Tappet clearance query

Alty Ian · Post by **Alty Ian** » Wed Mar 16, 2016 10:17 pm

Having done its first 500 miles after a rebuild I have checked the valve clearances. When I did the head they were all 4 thou inlet and 6 thou exhaust as the manual.

I had expected the clearances to widen but an experienced engine man I know predicted they would tighten up, and he was right with one exception.

Most are 1 thou less than they were ie 4 thou is now 3 thou and 6 thou now 5 thou. So my question is,..

.... is it worth adjusting a valve if the gap is 1 thou less than manual,... because the engine runs very smoothly with no tappet noise or issues I can see or hear.
(I know gaps are widened for racing engines). Or would tighter gaps affect the performance of the engine in some detrimental way?

Tony · Post by **Tony** » Wed Mar 16, 2016 11:44 pm

If it ain't bust, don't fix it

abowie · Post by **abowie** » Thu Mar 17, 2016 12:41 am

I'd leave it.

christopher storey · Post by **christopher storey** » Thu Mar 17, 2016 9:08 am

I also would leave it . It is extremely difficult to get the clearances accurate to 1 thou, and one ends up resorting to things like the 1/2 thou sized shims which you can get hold of ( I think Hutsons have them ) . The worst that is going to happen is some burning of a valve or two, and with a gap only 1 thou down from nominal that is going to take many many thousands of miles , if it ever happens at all . It also presupposes that one reliably ( and reproducibly ) can measure a sliding fit to an accuracy of 1 thou , which I am doubtful about . As far as performance differences are concerned, the effect on the valve timing of a 1 thou difference is minimal

chrisfell · Post by **chrisfell** » Thu Mar 17, 2016 9:08 am

Er,..... no. Tappet adjustment is routine maintenance. Under no circumstances would I leave it, or any other routine maintenance task. I wouldn't run tyres slightly under inflated. I wouldn't run the engine with its oil level slightly below the minimum. I wouldn't drive with brake pads slightly below the service limits.

Adjusting tappets is a necessary and, for my money, a satisfying and therapeutic task. It takes time and planning - proper tools and access to a stock of shims. And there is something hugely rewarding in finishing a job knowing that the result is bettter than when you started.

(Now, Granny, this is how I think you should suck eggs.....)

Tappets, that is the cam lobes and their followers, will not wear noticeably if the engine's lubrication is correct. However, tappet gaps will close slightly during running in as the various components settle in. After that tappets may close as each valve hammers itself into its seat over time - but we are talking 10s of thousands of miles. In the 45,000 miles since I've had my car each tappet has been checked each winter and adjusted probably once or twice. I know this because I have accumuated eight spare shims. Checking and adjusting tappet gaps allows us to monitor valve seat recession. When fuels had high lead content this was never an issue, and for our XK engines with hardened valve seats fitted by the factory, it shouldn't be a problem. However, it always pays to check.

Routine maintenance is important. It keeps our engines, which are already amongst the most bullet proof ever designed, going for decade after decade.

Proper tools. Spanners of the appropriate sizes to remove the cam covers, camshaft bearing caps, and (if adjustment is required) the cams from their sprockets. Feeler gauges - imperial. An accurate imperial micrometer. Beware, chinese and eastern european micrometers can be very inaccurate. Mine is second hand, bought from a retired engineer. Lock wire and pliers.

Shims, one gets from your favourite main Jaguar E-Type parts supplier. Most will post the day you phone. Other useful tools. A magnetic probe, useful for removing shims from the top of the valve stem. Flashlight, for seeing in the darker recesses under the bonnet. Clean rags, to stuff around the cam chain slot preventing bits of lockwire disappearing down and into the sump. Clean margarine tubs, for putting the various nuts washers and bearing caps in while they are being removed or replaced. And for those of us formerly with 20-20 eyesight, reading glasses, so I can read the scale on the micrometer, and do the fiddly bits with lockwire and pliers.

As for bullying Granny, I'm sorry, I just can't help myself.

christopher storey · Post by **christopher storey** » Thu Mar 17, 2016 9:12 am

There you are .... two diametrically opposed views within minutes of each other :D :D :D . The only thing I would say about ChrisFell's view is that the whole purpose of shimmed tappets is that tappet adjustment is eliminated from routine maintenance . The rest is hard to disagree with, save that for a 1 thou difference it is a counsel of perfection

Oh... and that I regard adjusting tappets with the head on the engine as a recipe for, sooner or later, a valve having an unscheduled meeting with a piston

Post by **mgcjag** » Thu Mar 17, 2016 9:18 am

Hi Ian....you have gone to a great deal of work on your restoration...its only 1 thou but thats 25% how do you feel about not having it spot on..... after all Jag never said + - 1 .....Steve

Alty Ian · Post by **Alty Ian** » Thu Mar 17, 2016 7:31 pm

Thanks for all the interesting replies,... I'm something of a perfectionist and on balance I think I would rather know its spot on,..... before I let my son loose and
start impersonating Stirling Moss with it

Gfhug · Post by **Gfhug** » Thu Mar 17, 2016 9:07 pm

Ian, did you consider fitting the later cams that have 12-14 thou gaps?
All reports say quieter and smoother running. Ken Jenkins normally has some good second hand ones for sale.

