tappets

[owen]

my thorneycroft t90 has been sounding tappety and on checking the clearances some were about 1/4 inch instead of 15 thou. the question is how do i turn over the engine while I adjust the tappets. thanks

<hr width=100% size=1>

 

[steve_cronin]

Big engine like that?

On the starter with the decompressors out would be my method if there isn't a handle but it takes patience to make sure that the cam followers are on the backs of the cams properly. This is a four cylinder, right? Each cylinder will have a partner that is "Rocking" ie one valve of the pair is opening and the other closing at the same time(or nearly) on the same cylinder when the partner's very surely are one the back of the cam and therefore closed so you can set the clearance. You will need to know your firing order to determine which cylinders are partners.

e.g. if the order is 1-3-4-2 or 1-2-4-3 then adjust No 1 when valves on No 4 are rocking and vice versa. The same is true for nos 2 & 3 in this case. If the order is 1-2-3-4 then 1&3 and 2&4 are partners.

There may be wear in the cam followers or the rocker cups so expecting the tappets to be quiet at what seems the correct setting may disappoint. If they are still unacceptably noisy, try a thin feeler (say 5 thou) in each gap whilst the engine is ticking over. If this removes noise markedly then tighten this tappet a bit to reduce the noise. Repeat for the rest but don't run the engine too fast with the rocker box off or you'll spray oil everywhere. I doubt though that there will be much wear so the normal noise might be acceptable.

Steve Cronin


<hr width=100% size=1>

 

[Evadne]

Sorry, I'm not familiar with your engine but I would have thought static clearances would be got by turning the crankshaft pulley with a spanner or long screwdriver through the holes (like on my old cortina), or even a quick dab on the starter motor?

<hr width=100% size=1>

 

[Gordonmc]

I also have a T90. Turning the engine is tricky to get the cam just right, but...
Engine must be cold. Pull out the decompressor and keep it out. I cut a slot in a wine cork to slip over the cable.
Use a socket on the crank pulley nut to turn the crankshaft (in normal direction of rotation).
Adjust clearances to 0.381mm or 0.015in.
1 with valve 8 open
3 with 6 open
5 with 4 open
2 with 7 open
8 with 1 open
6 with 3 open
4 with 5 open
7 with 2 open

Using the starter motor is not recommended as the startery battery should be disconnected.

<hr width=100% size=1>

 

[owen]

thanks for all replies.I have the sequence in the hand book so it was advice on turning the engine i was after so it looks as though a handle on the crank is the answer. incidentally does adjusting the tappets have any benefit apart from keeping things quiet?

<hr width=100% size=1>

 

[Heckler]

if the tappets are left slack the increased slop in the valve train puts a slight hammer effect into action, they hit instead of pushing, thats the noise you hear, the valves open at the correct point in the cycle when the gaps are correct, think about it if the rockers are taking up slack they aint opening valves, if they are too tight they prop the valves open and combustion pressure gets past the seats and burns them.
just a thought for you, why are the gaps a quarter of an inch? if they are truly that there is something expensive going wrong, either cam or followers are badly worn or no oil to rockers and pivot points are badly worn.
stu

<hr width=100% size=1>

 

[Birdseye]

yes, wide open tappets will cause a deterioration in performance since air has less time to get in and exhaust less time to get out.

question is, why are they wide open? tappets normally wear closed ie get tighter not slacker.

<hr width=100% size=1>

 

[Avocet]

Sometimes, the bearing face of the rocker isn't flat anymore - it has a slight groove in it the same width as the top of the valve stem. When you put the feeler gauge in you sometimes "bridge" the groove because the feeler gauge is wider than it. If this happens you will think your tappets are fine but it will still be noisy. Sometimes you can grind the sides of some feelers to make them the same width as the valve stem but then they tend to "buckle" if you're not extremely careful pushing them in. I've also heard of people using dial gauges to set tappets but have never seen it done.

<hr width=100% size=1>

 

[Avocet]

Sometimes, the bearing face of the rocker isn't flat anymore - it has a slight groove in it the same width as the top of the valve stem. When you put the feeler gauge in you sometimes "bridge" the groove because the feeler gauge is wider than it. If this happens you will think your tappets are fine but it will still be noisy. Sometimes you can grind the sides of some feelers to make them the same width as the valve stem but then they tend to "buckle" if you're not extremely careful pushing them in. I've also heard of people using dial gauges to set tappets but have never seen it done.

<hr width=100% size=1>

 

[clio]

what kind of oil pressure do you have. for a quarter of an inch, your cam shaft bushing's or camshaft may be worn. I would suspect the push rods my self take them out and see if they are bent.As for the efect on your engine the valves will be opening late and closing early with a quarter of an inch you are lucky it runs at all

<hr width=100% size=1>

 

[owen]

thanks again for advice and replies. the oil pressure is spot on as per handbook. Im surprised that as the system 'wears ' that the tappets become tighter?

<hr width=100% size=1>

 

[Heckler]

as the system wears the tappets become looser. and if you have a quarter of an inch clearance you have something seriously wrong, camshaft runs in bushes which can wear, vunlikely, cams are egg shaped, the pointed end of the lobe wears but not usually the fat end so unlikely to be the prob, cam followers can wear could be the prob, push rods bent? mmmh, not unless the timing has been out and pistons have touched valves. rockers wear on the pivot bushes or on the bit that touches the valve. all the wear points usually are cumulative and dont usually produce such a big gap, what will cause it is bad lube due to not changing oil regularly (and i mean leaving the same oil in for years) or lack of oil to the rocker box area and usually because the lube drains down to and lubes the cam followers this exarcebates the wear in the train.
stu

<hr width=100% size=1>

 

[david_bagshaw]

wide tappets can also cause an increase in performance, was a trick used frequently when tuning the bmc a series engine.

<hr width=100% size=1>David
<A target="_blank" HREF=http://www.euroboating.net>http://www.euroboating.net</A>

 

[Birdseye]

tappets can become looser if the wear ocurrs(?) in the valve train, but this usually takes a long time. they often become tighter as erosion occurs on the mating surface of the valve head. surprised me when i first found it, but thats what happens on the yanmars i have had. suppose it must depend on the characteristics of your particular engine exactly where the wear happens and which way the gap goes.

<hr width=100% size=1>

 

[vyv_cox]

I agree with Stu's analysis. I suggest that with tappet clearances of 1/4 inch you need to find out why, rather than simply adjusting them. It may be no more than the lock nuts loosening but it does need investigation.

One other possibility on the lubricant front. Cam/tappets is a difficult contact to lubricate because hydrodynamic lubrication cannot be achieved in the time available. In high performance engines this is the critical contact (remember Ferrari's reliability problems a few years ago?) Not so likely in a relatively elderly marine diesel, but using very cheap oils may be contributing to high wear rates. Suggest you use an API CD spec oil from a reputable manufacturer.

<hr width=100% size=1>

 

[steve_cronin]

As the owner of at least one BMC A series for the last 38 years (currently 2 MG Midgets) I would like to hear more of this.

Steve Cronin

<hr width=100% size=1>

 

[david_bagshaw]

Steve follow the link <A target="_blank" HREF=http://www.euroboating.net/how2/tappetclearance.jpg>http://www.euroboating.net/how2/tappetclearance.jpg</A> for a scaning from the book Performance Tuning in Thoery & practice by AG Bell isbn 085429 275 6,

also in the book is a good section on cyl head mods

have fun

<hr width=100% size=1>David
<A target="_blank" HREF=http://www.euroboating.net>http://www.euroboating.net</A>