Race Engine Forum UK

Hi Guy

A brief history: I have an ex-works cylinder head for my Skoda Felicia rally car (with the 136 all-aluminium 8 valve OHV engine with non-crossflow head), which I wish to use in my latest build. However, the cylinder head in question has been previously machined to use with domed-top pistons. For a number of reasons I can't use the original Group A domed-top pistons, but can source flat top ones which will be legal in Group A and work well upto the rev limit I will be using the engine at. However, to achieve the correct compression ratio and also to restore the original head's 'squish/quench' area, the part of the head which was machined away needs to be replaced.

I have seen a number of repairs carried out (indeed some of your own work with damaged heads), and am wondering if it is possible/financially viable to have this metal replaced by welding, and then machining back to achieve the right combustion chamber shape and then, obviously, compression ratio when all the measurements have been made?
Is this possible, or should I just give up and get whatever head work I can afford done to another head? (I have a brand new casting, but the difference between this head and the new one is night and day).

Welding - even with thorough pre-heating - induces a fair bit of distortion - in this case (yellow sector) everything about an inch from the infill will 'move'. and the seat inserts would have to be replaced post-weld. The plug thread will be at risk too though that can be recovered using some insert method (Recoil or Helicoil are good, avoid Wurth Timesert). The head face will bend a lot. The rule is - with big weld ops like that - get another head if you can. If you can't - you need a top facility with coded welder to expedite this.

GC

Many thanks for your precise and informative response, Guy, as ever.

How about aluminium cylinder head welding in less critical area?

I have this BMW M42 engine (4 cyl 16 v 1,8 l used in 318is) cylinder head which is going to be fitted with BMW S50 (6 cyl 24 v 3,0 l used in M3) throttle bodies. Port measurements are identical on both head and bodies but head has far larger cutout for injectors. When head and bodies are put together, there is quite large pocket formed on port because of cylinder head injector cutout, maybe 20 mm long, 10 mm wide and 5 mm deep which tapers away when you go closer to valves.

I guess I could enlarge the cutout in bodies to match the cylinder head cutout in size but I want to use original ceramic 8 mm thick spacers between head and bodies for heat insulation and I'm not sure how well they respond to modification attempts. I think it would be possible to use some kind of epoxy to fill the head cutout but I am really not comfortable with that idea, what if piece of epoxy falls out and drops into cylinder? So rather I would have the cutouts filled properly by welding but only if it does not compromise head integrity and reliability.

What route I should take?

In the case of every head I have worked on with inj ports in manifold and inlet port - that region makes no difference to the airstream so my advice is - actually - to just ignore it.

Hope that helps,

GC

I'm having some problem with a Porsche 924 turbo cylinder head, next to one exhaust insert the material have eroded, about 90-120 degrees of the insert is in air in a ~3mm depth, so I have to cut it and welded up or scrap the head. Have some other dots in the fire-ring zone to, on a boosted engine could make problems.

The problem is that I believe the head have Mg in it, not Si like the new ones, ant if its true welding it is more difficult. I saw some old race heads with special casting an lock the same, impurity, porosity, full with holes.

Maybe in aerospace they can weld mag alloy, I can't..

G

Mag alloy can be welded, but it is best performed inside an Argon filled environment (commonly refered to as a 'bubble') effectively its like an inflatable clear PVC bag but with long inwards facing sleeved gloves and sealable inlets for the torch and earth connections. Once the job is placed inside and the bag is sealed and filled with Argon. The welder can see through the bag to look at what he/she is welding. TIG welding in the inert gas shield can then take place without oxidising and burning out the magnesium in the Alloy. Unfortunately however, welding shops in posession of a bubble are few and far between.

If it's just an aluminium with an elevated magnesium content then it can be welded in the normal way (without need of elaborate inert environments etc) but use a mag alloy filler wire such as 5356 or 5183. Porosity, holes and general difficuty in achieving a clean weld are usually symptoms of insufficient cleaning or prep work prior to welding. Clean the item to be welded as thouroughly as possible, preheating on a large casting is essential, this also helps to avoid distortion, the best way to achieve this temp is with an oven. Most big fab shops posess one for heating/drying stick electrodes. The time in the oven also helps to de-gas the casting as the heat causes the oil in the pores of the metal to vapourise and the longer the casting can be left in the heat the better (we sometimes have to leave them in for 2 days to get a good result)

When you are ready to start, prep edges with a burr or a file, try not to use abrasives as the abrasive grit (usually aluminium oxide) and the adhesive used to bond it to the backing material is the last thing you want to be smearing into a welded joint. Clean al areas to be welded immediately with acetone (watch out for your eyes and unprotected skin, and the stuff is highly flammable so get it and any rags well away before you start to weld) wipe off any excess and allow it to evaporate for a few minutes (a well ventilated shop is essential)

Once you are ready to weld, set the balance to give a high degree of weld cleaning with minimal ammounts of penetration and do some experimental weld pools without adding filler, start very gently and allow the pool to form slowly, I cannot stress enough not to rush. If you get a lovely clean bright pool then by all means you can start to add filler and you will be away. If however the material starts to bubble or black dots of comntamination start to float to the top of your weld then you have a porous or dirty casting and you will need to take alot more time and care over how you weld it. A very very shallow weld pool with some low melting point ally filler can work wonders on poor quality castings but due to its generally runny nature it is useless for building up edges or replacing large quantities of metal. But it does make a good base layer between the parent metal and the higher melting point mag alloys that have more desirable characteristics when it comes to getting lots of metal in quickly.

Thanks the explanation Daveyboy, it is very useful and I'll try to do my best, hopefully come back with some nice photos...

How did you get on in the end?