As ever do this at your own risk. For most people you’re better off just getting heads machined by a specialist but if you’re reading this blog you’re probably already aware that’s not always the way I do it!
So this idea came from me wondering how I could easily clean up the cylinder heads on the V6 without additional machining. The heads were generally in good condition so I just wanted a fresh surface for the new head gaskets to seal well on rather than trying to remove any surface damage or warping. If you have this sort of damage this method is not for you.
When I came up against this problem I decided to do some research and found quite a few people online saying you could just do it with suitable abrasive paper and a sanding block. Now I get the idea but the engineer in me sees a good possibility of some part getting ground back more than another actually increasing the issues with the head that we’re trying to remove in the first place. Around this time I spoke with a few different people who have experience with engines and they all said much the same thing – machine skimming is safest and easiest but with enough care it should be possible to do a perfectly good job by hand, the problem is getting the whole thing completely flat which is very difficult by hand. Most people who had done this seemed to have done it on engines with small cylinder heads such as single cylinder machine engines which being small are easier to get flat by hand.
The problem was absolutely one of getting something suitably flat that would cover the whole head to get the whole thing even so I started looking into what might work. I was already aware of engineers surface tables which are used for checking flatness but these are large, heavy and very expensive as they’re often made of stone or tool steel. I then looked at getting some surface ground steel plate (where a thick steel place is ground to a precision flatness) but again this seemed to be expensive and awkward. After a bit of thought I had an idea…
Ok, so at first glance this seems like a daft idea but stick with me! The cheapest and most rigid precisely flat surface I could find was a piece of toughened float glass. Specifically toughened because it is created in such as way as to pre-stress the surface which makes it both stronger and stiffer but also much more brittle – this is the glass that breaks down into granules when broken rather than shards. Initially my plan was to just order a decent sized bit but that seemed rather wasteful so I thought about it and realised second hand furniture included quite sizeable bits of the glass. I began searching eBay and Facebook marketplace to see if I could find something suitable as cheaply as possible and after a week or so found this coffee table. It was nearby and listed as local collection only with some damage to the wood veneer and scratches on top surface of the glass (so the price was unlikely to go high) and 99p no reserve starting. Couple days later I was the proud owner of a £4 coffee table!
Now you may be wondering why we aren’t bothered about the surface scratches on the glass which is likely to be a problem with any similar furniture. The reason is twofold, firstly due to this being toughened glass any scratches are likely to only be very minor and secondly the glass has a whole other side which is unlikely to have any scratches anyway so we’re going to use the underside.
I took the table apart carefully removing the top and to give additional support I placed it flat on a 19mm (3/4″) thick bit of chip board carefully screwing an section of baton at each end to stop the glass sliding about while we’re working. This assembly when then placed on my carpet which is just very hard office carpet tiles on concrete so shouldn’t allow any appreciable movement so hopefully with that stack of support the glass should be perfectly flat even with a cylinder head on it! The abrasive I will be using is wet and dry paper in a range of coarseness, get a pack of each grade you plan to use, it will take a lot of it! I made sure the wet and dry would stay in place by spraying the back of it with spray mount adhesive and putting it in place. Spray mount should also peel off relatively easily when we need to change the paper. I used 3 sheets to create an area larger than the head face in both length and width.
So now we have our setup we need to prepare the cylinder head for this. When it came off the engine it was quite grimy as you might expect so this needed addressing.
So here you can see how it was when it was (almost) fresh off the engine but with the residual head gasket material scraped off. You can see the amount of grime isn’t too bad, I have already wiped some off the top half but the bottom is a bit more representative. You can also clearly see the outline marks from the head gasket that we’re looking to remove later. This step basically involved soaking the mating face in a de-greaser then wiping it all off carefully.
It’s not perfect but it’s a huge improvement on where we started. The combustion chambers are considerably better. There’s some residual on the face but its more staining than anything else and will be removed by the refacing. So now we’re ready to go.
First cover the wet and dry with your chosen lubricant – water should work but I found WD40 seemed to work better as it helped the head ‘glide’ more. A light oil like 3in1 would probably be even better as it’s a little thicker again and WD40 tends to dissolve the spray adhesive as you work making the wet&dry come loose. Take the head and place it on the wet and dry (I started at 120 grit) holding both sides lightly start to slide it across the surface. There are different approaches to this where you can angle the head first one way then the other to give a crosshatch pattern. In my case I generally moved it in a long oval and this seemed to give a nice even finish but as ever your mileage may vary!
You will find that as you work the oil and metal shavings will spread so I suggest doing it somewhere you don’t mind the mess!
This will take some time and effort. If the wet and dry wears down replace it. when you’re happy with the initial surface being clean of all the minor marks and debris you can go up to increasingly fine levels of grit for a better finish.
On the subject of finishes when I was doing this work I found the following information which relates the abrasive rating with the achieved resulting surface roughness – if anyone knows where this is from please let me know as I can’t seem to find out.
|US Grit||UK Grit||Ra µm||Ra µinch|
So in the context of this I’m working in UK grit. Unfortunately the only information I could find on the required surface finishes for head gaskets came from the US so is in Ra µinch (Ra being the roughness average of the surface) but luckily this table equates everything. Generally normal gaskets seem to need a surface finish of about 50-60 Ra µinch, modern multi layer steel head gaskets require 30 Ra µinch or smoother so we need to finish at a minimum of P320. I actually went up to P400 to be safe.
This is the comparison of the untouched head and the one with the first couple of grits done and so not quite finished but you can see the massive improvement made here.
I know judging by eye isn’t accurate but it’s clearly doing something good!
Keep going until you do all the grits you need and when you’re done then you need to check the flatness. I did this with an engineers straight edge (as opposed to a builders straight edge which is a big ruler) which cost £25 off ebay. This is a bit of steel that has been precision ground to be completely straight in one plane so is often only a couple mm thick but 70mm wide or more. You check the flatness by putting the straight edge perpendicular to the head (so it sticks up) and trying to slide a feeler gauge under the mid point (or as close as possible) of the area you’re checking. You need to check the width, length and both diagonals but also check across all the bores. The head should have no more than 0.075 mm off flat over the longest span on an iron head, or 0.05 mm off flat on an aluminium head on a V6. In my case the smallest feeler I have is 0.04mm so that proved it was good enough but to check it further I got some thin foil, checked the thickness with a digital micrometer which came out as 0.01mm and placed the foil on one of the central bridges of the head and put the straight edge on and it rocked on the foil. I then redid the test with the foil at the ends and the same again in other directions. Each time the straight edge was clearly resting on the foil first so the head must be flat to <0.01mm across the whole head. Unfortunately I neglected to take any photos of this stage but there’s plenty of information online.
That’s about as good as it gets so our £4 coffee table looks like a success. Plus I still own a coffee table – albeit with a few new scratches!