When are tight valve clearances too tight to ignore?

[esray]

Thanks to the stalwarts on this forum, Roy the Rover is purring out of his exhaust like a contented cat

However, I have checked the valve clearances and they are as follows:

Inlet (0.008 - 0.010) Exhaust (0.013 - 0.015)
1 0.007 0.011
2 0.007 0.011
3 0.008 0.011
4 0.008 0.010

Are these acceptable or too tight to ignore, please :?:

 

[redrover]

When are tight valve clearances too tight to ignore?

When the camshaft starts squealing at cruising speed because the back of the cam is constant contact with the bucket!
That's when you simply CAN'T ignore it!!

Your exhausts are tighter than ideal at present, but it's when they get down to under 10 that you start getting into problems. I've been there!
Others will have more experience than me of when to reset them.

Michael

 

[harveyp6]

At those clearances I wouldn't be panicking. (Yet)

 

[Keith Coman]

Specifically those clearances are below sub-minimums and I would be taking remedial action asap. [Generally one is right inside the measurement margin of error when getting down to one or two thou either side of minimum spec.]

FI: I set my gap target at two thou over maximum spec.

 

[chrisyork]

Definitely the book answer, Keith! But also remember that these clearance limits are those recommended by Rover for when setting up the engine at a sealer service. So the assumption will have been that they would get a tad tighter before the issue was cought at service.

That said, I'd be making a plan to address them at the next convenient opportunity, say within 6 months depending on what mileage you do. Perhaps Harvey can suggest any way that you can get prepared by guessing what shims will be required in adnance?

Chris

 

[harveyp6]

Perhaps Harvey can suggest any way that you can get prepared by guessing what shims will be required in adnance?

There's no way of knowing what's in there, so no way of knowing what you'll need.

It won't be doing a high mileage I wouldn't think, so I'd check them again when I felt the need depending on usage, but I certainly wouldn't panic yet. I'd be adjusting when the exhausts were less that 10, and inlets less than 6. and we're not there yet.

I bet if you checked all of the cars on here that hadn't been recently checked most would have a couple of tight ones.

 

[JVY]

Sorry if it's a silly question but was the engine stone cold when you did the check? Even if left to cool for a few hours, you might still get misleading results as the gaps close up pretty quick when the engine is warm. Just wondered, as you said your car was running OK.

 

[esray]

Sorry if it's a silly question but was the engine stone cold when you did the check? Even if left to cool for a few hours, you might still get misleading results as the gaps close up pretty quick when the engine is warm. Just wondered, as you said your car was running OK.

My thanks to you all - I now feel more comfortable and I will have another good look in 6 months time! 8)

JVY - Yes, checked them stone cold (well, as cold as they get when its nearly 40 degrees celsius)!!

 

[Dave3066]

JVY - Yes, checked them stone cold (well, as cold as they get when its nearly 40 degrees celsius)!!

Wish I had that problem to contend with :roll:

My valve clearances are tighter than that and I'll be addressing them when time permits. Doesn't seem to be having a dramatically adverse affect at the moment and I probably do more mileage than most. Of course I won't really know the full effect until I sort them.

Dave

 

[harveyp6]

I'd be adjusting when the exhausts were less that 10, and inlets less than 6. and we're not there yet.

Having re-read that it doesn't sound quite how I meant it. It's unlikely that they will all get down to 6 &10 at the same time, even if they're all a bit tight, but once I found one down to that, that's when I'd do them, and obviously do all of them at the same time. Otherwise I'd give the owner the option. Unless of course I was short of money at the time, in which case anything remotely approaching one thou tight would mean they all needed doing immediately. Can you leave the car with me sir? :wink:

 

[esray]

Dear All,

I am probably missing something really obvious here, but why does the gap get smaller with wear? If the tappet wears, or the shims compress, surely the gap would get bigger? Is it wear in the valve seats allowing the valve stems to return higher when fully closed?

