Cylinder Head Thickness

[Westfaling]

I've finally got my cylinder head back from the machine shop after a light skim and fitting of thread inserts. Interestingly the shop would not use Helicoils in the alloy heads and insisted on solid inserts called "Wurth" from Germany.... hence the long wait. I see you can buy these in Blighty.
My question is do I need thick head gaskets? I searched the Forum and could find nothing concrete. There were however lots of posts from people who no longer seem active on the Forum!

The good old interweb produced the following: http://www.stag.org.uk/technical/page0034.htm
Seems like good advice. Thoughts?
The photos show the cylinder the least clearance from the valve guide.. others are slightly better with about 1mm clearance.
Nick

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[dasadrew]

Nick,

I can't remember exactly where I got some of this stuff from (probably the same source as yourself), but I copied it into my personal Stag Data Log (!) a year or so ago. Might be of help.

Drew

(By the way, the inserts themselves are not called "Wurth" - "Würth" is about the biggest and most renowned German company for fixings an attachments)

Head Skimming (copied from somewhere)
In order to determine by how mich the heads have been skimmed, the head height between the bottom of the camshaft bearing (half bore) and the face of the head should be measured. A new head is 112.64mm, the specified minimum is 112.40 (obviously, not much skimming allowed). Any amount of skim below 112.40 should be compensated by thicker gasket(s) which add 0.5mm or, if still not sufficient, by addition of a shim (Tony Hart sells "Savershim" which works very well, in my case). A combination of standard gasket on one head (not overskimmed) and thick gasket on the other (if overskimmed) can also be used.
Other sources from Triumph (tech reprints) say minimum is 4,427" = 112,45mm
My Head before skimming = 112,45 mm = exactly the minimum
10thou = 0,254mm
Thick gaskets are 20thou thicker = 0,508 mm thicker

My measurements:

My RH Head after skimming
RH Front 112,00 mm = 0,45 mm under minimum
RH Rear 112,15 mm = 0,30 mm under minimum
With Thick Payens, my RH head is then effectively:
RH Front 112,00 + 0,508 = 112,508
RH Rear 112,15 + 0,508 = 112,658
My LH Head after skimming
LH Front 112,05 mm = 0,40 mm under minimum
LH Rear 112,15 mm = 0,30 mm under minimum
With Thick Payens, my LH head is then effectively:
LH Front 112,05 + 0,508 = 112,558
LH Rear 112,15 + 0,508 = 112,658

[jleyton]

Hi Nick,

Even though I totally disagree with your machine shop about helicoils (as I posted in another thread), that head looks brilliant. I just hope that whenever I take mine off they can come up as good as that one

Cheers

Julian

[73stagman]

We use helicoils and the Wurth thread inserts at work. We use helicoils in ally because of the load ratings of the base material and the size of the threads. We do however use the Wurth inserts in Steel collars because they will take seriously more load than threads in the raw base material and the size of the threads (M16) We use the collars to tighten up subassemblies as we build our oil well drilling tools. Upto 45000 ftlb.... to do up and 60000 ftlb to undo!

Also Helicoils are pretty "thin" and if the original holes are badly damaged and not just the threads damaged then the helicoils wouldn't necessarily provide a full thread load capability. The Wurth style allows the original hole to be opened up and a high tensile threaded collar to be inserted and locked (the four light coloured pins you can see at the outer edge of the inserts. Bottom left on the manifold piccy not sure where they have gone on the other holes?) I guess its about reputation and the machine shop is obviously proud of it and wants to keep it that way. I doubt if you will ever damage those threads again! Great looking job

[Bob Heritage]

Our local supplier sells a Helicoil equivilent named Re-coil, this is made in Australia- could have saved you the wait

[Ian Durrant]

Hi Nick,

When you put the valves in do they stand proud of the head surface when seated. If they do you will either need to use the thick head gaskets or pocket the pistons. If you have the lower compression pistons without the "button" in the middle you should be OK compression wise via either method. If you are after top power outputs though I would steer clear of thick head gaskets!

