Shaved Head or Gasket Mod

Hello, I have a 1982 XJ550 Maxim project. I am interested in adding compression. I don't exactly know of a shop to take the header to get milled. I also have read that using a thinner gasket would work too, or I would have to build my own mill… which is not as far fetched as it seems, just time consuming.
It would obviously be simpler if there were a thinner header gasket and where to get it. Or if there is a way to cut a new stock gasket to be thinner? Thank you in every way!

 

actually what your trying to do is up the compression ratio. If you want to raise the compression look into a three angle valve job and new pistons, rings and a one over bore.
you might build your own mill......ok

 

If you shave the heads, your whole shim clearance system becomes an issue. Regardless of what the chart says, your valves will be closer to the head. So, you'd have to be very careful to account for the closer tolerance now.....

I would listen to Polock. Don't shave the head. My opinion. Maybe I'm over-thinking it, but it seems to me that if the head and/or block is shaved to re-level it, then a gasket that is thicker to make up the difference would be needed in order to maintain chamber size, as well as piston-to-valve minimum clearance.

dave

 

An over-bore will have no effect on compression ratio. Skimming the head will do the job, but will require a machine shop to see the whole engine to determine if the valve pockets will need to be cut deeper into the pistons (and if there is enough meat to cut into).

The real question is why do you want more compression? What is the goal?

You also need to consider what the connecting rods can handle before they get all bendy on you, along with a host of other things that you may be pushing beyond their ability to cope with your modifications. If more oomph is what you're after, there are cheaper ways to go.


FWIW my father was a machinist. In order to build a decent mill for engine work you will need to have a decent machine shop and foundry to build it with.

 

Sure it will......if you over-bore, you're making a larger cylinder volume that will still get squished into the small chamber. That's going to alter your compression ratio.

 

A +1 overbore might account for a 1 to 2% increase in compression ratio. It's not hardly enough to notice. So you are right, but I don't consider it to be a reasonable method for achieving a higher compression ratio if that is the only reason for the overbore.

 

Now, I will agree with what you added to the first post and the second. Yup, may not be enough to notice, but it does have an effect....that's all I was saying there. Agreed.

 

Cool beans then.

Just as an example for Sneezus's benefit: I built a scooter engine a few years ago. I needed a bit more oomph to get on the highway since this is a pretty small town. The easiest method was an overbore kit; bumping the displacement from 125cc to 155cc just by increasing the piston and bore size. The engine's compression went from 10.0:1 to somewhere around 10.5:1 (this is from memory as I don't have the logbook for it anymore) with a bore increase of 6mm.

 

In a 351W, going from 10:1 to 10.5:1 is a pretty good increase!

 

Thank you everyone for the info and 2-cents It is greatly helpful! Even though now I am in the tool room at work and have the equipment, just no access yet haha. I have decided to just put it back together without modification. I was under the impression that higher compression is more power, but I didn't take into concideration that the timing could be off. I am now just going for a higher tooth count on the front sprocket and a lower tooth count on the rear sprocket, one reason is to give lower rpm at highway speeds. Any other modifications anyone willing to share? As you may have noticed, I am no expert but am willing to listen and learn. Again thank you!

 

more in the front and less in the back might make a huge change, maybe too much. There's places out on the innerweb that have ratio charts and programs that will tell you a % change for rpm and mph.
1 tooth in front is quite a bit, 2 in front and you'll be getting a bigger back sprocket or going to the salt flats
in order to do it right you should change the sprockets and chain together or one will ware the other out faster than normal

 

The engine doesn't need lower RPM for the highway. One tooth on the front will be plenty if you really feel the need.

 

Yes, this is the site I use http://www.gearingcommander.com/
My stock setup is 17t front and 45t rear. Aren't most stock xj550s 16t front? I don't remember where i read that. I was looking into 18t or 19t front and 44t rear, and yes all brand new including the chain.