flycutting a head

[bob johnson]

how much can you take off a head for a 1988 70 hp evinrude??

there is just a little pitting in between cylinders. plus there was some debris smashing into the the head by the piston after a recent rebuild...

it seems pretty flat now....

I can cut it at work with not much set up...but i dont want to throw off compression too much

the block is bored .040 over already... from a previous to me rebuild.

I am putting new rings and one new piston in..

also the reed things....what should I be looking for , to see if they are in good shape.

they are nice and clean no pits or burrs...nice round edges...


one more question, do i need a special heat resistant primer and paint for painting the head... and block

I have the head all sand blasted now....

I have been doing every pieces i can seperate and have been priming all aluminum pieces with INTERLUX Primocon underwater primer.

I am on the motor parts now which mostly were black.

previously I over painted with Evinrude blue for all the non power head parts

thanks guys

bob

 

[Faztbullet]

Re: flycutting a head

About .020 is all you need to cut. Make sure there is no steel debris from rings/bearing embedded in head to create hot spots. I use plasticote high temp engine paint

 

[bob johnson]

Re: flycutting a head

About .020 is all you need to cut. Make sure there is no steel debris from rings/bearing embedded in head to create hot spots. I use plasticote high temp engine paint

wow, I would have thought .020" would have raised compression....especially since the cyliner is over bored .040"

there is more volume being compressed , just on the over .040 cylinder..I was worried about cutting the head down and raising it more..

but you think .020" off the head in this over bore motor is still good?

I'll stop as soon as it is clean, if it is under the .020"

bob

 

[reelfishin]

Re: flycutting a head

I would be tempted to make both heads the same to preserve equal compression?

 

[bob johnson]

Re: flycutting a head

I would be tempted to make both heads the same to preserve equal compression?

there is only one head on this three cylinder 70 hp.

bob

 

[Dhadley]

Re: flycutting a head

Personally I wouldn't cut more than .010 especially if you don't know if the head or the block has been cut before. Since it's been rebuilt a couple of times already. Todays fuel has a lot to do with it.

 

[Faztbullet]

Re: flycutting a head

Let me clear my post from earlier... only cut to a max of .020. You are correct
to remove as little as possible up to .020. Cutting head and overbore you might see a 3-5 psi increase.

 

[bob johnson]

Re: flycutting a head

do you guys think that polishing the chamber will reduce carbon deposits?

one chamber( the top) has a slight imperfection in the chamber..it is like a rolled edge on one side...you can catch a fingernail on it...almost looks like a smashed burr...but it is a cast surface!

I can program a ball end mill on my cnc miller to clean up ever so slightly the surface in the chamber..

if it will help with preventing carbon buildup..or i needed it to reduce compression.

thanks

bob

 

[Faztbullet]

Re: flycutting a head

Polishing wont help carbon deposits, piston tops when new as slicker than a babys behind and it still builds up, nature of the beast and fuel mixture(gas/oil). Imperfection wont hurt nothing as I have see factory heads with such bad casting they look like they was hit with a shotgun.

 

[reelfishin]

Re: flycutting a head

there is only one head on this three cylinder 70 hp.

bob

I didn't catch the 70 hp part. Either way, cutting the head is fine, don't worry about polishing it. The goal is to have all the chambers the same size, and the gasket surface flat. I'd go no further than to remove any damaged aluminum bits that may break off if the cylinder head is battered from a previous failure.

 

[kenmyfam]

Re: flycutting a head

You can polish the ports, the faster the exhaust gas gets out , the faster the fuel / air get's in.
Used to do a lot of micro tuning on 50cc motorcycles many years ago. I personally wouldn't try it with an outboard though.