ATB Ray

 

[redrover]

Dear All,
I am probably missing something really obvious here, but why does the gap get smaller with wear?
ATB Ray

Simple reason, complicated answer.
Unleaded fuel burns far hotter than leaded (hence why lead was originally added to it),and causes the valves (especially exhaust) to become super hot when the gases are leaving the combustion chamber. When the valve closes, the hot valve makes contact with the valve seat, micro welds itself in place, and then gets pushed back open again by the cam. As this process goes on and on for millions of revolutions, microscopic bits of material are gradually transferred between seat and valve, and ultimately dislodged altogether. The result is valve seat recession, where the seat gets thinner and thinner, and the valve has to travel further 'upwards' in order to make contact with the seat, thereby closing up the gap between the bucket and cam lobe.
The second symptom of this is lost compression due to a poor valve seal.

I think there's a bit more to it than that, but that's the basic gist anyway.

In my experience, fuel additives only delay the effects and it is difficult to get the right measure. A new set of hardened valve seats (which is all that 'unleaded head conversion' means) is the only cure.
Some engine, such as the Jaguar XK, had valve seats that were hard enough to cope when unleaded came on stream anyway, so don't need the 'conversion'.

Michael

 

[esray]

Thank you Michael,

Very interesting.

I assume then that as the valve seat recession reduces the gap, you simply reduce the thickness of the shims to reopen the gap?

 

[redrover]

I assume then that as the valve seat recession reduces the gap, you simply reduce the thickness of the shims to reopen the gap?

Got it in one.

Adjustment by shims makes adjusting clearances a very difficult task from a maintenance perspective, but for an OHC engine where the cam is acting directly onto the valve (the Rover setup), it's probably the most effective way of doing things.

In a SOHC engine with a hemi-head (necessitating inclined valves), rocker gear is needed which could have the screw up/down adjusters you get on push-rod engines incorporated into them, thus making adjustment far easier.

The only similar solution for a direct acting cam is the off-the-wall Alfa Romeo approach: instead of having one big fat cam pushing on the whole top surface of the bucket, they had two very thin cams pushing on the extreme left and extreme right of the top surface of the bucket. In the gap in between, you slotted in an Allen key and screwed an adjuster that was contained within the bucket. A clever way of doing things, but it's much harder to get the buckets to rotate consistently, and if the cams wear at different rates you're buggered!

I suppose they could have incorporated the adjuster part of the Alfa type into the Rover type setup, but given that you would still have to go through the 'inconvenience' of removing the camshaft for access, the 'convenience' of having a screw-up adjuster is negated. This coupled to the fact that threads tend to rotate, and shims don't squash so easily, means that the maintenance difficulties are offset against accuracy.
And if you had new hardened valve seats fitted, the maintenance aspect would be deleted altogether.

Probably not a very useful musing, but it kept me entertained on my commute home!

Michael

 

[Keith Coman]

Earlier this year I had hardened valve seats installed on my '69 TC but recycled the pre-existing valves after having them cut to match the new seats. Valve gaps were set as close to factory spec as was possible given my supply of shims, erring on the wide side when it was impossible to come up with a shim combo that would not comply. Within 2 weeks of regular use [to see how things went] all the valve gaps had closed up to below minimum factory spec - to a point where the car had no power above 2000 revs. I put this down to some natural bedding in process.

In resetting the gaps, I reverted to my normal practice of setting the valve gaps over-size by a target of 2 thou over factory maximum. This was not universally possible again due to shim availability restrictions, so a couple of valves were 3 thou over maximum. So far [touch wood] the process has worked and the valve gaps have showed no change - hoo-rar! OK - the engine is a bit rattly but not over-much and I suspect that factory valve settings were a compromise between engine noise [not to frighten the customers] and mechanical ideals.

To be perfectly blunt I find Rover's use of shims to be a real PITA from a long term ownership and maintenance point of view. It probably made good sense in the 1960's, but how much easier it would be had they adopted a system such as that used by Ford in the Pinto engine! As a result I'm very old school and subscribe to the mantra "A rattly tappet is a happy tappet" - because one with a tight setting is certainly likely to be wearing a frown :cry:

Yours in shims
Keith

 

[redrover]

Hi Keith,

Strange that your clearances tightened up after having hardened seats. My experience was the opposite! My clearances were set up on a jig with a broad selection of shims at hand to get them spot on. I checked them again after fitting the head, and after the first heat cycle, and they had remained the same. However, over the first 500 miles I developed a helluva lot of valve chatter, and on checking found most of the clearances to be between 2 and 6 thou OVERsize. Now reset, and they've been perfect ever since (1300 miles).

