Race Engine Forum UK

Hello Guy,

I am rebuilding a Delta 1498ccm (SOHC) cylinder head. This is the same cylinder head as used on early Uno, Ritmo, Regata etc. with 1.5 engine. What is making this head a bit rare is that it uses 10x M12x1.25 head bolts in comparison to newer type heads that use 10x M10x1.25 AND 4x M8x1.25 bolts.

Unfortunately, the head has been damaged:
a) due to use of tap water in cooling system, there are two oxidation pits;
b) there is a minor crack between valve seats on one cylinder.

In my humble opinion none of the defects described above would require welding as:
a) The pitted area is located under the much bigger coolant passage on the engine block, so it is not serving as sealing surface in any case (this is how water got there, on the first place).
b) The crack between the seats is tiny, and quite common on these engines. The head has not been leaking water.

However, before I invest more time and money in this head, I would like to know what do you think about the damage. In the case the head should not be used without welding, I will look for another one, as the welding would most probably be more expensive than another head, and quality of TIG welds is always questionable in Serbia.

Photos are attached. Many thanks,
Nikola

Don't worry about the crack betw the seats, many heads exhibit that tendency.

The corrosion pit doesn't undercut the fire ring nor lead to an oil gallery thus is not functionally significant. If you can't find a top welder I'd be tempted to chase out the corrosion a bit, fill the hole with chemical metal like Loctite Hysol or Devcon and then get it milled flat.


GC

Thanks Guy, and sorry for my delayed reply, as my account was deactivated as a result of the email address update... and afterwards I was on holidays.

Back to the head, I have drilled out intake seats, as I wanted to use bigger intake valves. I have prepared cast iron seats, with 42mm OD, and 0.125mm interference fit. Heated the head to 200°C, and pressed the seats in. However, the crack (discussed above) got bigger as the head was cooling down, so I decided to discard that head and look for a healthier starting point.

When the intake seat next to the crack was drilled out, the crack was present only on top end of the seat counterbore (viewed from the CC chamber), some 2.5mm in length, measured from the CC face. After the new seats were installed, it opened up significantly, so I assume it has propagated much deeper and it simply didn't seem right to invest any more time / money in that head.

Off topic, in your experience, what is the best valve back shape, to be used with Fiat SOHC heads, with valve seats prepared according to your instructions? In your book (although for twin cam series), you recommend 30 degrees back cut, together with waisted stem for inlets, and bigger radius for exhausts - which I used in the past, but I was wondering if there were some new SOHC specific developments?

Many thanks,
Nikola

Hullo Nik,

if I had known you were going to bore it out I would have said 'don't do it' because of course a crack will grow if you generate a new interference fit. You might have got away with using copper alloy inserts because they only need 2thou or so, but either way any kind of alterations to the seat insert region on a cracked head it is highly inadvisable! You are the living proof!

G