Valve-clearance checking necessity

[hatman]

Sorry to parade my ignorance (but, as they say, 'If you've got it, why not flaunt it?') - on this occasion it relates to the necessity, quoted in the Owners' Manual, to check valve clearances after routinely torquing up the head bolts. My question is, simply, why?

With pushrod engines, tightening the head bolts does of course squeeze the whole shooting works closer together, valves and all - obviously the clearances get closed up. But with the twinc, the valves, camshafts et al are all housed together in the head and, as far as I can visualise, tightening the head bolts merely compresses everything below the cams/valves (and their clearances, which are dialled in to the arrangements within the head) so how can that affect the clearances?

Another aspect of this is that, having carried out the recent head gasket work, we installed the new valves and springs and shimmed the whole lot up to tolerances on the bench, following which we re-installed the head and torqued up the head bolts, it being far easier to shim up etc with the head on the bench rather than back on the car.

Clearly, the manual, being the fount of all knowledge, is right, so what am I missing? (apart, that is, from many of my grey cells).

[rgh0]

I often see a 1 or 2 thou change in valve clearances between when I shim a head on the bench and when it is installed. The slight distortions of the head as it is bolted down or may be the greater rigidity of the head on the block probably cause this.

I never see any measurable change when retorquing an already mounted head. I always check the valve clearances as the last thing I do before putting the cam cover back on an engine but thats more a double check to ensure I have not screwed anything up while I was doing what ever work had the cam cover off in the first place.

Rohan

[iain.hamlton]

Howard,

I am guessing here, but, perhaps as you have the cam cover off, you may as well check the clearances. Also, if you have run the engine a few hundred miles before pulling the head down again, the valves may have "settled in" a bit. Agree though, the reason for doing it isn't as obvious as with push rods. I wouldn't expect you to find any adjustment needed. I'd be interested to know what you find.

Are you still going to use Tetraboost?

best regards, Iain

[hatman]

Yes Iain - with new valves and freshly ground seats there's even more reason to use plenty of lead in the fuel to put a good healthy coating on the new stuff so I can bounce off the rev limiter without worrying about valve recession!

Interesting views on the likelihood or otherwise of valve-gap variation on torquing down the head, particularly as they seem to bear out my own bemusement at the stated necessity in the manual. What I can say is that, having shimmed up to book tolerances on the bench, a subsequent in-car check shows that they've all closed up by about three thou - bugger!

Surely a virtually uniform close-up rate across the whole lot can't be explained by head distortion on bolting down, can it Rohan? (especially so as the head's just been skimmed and is now as flat as the proverbial).

[rgh0]

I have rarely seen it that much and totally consistent across all valves but its similar to what I see i.e. The clearances usually close up

On thinking about it a bit another possible mechanism is the head being compressed by the head bolts load. This would close up the gaps. I have not done the calculations but 3 thou seems possible given the loads involved.

I always assumed with a straight head it was due to the head flexing under the non symetrical spring loads onto the cams and this affecting the clearances on the measurement when the head was on the bench and this flexing dissapearing when the head is bolted to the block.

Rohan

Rohan

[steveww]

Not too sure of why but my experience some of the gaps moved up to 4 thou after a 1000 miles after new cams, valves, seats and guides were fitted. Settling in as someone suggested. As it only takes a few minutes I always check the gaps whenever the cam cover is off.

[hatman]

Aye, Steve, it may only take a few minutes to check but removing the cams and re-shimming is a laborious pain in the backside - unless of course I'm doing it wrong.

[steveww]

No you are doing it right, it is a pain. Of course you never have the right shims to hand and have to order them in.

[poiuyt]

My recent experience (search on Tappets in the elan group) suggests a gap check is the thing to do whenever you have the cam covers off. The head was re-done at a race shop with experience on the twin cam but one of the tappets was destroyed after the gap went away about 8000 miles after the re-build and the other intakes were at around .004.

Yes, it's a pain to re-gap, but easy just to check, and it may help you avoid a breakdown far from home. My recommendation, when you do have to re-gap, is to set the gaps at the wide side (.007 - .008 intake) and the same with the exhausts (.010). This will allow for some valve recession without causing any problems.

Our Elan was out of commission for 10 weeks because the problem developed at the start of the tax season and we had no time to repair it until a week ago.

Steve B.