Regarding your preference for oversize gaps... I can see your logic, but if you have unleaded seats what's the point? It's just compromising volumetric flow and generating noise. If anything, you could afford to get the clearances a few thou tighter. Let's face it, 10 thou is still way beyond the range of heat expansion, and would contribute to greater valve lift. There shouldn't be any worry about the extra spring stress as the springs are the same for inlet and exhaust, and the inlets are factory recommended down to 8 thou, so why not go that far on the exhaust? As Chris mentioned earlier, the exhaust clearances are only 13-15 thou to compensate for worst-case-scenario valve recession between services - an issue you have pretty much eradicated in fitting hardened seats.

Just a thought. I appreciate experimentation is the mother of complication, so better safe than sorry!

Michael

 

[esray]

I assume then that as the valve seat recession reduces the gap, you simply reduce the thickness of the shims to reopen the gap?

Got it in one.

Adjustment by shims makes adjusting clearances a very difficult task from a maintenance perspective, but for an OHC engine where the cam is acting directly onto the valve (the Rover setup), it's probably the most effective way of doing things.

In a SOHC engine with a hemi-head (necessitating inclined valves), rocker gear is needed which could have the screw up/down adjusters you get on push-rod engines incorporated into them, thus making adjustment far easier.

The only similar solution for a direct acting cam is the off-the-wall Alfa Romeo approach: instead of having one big fat cam pushing on the whole top surface of the bucket, they had two very thin cams pushing on the extreme left and extreme right of the top surface of the bucket. In the gap in between, you slotted in an Allen key and screwed an adjuster that was contained within the bucket. A clever way of doing things, but it's much harder to get the buckets to rotate consistently, and if the cams wear at different rates you're buggered!

I suppose they could have incorporated the adjuster part of the Alfa type into the Rover type setup, but given that you would still have to go through the 'inconvenience' of removing the camshaft for access, the 'convenience' of having a screw-up adjuster is negated. This coupled to the fact that threads tend to rotate, and shims don't squash so easily, means that the maintenance difficulties are offset against accuracy.
And if you had new hardened valve seats fitted, the maintenance aspect would be deleted altogether.

Probably not a very useful musing, but it kept me entertained on my commute home!

Michael,

Michael

Thank you very much - I have actually understood what you have been telling me - You have a talent there!

 

[OrganDoctor]

Dear All,
I am probably missing something really obvious here, but why does the gap get smaller with wear?
ATB Ray

Some engine, such as the Jaguar XK, had valve seats that were hard enough to cope when unleaded came on stream anyway, so don't need the 'conversion'.

Michael

Not sure about this, as I quickly burned out valves in my XK120 back in the late 60's when AMOCO sold "white Gas" which was a high octane unleaded fuel.

 

[esray]

Dear All,
I am probably missing something really obvious here, but why does the gap get smaller with wear?
ATB Ray

Some engine, such as the Jaguar XK, had valve seats that were hard enough to cope when unleaded came on stream anyway, so don't need the 'conversion'.

Michael

Not sure about this, as I quickly burned out valves in my XK120 back in the late 60's when AMOCO sold "white Gas" which was a high octane unleaded fuel.

Interestingly, I am using LUK unleaded 100 with with a lead additive - Based on previous threads, I now wonder whether it actually matters!!

ATB Ray

 

[Keith Coman]

Thanks redrover for your feedback. I was intrigued to learn that your gaps opened whilst mine closed following valve seat work. I wonder whether this was due to the fact that my new valve [unleaded petrol] seats were made of a bronze-like material [pale gold - almost brass - in appearance]?

Working hypotheses:
1. Current valve seat technology favours soft [bronze] seats so as to allow normal/conventional grade valves to be used? Or....
2. Valve seats can be replaced with ultra hard material and specially hardened valves? [The traditional "head conversion" process for unleaded petrol.]
3. The use of bronze seats shifts recession resistance [cushioning] from valve and seat in combination largely to the seat itself?
4. In new bronze seats, there is a bedding in process until the seat surface is "case hardened" that causes a mild form of valve recession during the first few miles of running and which thus closes the initial valve gap up?

The odd byways Rover engines doth take us eh?