head with too close of a shave..copper gasket fix?

[rudderless]

First post after 3 years lurking. So..I bought an ebay head for my 76 omc 9.9/15hp. Year 1989 with thermo on top to replace the poorly designed 76 head. After checking head space and looking over the head I see that someone got out of control and took off .040 or so. The piston hits the head.

I am in internal discussions with myself about using a copper head gasket. I have made them in the past with success. But this one needs to be .080in or so.

What does my minimum head space need to be for a cross flow motor? I am not looking for 200 psi or anything. Just want to use this head in my mota. The head hits the piston at the exhaust side of the diverter.

Maybe 2 stock gaskets?

[Umassot50]

First post after 3 years lurking. So..I bought an ebay head for my 76 omc 9.9/15hp. Year 1989 with thermo on top to replace the poorly designed 76 head. After checking head space and looking over the head I see that someone got out of control and took off .040 or so. The piston hits the head.

I am in internal discussions with myself about using a copper head gasket. I have made them in the past with success. But this one needs to be .080in or so.

What does my minimum head space need to be for a cross flow motor? I am not looking for 200 psi or anything. Just want to use this head in my mota. The head hits the piston at the exhaust side of the diverter.

Maybe 2 stock gaskets?

I dont like thick copper gaskets. They tend to leak in my experience. Also two gaskets not a great idea. Can you just find another head? How expensive is a 15cc head? If 80 thousandths has been taken off the head compression will be way to high anyway.

[olboatman]

OMC or the aftermarket may have made a lower comp.(thicker) gasket for these engines as they did for other motas when the quality of fuel went down. I would check with BRP as they might have some nos. Gary

[olboatman]

BTW If you do make a copper gasket make sure you spray BOTH sides with Permatex Copper coat . We had good luck with the Copper coat on high comp. sled motas in the 70s.:thumbsup: The only up side to gettin old!!!!:eek: Gary

[rudderless]

BTW If you do make a copper gasket make sure you spray BOTH sides with Permatex Copper coat . We had good luck with the Copper coat on high comp. sled motas in the 70s.:thumbsup: The only up side to gettin old!!!!:eek: Gary

Yea..Hearth, Sacks, rotax..memories from the 60s and 70s...In fact I made copper gaskets for some of my sleds back then. I have over ported many piston port motors..a lot of spit from the carb..add no air box and my parents hated the gas smell from my sno suit.

[Chaz]

Any time you clip a head , be it a little or a lot , you need to re establish the quench area back out to the size of the bore . It's just part of the job .

http://i.imgur.com/lwzKPMEl.jpg

[olboatman]

I see what your sayin Chaz....Some of the OMC crossflow heads I have cut are cast with the bore size into the head quite a distance. I cut a 15hp 13ci (like the one in question) 30 thou without decreasing bore size in the head. Help me out if I miss understand your point as I'm always learnin. Gary

[rudderless]

Any time you clip a head , be it a little or a lot , you need to re establish the quench area back out to the size of the bore . It's just part of the job .

http://i.imgur.com/lwzKPMEl.jpg


I ruined my dads belt sander back in 1976 or so shaving a couple of heads on a twin cyl motor on a snowmobile. Had the sander clamped upside down and the shavings got into the motor. Sparks started to come outa the motor so I stopped, cleaned the al shards and put it back in the case. A week later I heard a yell and was called down stairs by dad. Yea..you know what happened..

A question..how close on a cross flow should I be? Like .040 or so..like on loopers maybe..just lookin to be safe..

[Chaz]

Some of the OMC crossflow heads I have cut are cast with the bore size into the head quite a distance. I cut a 15hp 13ci (like the one in question) 30 thou without decreasing bore size in the head.


rudderless :
A question..how close on a cross flow should I be? Like .040 or so..like on loopers maybe..just lookin to be safe..

My first thought was , how much piston to wall clearance does the motor have . Because the looser the bore the more the piston can rock along the thrust line as it approach's and descends from TDC, which is really where you look when trying to set one up with a tight quench area .

But for normal every day production line assemblies , .035 - .045 seems to be what the factories shoot for .

Gary , it really comes down to what you have as a total number . I don't know if those motors have a head gasket or not . But lets just say it does and use some hypothetical numbers .... :smiletest:
Piston is shy of coming up to flush with the deck surface .030
Gasket is .040
And the head has a recess pocket .050 deep .
For a total of .120
And a compression gauge says it pumps 90 PSI

You might just clip the head .050 never need to touch the quench taper .
Have .070 clearance
Pump 110 on the gauge

Add decking the block .030
Have .040 clearance
Pump 130 on the gauge
Run 91 octane ... big prop , haul butt .. :D
All hypothetical ... but you get the idea .... :thumbsup:

For someone who want's to really do one up like this , there are plenty of programs out there that if you feed it good info it will give you real world numbers to shoot for . Lots easier than when I started out , we had to do everything with old fashioned math using pen and paper ... :D

[rjdubiel]

I always assumed you check your squish clearance on a crossflow at the corner of the cylinder and head on both the exhaust and intake side. Is that correct and where the piston is hitting the head in this situation? Or is the Deflector on the top of the piston hitting the head as well?

[powerabout]

Any time you clip a head , be it a little or a lot , you need to re establish the quench area back out to the size of the bore . It's just part of the job .

http://i.imgur.com/lwzKPMEl.jpg
Is this head going on something with custom domes, looks like a huge angle on that?

[rudderless]

I always assumed you check your squish clearance on a crossflow at the corner of the cylinder and head on both the exhaust and intake side. Is that correct and where the piston is hitting the head in this situation? Or is the Deflector on the top of the piston hitting the head as well?

Upon further inspection..its hitting at the deflector corners. The old head is back on for the rest of the summer so I can't do any real measuring until winter but it looks like .040 is missing.

[Chaz]

Is this head going on something with custom domes, looks like a huge angle on that?

Power ,
That is a 300x head that had a little rash ... :eek:

Tell me , what is the stock angle .. ?

And what you super keen eyeball inclinometer tells you this one is ... :D

[powerabout]

12.5degrees, lol

[Chaz]

12.5degrees, lol

Survey says :

https://maymayhem.files.wordpress.com/2015/05/family-feud-board-x.jpg (https://maymayhem.wordpress.com/2015/05/06/round-1-day-2-2015-may-mayhem-tournament-of-champions/)

[powerabout]

Upon further inspection..its hitting at the deflector corners. The old head is back on for the rest of the summer so I can't do any real measuring until winter but it looks like .040 is missing.
Look up 15A race engines, I think the guys trim the deflector

[powerabout]

Survey says :

https://maymayhem.files.wordpress.com/2015/05/family-feud-board-x.jpg (https://maymayhem.wordpress.com/2015/05/06/round-1-day-2-2015-may-mayhem-tournament-of-champions/)
wait..I am phoning a friend

[Chaz]

Ppppffffttttttt .... this is the only lifeline the game rules allow reaching out for ... :D

And only one per customer .. :smiletest:

https://tse1.mm.bing.net/th?id=OIP.Q-2NKOwZFCD5W4NwNEzm_QEsCj&pid=Api

[powerabout]

Ppppffffttttttt .... this is the only lifeline the game rules allow reaching out for ... :D

And only one per customer .. :smiletest:

https://tse1.mm.bing.net/th?id=OIP.Q-2NKOwZFCD5W4NwNEzm_QEsCj&pid=Api
how about 3 capsule suits, and a foam esky thats all I've got?