[elanman999]

Yes it is a pain shimming the tappets, what you need is a tool to hold the sprocket so the chain does not go loose. I have made one and if you pm me I could make you one too.
John E

[steveww]

I can not be bothered messing about trying to keep the cams in place, it is more of a pain than just lifting the cams right out. Timing the cams is not difficult and adjustable sprokets makes it easier.

[ppnelan]

I too have had the valve clearances close up when the head is installed, and it seems to be caused by wear in the cam bearings (or possibly camshaft journals?). When the timing chain is fitted, it pulls the end of the camshaft down hard against the bottom bearings, particularly at the front ends. This reduces the clearances. When measured with the chain off, the valves & springs push the camshaft up against the top bearings giving a larger clearance. If the clearances change by much, it probably means it is time to get new cam bearings !!

That's my theory anyway....
Matthew

[denicholls2]

No insight on the clearance issue. But if you are adding a lead supplement to your fuel to avoid valve recession, it's highly likely that you are shortening the overall life of many of your engine components significantly by trying to avoid a problem that does not exist, i.e., valve recession.

Decades of unleaded fuel use in the U.S. without special modification to classic car valve seats has not pointed to an increased propensity to "recess" valve seats in older engines. If it was a problem, we would know about it.

However, the use of lead in fuel demonstrably and significantly reduces the life of an engine between rebuilds.

You can find more discussion of this on the Yahoo Europa site. My advice is don't pay extra to wear out your engine faster to avoid a problem you will not have. If you really need higher octane, try to get there without adding lead.

Until I joined Lotus lists and started hearing this talk of using lead in Eurpean countries to avoid valve recession, I'd never even heard of valve recession, and I've owned lots of aluminum head engines running on unleaded regular, many of which were manufactured before unleaded fuel became the law of the land. None of these cars were retired due to engine failure, and all logged over 100,000 miles of service. The only machine that ever warned me against the use of unleaded gasoline was my Honda SL-350. Thirty some-odd years on, I still have that bike and it doesn't have recessed valves.

My belief is that opportunists are using the transition of European countries to unleaded fuel and the FUD of older car owners to make money selling snake oil. Most snake oil won't damage your engine, however, and leaded fuel will.

[hatman]

Interesting post dnicholls2 but, without trawling through the Europa site you mention, how can leaded juice create engine damage? Care to give us a precis? And, on that premise, are you/Europa site claiming that for the nearly 100 years that leaded petrol was the staple diet of engines on this side of the ditch it was actually surreptitiously destroying them? Also, FUD?

Theoretical engine damage, interesting though it may be, cannot hold a candle to actual, real-life damage, which is what I've now got as a result of trying to sort out the closed-up valve clearances on my blessed twinc. In a nutshell, I've succeeded in breaking off one of the camshaft bearing shell-cap hold-down bolts and, of course, it's sheared just inside the cylinder head. What the **** do I do now? All suggestions (preferably helpful but also any truly amusing ones) will be gratefully received.

[M100]

The only drawback of leaded fuel is that the lead scavengers, used to stop excessive build up inside the engine, combine with moisture in the exhaust and corrode the mild steel relatively quickly.

I've run my mid 80's rebuilt twincam (i.e. way before unleaded was even heard of in the UK) on unleaded with no additives for 10 years or so. I can't measure any valve seat recession in around 70,000 miles.

To get the broken stud out, can you remove the dowel to give you something to grab on with some snipe nosed pliers?

Alternatives, leaving the head in place:

Use a centre punch and try and tap the stud towards the outer edge to unscrew it.

Weld a chopped down nut to the remains of the stud with a mig welder

Use concentrated nitric acid. This will dissolve the steel stud and leave the aluminium intact.

Drill down the middle of the stud and use a parallel fluted extractor. Don't be tempted to use anything else particularly those with a taper or it will all end in tears!



You might strike lucky at a good motor factor/tool merchant. The only place I know you can definitely get them is from snap on

http://buy1.snapon.com/catalog/item.asp?P65=&tool=all&item_ID=10459&group_ID=1260&store=uk&dir=catalog

By using this type of extractor you only drill a 1/8" hole and the risk of the stud spreading or the extractor breaking are much lower than with any other type. You might be tempted to buy some more goodies at the same time They will also send you a catalogue for free. Postage is a fiver but compared to taking the head off it's cheap!