack it ok.

PB

[stuey]

I think the cooling is fine now, I made a slot under the spinner and have a large exit hole at the rear. The original cooling problems I had were probably down to still being tight, and running a bit lean (my mistake). The engine did run ok, and was perhaps a little rattly. I'm pretty sure it will be fine now.

Incase it helps, the SC instructions were as follows. Turn the engine over with the prop until you feel compression, then turn a further 1/4 turn. At this point the both valves are closed. This is when I realised the gaps were a bit big and I had made a mistake!!

I think when I had the dot on the cam lined up, I had it directly under the pushrod instead of the bottom in a straight line through the centre if that makes sense?

[Peter Beeney]

stuey,

Looking good, as you say, it should now run like the proverbial sewing machine!

There is another quick tappet tip, I’m sure well known, but might help someone in the future; and you don’t need any timing marks.
First, a wooden or plastic pin in the plug hole. This gives you a perfect indication of TDC as it goes up and down. Then watch the valves, when they are moving very briefly together at TDC that is the induction stroke, turn the engine until the piston reaches TDC again, this is now the power stroke, both valves are fully closed and you can adjust the tappets at this point.
The reason for the for valves moving together is because the inlet valve opens very slightly before TDC on the exhaust stroke and the exhaust valve doesn’t close completely until after TDC on the induction stroke; and one reason for all this is to try and ensure that the valves are open for as long as possible. If all this makes sense!

I just simply turn the engine over until one valve is open. I then consider that the other one is closed; so I can then adjust it. But again you have to be a tiny bit careful here, because of the valve timing it doesn’t take much movement to alter things. I reckon that when the inlet is just closing the exhaust is fully closed, and when the exhaust is just opening the inlet is fully closed. I hope!

So, all set for a bit of the ‘foot on the loud pedal’ stuff now?

Good Luck.

PB