milling the head on a 400ex?

[waytooslow]

I want to mill the head on my 400ex, how much material can I remove safely?

[Florida400EX]

Don't bother milling the head. Just use XR400 gaskets, that will get the head just about as low as you would want it.

[F-16Guy]

Originally posted by waytooslow
I want to mill the head on my 400ex, how much material can I remove safely?
What you really want to do is deck the cylinder. The piston sits a little low in the hole when using a piston with stock wrist pin location. You want have about 0.040 deck height including compressed gasket thickness. This will reduce the squish area between the edge of the piston and the head, which helps force the fuel/air charge toward the center of the combustion chamber. I'm not an expert engine builder, but I'm pretty sure the effects are: more complete and efficient burning of the fuel/air mixture, and better flame travel in the combustion chamber which allows a higher compression ratio with a given octane fuel due to the lower combustion chamber temperatures. Decking will also raise the compression ratio slightly. Milling the head will only raise the compression, not affect the deck height.

[ill_lil_romey]

If you eleminate the base gasket and use a xr head gasket, it should get you the perfect squish area with most pistons.

[F-16Guy]

Originally posted by ill_lil_romey
If you eleminate the base gasket and use a xr head gasket, it should get you the perfect squish area with most pistons.
You can do it that way and get pretty close, but I'm a little leary about using gasket maker anywhere near the oil passage. If any squeeze-out partially blocked the passage, or even one of the holes in the cam lobe, it could be trouble. If I were going to do it, I'd do it right.

[waytooslow]

Can I just get a high compression piston? Will that do the same thing? Could I use a high compression piston with a XR400 gasket?

[daimon1054]

1 get a better piston because the stocker was not made to run at higher compression ratios
2 when the jug is off get it milled, you will want to have the deck checked anyway so the money diff is not much.

As said skip the gasket material and find a good shop, the cost will not be much more and could be ton's less because the chance to screwing something up by doing it right is much less than trying to get out cheap.

I don't think you will be happy in the end though and would say spend a few bucks more and get the 416 or 440 kit.

[F-16Guy]

Originally posted by waytooslow
Can I just get a high compression piston? Will that do the same thing? Could I use a high compression piston with a XR400 gasket?
Definitely, that's what about 99% of people do. If you're just looking for some extra power, a piston, cam, and gaskets would be about all you'd need (assuming, ofcourse, that you already have at least a slip-on pipe, a good filter, and access to larger jets). If you were building a race engine and you needed to extract every last HP possible, you would need to worry about doing the measurements and machine work necessary to achieve the right clearances. Ideally, you'd want to do it that way anyway, but few people have the patience to do it right. What you could do is call Cometic and order their thinnest base gasket for that engine, as well as an 87mm Flexsteel XR400 head gasket. Most people use the stock size, but I'm pretty sure they make one in the correct size; If not, I'd use an 88mm so you don't have gasket material extending into the combustion chamber.

[F-16Guy]

Originally posted by daimon1054
...I don't think you will be happy in the end though and would say spend a few bucks more and get the 416 or 440 kit.
Good point. You will most likely want to freshen up the bore (I was assuming you were going to bore it to a 416, or 87mm). It shouldn't be more than $40-$50, and the oversized piston and gaskets are the same price.

[waytooslow]

OK. So here is what you are saying to do...buy the xr400 head gasket, buy a super thin base gasket, buy the 416 kit with the 11:1 piston...and I should be able to go fast. I hope I am getting all of this right.

[daimon1054]

Find a 416 kit and get the compleet kit. It will have everything you need. If you have any questions call the tech support people who sell you the kit and they will save you time and money and make sure your are happy with the end results.

[Quadzilla]

Originally posted by waytooslow
OK. So here is what you are saying to do...buy the xr400 head gasket, buy a super thin base gasket, buy the 416 kit with the 11:1 piston...and I should be able to go fast. I hope I am getting all of this right.

C&D sells custom Cometic gaskets that are 8 thousands thinner than stock XR on both the base and head gaskets. That is what I used on mine. Gets the squish almost perfect and also increases the compression a little over what the piston is rated for. For example, I have a Ross 10.8:1 piston, but I'm prob running 11:1 or slightly higher because of my gaskets...

[kgbg]

Which ever way you go about it, I had mine decked, head milled, and chamber welded up, to clay the piston, to make sure you have proper vlave to piston clearances. I had very high comp (13:1), with a flat top piston. Make sure you have clearance. I did not and it was ugly.

[Quadzilla]

If you run w a lower compression (less than 12:1) you should not have any clearance issues. Anything over 11:1, you will have to run a race race/pump mix (or just race gas) and also have to get heavy duty studs for your head...