Cylinder head machining?

Bus_Boy · 30 October 2010, 10:39

I am replacing the cylinder head gasket on my 30 Tohatsu 2 stroke.

Originally i wasn't going to have the head machined as i didn't think it was nessacary, and there isn't really a whole lot you could skim off even if you wanted to.

But none the less i have been advised that i should in fact have it skimmed off before fitting the new gasket.

So i am trying to find out what is a safe amount that can be taken off. It is advised for the 50 hp models in racing classes, that you can take up to

-1mm of the cylinder head
-0.5mm of the engine block

Now i know that is for a 50, but it's all i can find. Has anyone had to machine the cylinder head on a 30 before, and if so do you re-call how much was taken off?

If i do take 1mm off, will i have to use a higher octane fuel? How safe is it shaving the engine block itself, even if it is only 0.5mm?

If i can't find any exact info, then i will just have to go with what i have found for the 50 models, but i would like if someone experienced could jump in here and give me a bit of info.......

jwalker · 30 October 2010, 13:09

1mm is a lot to skim off and unless there is bad damage, unnecessary. The amount you can skim from the block will depend on the piston height and combustion chamber shape.You probably don't want the piston crown protruding from the block and striking the cylinder head, do you?

This is irrespective of the increase in compression you will have.

If it's a simple refacing you need to give a good surface for the gasket to mate to and the cylinder head face is just surface marked, it's quite possible to do this using a stout, flat block of wood and emery cloth. Use a straight edge or steel rule to check the surface for flatness as you work it and lubricate the emery with paraffin, white spirit or, my preference, diluted oil. Work in one direction then across that direction alternately and you can see where you are working the block by watching the direction of the emery marks. Finishing with a circular motion is a good. Take care not to remove more from edges than the flat parts.

Bus_Boy · 31 October 2010, 04:58

I had a bit of a go cleaning up both the surfaces. The cylinder head surface was quite easy, but i'm having a lot of trouble with the block surface.

I placed the cylinder head back on to the block, and put a small LED light in through one of the park plug holes, taped around it and put a plug in the other. I can see some light coming out form a few area's....

Obviously once the new gasket is in place, these small differences would go away, but without the gasket should it still seal %100 ?

The block itself is level with with the sleeves, if the block was to be machined is it okay that the sleeves are machined also?

The crown is allready exposed from the sleeve at TDC which makes me wonder about taking 0.5mm from the block itself...(as you said JWalker)

I will try and get a picture of the block up today aswell...

9D280 · 01 November 2010, 10:09

I don't know these engines at all, but as a general comment I would have thought that a 1 / 0.5mm skim would be the absolute max you could skim rather than a suggested, more than that it's a scrapper.

If you have access to a machine shop, if you ask them to tidy the face(s) and tell them the max removable material, they should skim it in ridiculously small increments until the whole surface has been "cut".

You may find a flat lap is all it needs, which will remove fractions of a tenth of a mm.

bosun · 01 November 2010, 19:01

I have little experience of outboards but owned a garage and was a car mechanic for 30 odd years, 1mm is a vast amount to remove from a head and would never usually be done,except in special high performance tuning circumstances. Likewise machining the top of a block,known as ''decking'' is never normally done as blocks don't normally warp,decking of blocks usually needs pistons to be machined down too to preserve an acceptable compression ratio. Flatting the head as previously suggested should do the job,you can do the same thing to the block too if you turn the crank so that the pistons are part way down the bores, I would be inclined to use ''hylomar'' on the gasket (not silicone) a thin head like the one on your outboard will pull its self down straight a certain amount when torqued back down provided the bolts are done in the correct order.
However outboards may be different..............???
Paint stripper will get the remains of gaskets off without scoring the metal and also decarbonises very nicely.

martini · 01 November 2010, 19:57

Quote:

Originally Posted by jwalker

If it's a simple refacing you need to give a good surface for the gasket to mate to and the cylinder head face is just surface marked, it's quite possible to do this using a stout, flat block of wood and emery cloth.

I use a piece of thick glass for this type of job, just a tip