[Westfaling]

Ian
I'm about to put the valves in to check and measure combustion volume. I have the flat top County pistons I was hoping not to have to use thick head gaskets as I'm only going to be around 8.5 - 8.8:1 C/R. I guess that will only leave pocketing the pistons if the valves are proud of the head. I'll report back my findings later this weekend.
Regards
Nick

[elvimto]

I use the inserts at work on many different applications they work great, I also used them on my cylinder heads well you could not beat the price

cheers Stuart

[Princejohny]

Some pics of you measuring combustion volume would be good, Nick. Never seen that done before.

On another subject, my boy's recently started 3 year Phd at Lincoln University NZ. I think that's just a couple of miles away from you. Hope to see your Stag on the road when we visit next year!
Johny

Ian
I'm about to put the valves in to check and measure combustion volume. I have the flat top County pistons I was hoping not to have to use thick head gaskets as I'm only going to be around 8.5 - 8.8:1 C/R. I guess that will only leave pocketing the pistons if the valves are proud of the head. I'll report back my findings later this weekend.
Regards
Nick

[flying farmer]

Check the protrusion of the valves with the camshaft fitted and correctly shimmed as the valves are slightly open at TDC.
There should not be too much problem with a standard camshaft as the dish in the pistons is quite deep, and the overlap not great.If you can place the cylinder head on top of the block with everything timed up and no head gasket fitted, that will give you 40 thou clearance with the head gasket fitted.
Be aware though, that the cam timing will probably be out with this much skimmed off the head, and you need to check that the timing marks do properly indicate TDC as I have known them be as much as 4 degrees out. It is not a difficult job to redrill the cam sprockets to correct the cam timing, just a matter of thinking about the direction of rotation, and correctly measuring the amount required
Neil

[Westfaling]

Johnny
Get in touch when you're closer to visiting NZ. You won't be disappointed in NZ the South Island is an amazing place .... bit shaky though! Picture below is interesting of the corner of Colombo St and Kilmore St. down town Christchurch. (Feb 22 was the biggest aftershock to our Sept 4th earthquake - killed 180 people) Largest acceleration ever recorded under a metropolitan centre anywhere up to 2.2G.

Neil
Thanks for the advice on TDC I guess I could correct any error with an offset woodruff key on the crank pulley. I've had to do this on my MGB to put the US spec cam timing back to pucker UK timing.
Nick


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[V Mad]

Helicoil also make those solid inserts, as used in the Draper Expert Helicol spark plug thread repair kits.

[Westfaling]

Well it looks like it's thick head gaskets. Cylinder head thickness was 111.65 and 111.35mm (minimum being 112.45mm (4.427") ). also the inlet valves sit a maximum of .28mm (11Thou) proud of the head.
Ho Hum I've now got two sets of standard Payen gaskets going spare. Oh well I guess I can save one set for next time when it looks like new heads will be required.

[Ian Durrant]

Well it looks like it's thick head gaskets. Cylinder head thickness was 111.65 and 111.35mm (minimum being 112.45mm (4.427") ). also the inlet valves sit a maximum of .28mm (11Thou) proud of the head.
Ho Hum I've now got two sets of standard Payen gaskets going spare. Oh well I guess I can save one set for next time when it looks like new heads will be required.

If mine get to that stage i'll pocket the pistons (or get a machine shop to do it). Have you tried it as Neil suggests? You might not even have to pocket them as you have the dished county pistons.

[flying farmer]

I would still measure it all before fitting thick gaskets. The engine on my efi stag had valves standing 5 thou proud, and then had 8 thou skimmed off the block to get rid of distortion/corrosion of the deck faces. That is how I ended up with 10.1 :1 compression with the high compression pistons. I had to rest the head on the piston crown as they are 15 thou above deck height, then work the 40 thou clearance from there
Neil