Geoff

Alty Ian · Post by **Alty Ian** » Thu Mar 17, 2016 11:25 pm

Hadnt even considered that, its very quiet and smooth now :)

christopher storey · Post by **christopher storey** » Fri Mar 18, 2016 8:51 am

Ian : there is one other thing you might consider. I imagine your measurements were taken stone cold , as required. Before going to considerable trouble, I would take another set of measurements with the engine hot. There is one school of thought that with alloy heads, the clearances actually increase when hot

Gfhug · Post by **Gfhug** » Fri Mar 18, 2016 9:04 am

Christopher, your comments of increasing clearances seems to be what I got.
I have fitted the later cams, stone cold all but one clearance was 12 thou, one 13.
When hot the clearances were 13-14 thou.

Anyone else got similar results?

Geoff

Alty Ian · Post by **Alty Ian** » Fri Mar 18, 2016 12:17 pm

Good points, all mine were measured stone cold.

Just for interest I might put the covers on warm it up and remeasure them all. Give me something to do

Gfhug · Post by **Gfhug** » Fri Mar 18, 2016 12:31 pm

Ian, if you're going to warm it up at least go for a drive to justify the time and fuel burn.
Far more fun than just listening to it ticking over (which, admittedly, is rather nice anyway) :D

PeterCrespin · Post by **PeterCrespin** » Sat Mar 19, 2016 1:46 am

Alty Ian wrote:Hadnt even considered that, its very quiet and smooth now :)

Well it would be with tight clearances! :-)

It goes to show the wisdom in so many aspects of engine building, that if you're going to settle for a slight error one way or another, it's almost always better to be slightly slack than too tight.

As for measuring for a cold spec on a hot engine, that's naughty. Like measuring for a cold spec pressure on an underinflated tyre once it has warmed up a bit...

christopher storey · Post by **christopher storey** » Sat Mar 19, 2016 7:57 am

Peter : I don't think i can agree that the cold/hot tyre is analogous to the question of alloy heads . Conventional wisdom is that clearances close when hot,hence the danger of valve burning caused by leakage and/ or too short a time for heat transfer from valve ( particularly exhaust valve ) to seat . However, empirical evidence shows that the reverse happens on some alloy heads : because the alloy expands more than the ferrous components , the valve clearances expand rather than contract and the dangers of inadequate clearance are eliminated

PeterCrespin · Post by **PeterCrespin** » Sat Mar 19, 2016 9:55 pm

My point was not deadly serious, and was as much qualitative as quantitative.

To wit, fudging the value of one variable (in this case temp) to massage another known errant variable measure close to factory spec (be it pressure or tappet gap) is somewhat delusional. Perhaps a better example would be if, say, a given interference fit for an outer race in an alloy casting was sub par at room temp, once could chill the casting in dry ice, or warm the bearing in boiling water, and hey presto - suddenly it's in spec!

Except that it isn't.

Alty Ian · Post by **Alty Ian** » Sat Mar 19, 2016 11:57 pm

In the end I just came to the view that I want it bang on, no matter what.

Its almost midnight, my backs aching, my fingers are sore and now every inlet valve is 4 thou and every exhaust valve 6 thou. I got rather carried away and started sanding down half a thou off some shims to get them perfect.

Something else I noticed,... just by not torquing down the cam bearings to the correct setting (2.0kg m) added a thou or so extra clearance.

Timing is set up bang on. I was surprised how much the new timing chains had stretched over just 500 miles, nearly half a turn of the adjusting tool.

Its going for a run tomorrow now

8)

Link:
BBcode:
HTML:
	Hide post links

Link:
BBcode:
HTML:
	Hide post links

Link:
BBcode:
HTML:
	Hide post links

Link:
BBcode:
HTML:
	Hide post links

Link:
BBcode:
HTML:
	Hide post links

The 'E' Type Forum

Tappet clearance query

#1 Tappet clearance query

#2

#3

#4

#5

#6

#7

#8

#9

#10

#11

#12

#13

#14

#15

#16

#17

#18