sandblast cylinder head

[mirage]

i send my cylinder head to the machine shop to be resurfaced and they recomended me to sanblast it.question is when it came back they did sanblast the combustion chamber as well and its like met grey colour and alittle rough.is this ok....pls advice...or do i need to polish it...

[maggspower]

Its always good to have a head cleaned, as it helps with the visual inspection, looking for cracks, wear ect......

I would have thought that sand blasting is more suited to cast iron pieces Aluminium is vey soft and is easily pitted by sand blasting, I would of recomended a hot dip chemical treatment, although it is getting harder to find places that do this due to legislation of the chemicals.

The problem is the surface texture that you now have will encourage carbon to build up in the chambers, giving you problems down the line, thing like detonation, which to be honest aint good.

Good machine shops are hard to come by, it took me a good few goes to find one that i'm happy with, your one shouldn't be doing things that you havent asked for, it only takes a phone call to check.

Anyway, I personaly would polish the chambers. You will need to protect the newly skimed surface with gaffa tape or somthing simular, and most important the valve seats this can be done with some old valves. What have you had done to the head so far?

[no2lurch]

isnt a rough surface good for causing turbulance and fuel/air mixing?

[maggspower]

isnt a rough surface good for causing turbulance and fuel/air mixing?

Yes it is, in the inlet port. We're talking about the combustion chamber, carbon buildup here can cause the chamber to shrink, raising compression ratio and causing hot spots eventualy leading to detonation.

It is best to polish the chambers, the exhaust ports can also be done for the same reasons. But like you said not the inlet ports.

[mirage]

thanks for the comments guys....what can i do from here....i have cleaned the head with chemical then pressure tested for cracks and resurfaced it 6 thou and the head height is 125.05mm after that..lastly sandblasted it except the gasket surface.....my worry is the combustion chamber ....pls advice...BTW im replacing new rockers and sourcing for cam as they were slightly worn....

[gudgeon]

Good reply from Maggspower, I was going to mention about rough surfaces being good for fuel-air mix but instead learned something from Magg's post.

Mirage, I'm doing a head at the moment and as far as I'm concerned valves, seats and guides are #2 on the list after pressure testing. Obviously if you are replacing the cam you'll need to do cam bearings. Are the rocker shafts still good?

I am also considering having the inlet manifold matched and polished to the head, which is not a very big deal by all accounts but can pay dividends. The exhaust ports can also be matched to the exhaust manifold.

[maggspower]

Yes its a good idea to get the valve guides looked at particulaly the exhaust side as they wear quicker and get hotter. The cam runs directly in the head no bearings, so if the head is damaged here its scrap.

If you are changing the rockers change the shafts too.

Matching the head to the inlet manifold is a great idea, and if you do it yourself free horsepower, hmmm my favorite But matching the exhaust manifold to the port will lose you power, the port should be smaller by about 1.5-2mm all the way round. Its to do with pressure waves and their resonance, I'm not going to explain it I cant be bothered, get a book

@mirage, what is your worry about the combustion chamber?

[gudgeon]

Thanks Maggs, still learning.
No cam bearings.. really! Haven't removed the cam on mine yet.

I got the manifold matching stuff from a book, seems to be a difference of opinion there but I'm inclined to believe what you say.

[maggspower]

Is the book written by an american by any chance?

[gudgeon]

Yup.

[maggspower]

[gudgeon]

30 years experience of fekin up peoples exhaust ports by the sound of it!
!God save America!

[town325i]

bead blasting is more for aluminium as its not as harsh as sand and it doesn't damage alloy like sand can. the the company i use to do work on my heads uses the chemical dip to clean and it does a really good job way better than anything ive seen before

[mirage]

thanks for all your reply.....ok i m getting the rocker shaft too....my concern about the combustion chamber is its alittle rough and matt in colour opposed to polished...is this ok or do i need to polish it or do i need to find another head...im worried about carbon build up.....but correct me if im wrong the carbon build up is only initial when its rough subsequently its as prone as polished right?polished head too can have carbon build up...

[town325i]

it should be fine

[maggspower]

Like i said, I personaly would polish the chambers. Think of it like this, if you want paint to stick to something you need to give it a "key" for the paint to stick to, this "key" means you rough it up a bit.

The chambers in your cylinder head at the moment have a good "key" for something to stick to, the thing is though you dont want anything like carbon to stick to them for the reasons in my posts above. If you polish them the carbon finds it harder to stick to it.



Heres a pic of a 325i piston that I have started to polish, still needs finishing in the pic. The chambers where done the same.

town325i said "it should be fine" and hes right it will be fine for a good while if you dont polish the chambers, carbon build up doesn't happen over night. But if you have it there infront of you now why not spend a couple of hours getting it perfect. Then you can say "it will be fine"

Sorry for being a bit OCD about it all I just think if you can make something better by only spending time on it it cant be bad

[mirage]

ok got it....will try polish it...any tips on how do i do it correctly..machine on manually?autosol?

[mirage]

ok got it....will try polish it...any tips on how do i do it correctly..machine on manually?autosol?

[maggspower]

very fine wet and dry, 1200, then a scotch bright pad, and yes some anusol You could use a dremmel or simular for the polish, but until then just go at it by hand.

What else have you had done so far to the head?

[mirage]

ok will try and let u guys know....what have i done to the head?nothing much other than what i have mentioned...any recomendation?

before


after degreaser


after sandblasting

[maggspower]

Right, lets start from the top

Its clean, it needs to be checked, visualy first for any obvious signs of wear, so the cam bearings are in the head If these are knackered the head is scrap. Next the valve guides, put a valve in the guide and check for lateral movement(off hand I can't remember the service limit) The exhaust guides will be the worst. Next the valve seats looking for cracks.

The head has to pass these checks, or you have to know about the problems so you can judge how much it is going to cost you, worn guides can be fixed, so can seats.

Next the head can be crack tested, it is pointless paying to have this done if there are other things wrong with the head e.g badly scored cam bearings. If it passes the crack test then you can start having the work done.

The valves need to be checked, stems need to be within tolerance and striaght, to check this put the stem of the valve on a flat surface with the head overhanging the edge then roll the stem along the flat surface, a bent one is east to spot. Check the seat for pitting and the top of the stem for wear from the rocker.

Whith your head now I would suggest getting rid of all the sand that is probably lurking in the water/oil ways.

Then lets assume that the valve guides are ok, you will have to get a machine shop to check them though. The seats will have to be recut as it looks like the head was blasted without the valves in situ. This means cutting new mating faces on the valve and the head and making sure that they are sealing.

Once you get to this stage, you are getting there

Where abouts are you? I may be able to recomend a good machine shop

[mirage]

thanks for the tip....head has been pressure tested allready and all ok..valve guides on cylinder no one and two are worn...some scars on the cam bearings/journals...looks like im better of looking for another head...getting one from a friend who is swapping his for a M30...will need to check that first and deside cause mine look like its too much of work... appreciate the recomendation of machine shop but unfortunately im in the other side of the world "malaysia".....thanks again..