Topic

Have had a few people ask me about the shim I used to save my head so thought I'd put some info down to let people know now that its been 9 months, 4500 miles and I know it works.

At this point, 6 months after changing the head gasket for an MLS one I knew I had some problems..


Took the head off the car, cleaned it up and had a look. Over heating had caused faint rings to become quite deep, the fire rings being the circles you can see where the cylinders sit against the head.


Knew I would need quite an aggressive skim to get down to a flat surface but knowing the K series head and the fact that I have fire ring indentations already confirmed to me that the head had dropped below the magical 95 brinell hardness rating needed to prevent them from occurring in the first place.
So I had a few options, gamble on a scrap yard head, or pay for a tested VVC head at £600...
Gave it a little thought and decided I just needed to make the head face harder, stainless steel is about 395 brinell so it seemed such a simple idea, a stainless steel shim, thick enough to transfer the concentrated load of the cylinder.

So I got onto Gosnays and got a shim that they produce for use in k series turbo applications where they need to lower compression and ordered this. The 1.9mm stainless steel shim.



Now there is a shim that comes part of the MLS gasket set, bit it is too thin for what I needed. Not only do I need to to protect the soft ally of the head but I also needed it to be thick enough to spread the concentrated pressure of the cylinder pressing into the head.
This is the shim from the MLS gasket set.


This it the 1.9mm stainless steel shim.


Realized why I'd not put this up sooner, because my camera packed in and so have no more pictures!

Anyway, I did some work on the head and then took it for a skim. Now I could probably have just fitted the shim over the rings but its going to lower compression as it is designed to do in turbo applications.
So the more I could skim off the less of a difference I was going to make when the shim went on.

Cleaned the skimmed head and the shim with ethanol and then bonded the shim to the head face using Loctite 5920.
http://www.loctite.co.uk/cps/rde/xchg/henkel_uke/hs.xsl/fullproduct-list-loctite-4995.htm?countryCode=uke&BU=industrial&parentredDotUID=productfinder&redDotUID=1000000IYQW

Have worked will all types of Loctite but it was the one that stood out for what I needed and as I said, its working...

Clamped the shim onto the head just so I could clean up any excess that squeezed out, you could do the same with just pressing it down by hand.

10 mins later I bolted the head to the vehicle and torqued it up, I then covered the engine and left it for 24 hours to fully cure before fitting all the other bits.

As pointed out its been 9 months now and the engine is fine, I don't know exactly how much the Gosnay's shim was as I just put it on my card but I don't think it would be much more than £30-£40. A lot better than the £600 quoted for a tested VVC head!

I should really do a compression test to see what the effect has been, will try get that sorted next time I change the plugs.

Interesting. Well done, 9 months proves it.

Can't believe this hasn't been thought of before. The K head is ball peened to harden it isn't it? This would seem to me a good move for any head that needs a skim, if it lasts. Did you use the mls gasket as well?

Yes, used the MLS gasket just didn't use the shim that comes with it, substituted it for the stainless one.

You have earned a bow and applause Darren this is amazing news... I might be tempted to call you if my car goes BOOM

That is brilliant, was the loctite just to keep the shim attached to the headduring rebuild or does it do any sealing? Do they recommend this in the turbo application? ii would guess a machine shop could put a micrometer across the shim and just take that much off of the head to keep the CR constant. So if you could get a gas flowed but soft head this is a really good way to up the output a bit

In Turbo applications it is only there to lower compression a bit to get the right compression ratio for what they need. I only used the loctite to stop the gasket from sliding about under heat and vibration. Once the head bolts are it it would probably be able to move about 3 mill in each direction if there was no clamping force, just the same as the MLS gasket itself.

You cant take 1.9mm off a head as the maximum you can take off safely is 0.75mm, so you're left with 1.15mm difference.

I also took the time to fettle the head a little bit whilst I had it on my kitchen table, opened up the valve seats a little and polished up the combustion area. This will have lowered compression a little more as I was taking material away.



From this:


To this:


Hoping a remap can take advantage of everything I've done at some point, but till then it runs fine.

I have heard of this in other cars but assumed that the engine builders didn't do this on the k-series for a reason. It was an option when I wanted to lower the CR too, but I went with custom pistons.

Kudos to you for trying it and finding it works though as I am sure many will follow now they have seen what it involves.

Yea took me a lot of reading up on how metals react next to each other, how vibration is effected having to pass through different metals etc etc.

Thing is the shim is designed for that purpose, designed to sit there and deal with the heat and the pressure so a lot of the research was just conformation of what the manufacturer was quoting.

Even went so far as to work out how it would effect the torque setting at the head bolts.

Thing is you hear of loads of heads being scrapped just because they are soft, or a skim has revealed an area of porous metal so as I'm not scared to take my engine to pieces every now and then I thought I'd give the shim a go, if it had failed I'd only be £40 out of pocket and would have to buy a new VVC head anyway.

If it saves someone the cost of a brand new head then it was worth the writeup.

OK thanks for that, I think you can take a skim from the block as well, but by then you are starting to get to the point where a correct hardness head makes more sense. . The same applies to high compression pistons as well I suppose. however this remains a very useful tip. With stainless the metallurgists can play tunes with the temperature coefficient of expansion so I wonder if the got it close to LM25?

cjj wrote: wrote:

I have heard of this in other cars but assumed that the engine builders didn't do this on the k-series for a reason. It was an option when I wanted to lower the CR too, but I went with custom pistons.

Kudos to you for trying it and finding it works though as I am sure many will follow now they have seen what it involves.

There is an engineering company near me that will weld a new face onto a K series head if it is soft/porous or been skimmed too much already. They claim a 100% success rate.

cjj wrote: wrote:

I have heard of this in other cars but assumed that the engine builders didn't do this on the k-series for a reason. It was an option when I wanted to lower the CR too, but I went with custom pistons.

Kudos to you for trying it and finding it works though as I am sure many will follow now they have seen what it involves.

Low CR pistons are the better way to do it as spacers can do funny things to the squish during combustion. That said, we are running a 3mm shim in my supercharged engine, as we already had the Scholar bottom end running NA so a shim was much cheaper (even though it had to be custom made and cost a lot mroe than 40GBP). Dad's been reading up on things and thinks with a slightly redesigned shim we can improve the squish so we'll see.

We use a special head sealing paint between the head and the spacer (I wouldn't be keen on using nothing), it's a bugger to use but seals it very well. Use a normal Payen elastomer gasket on between spacer and block.