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**Billy Ricks** · 10-14-2011, 02:36 PM

Originally posted by Gizmo View Post

I am sure that this 572 cc engine is interference fit. My question is how much can be milled from the head safely to bump up compression without risking a valve hitting a piston. Thanks Much Gizmo

I'd start by checking deck height of the piston. You can usually take a little off the top of the block to bring the piston near that limit. Depending on how much is taken there you may not need to be milling the head.

**Gizmo** · 10-15-2011, 11:12 AM

Thanks I'll keep that in mind when I am ready for boring and putting in oversize pistons. :-) Gizmo

**Big Jay** · 10-15-2011, 08:38 PM

You are going to want .045" intake and ,085" MINIMUM PTV clearance. So measure what you have now, and you will know how much you can lower the valves.

Don't forget, when you mill a head, you are not only lowering the valves, but also moving them out from centerline.

**240punk** · 10-21-2011, 11:01 PM

Originally posted by Big Jay View Post

Don't forget, when you mill a head, you are not only lowering the valves, but also moving them out from centerline.

Im not sure what you mean by this.
Is that an issue? Can you explain more in depth please?

**TeamDar** · 10-22-2011, 07:49 AM

You will be retarding the cam timing due to the shorter distance from the crankshaft to the camshafts. No problem if you are going to degree the cams.

**Billy Ricks** · 10-22-2011, 08:17 AM

Originally posted by 240punk View Post

Im not sure what you mean by this.
Is that an issue? Can you explain more in depth please?

It's how close the valves come to hitting the pistons. Some guys use modeling clay in the valve recesses on the pistons then hand crank the engine to mash the clay to get a thickness on it. It means disassembling and reassembling a couple of times to get the measurement.

http://www.racingheadservice.com/Information/Technical/PistonToValveClearance.asp

**trippivot** · 10-23-2011, 08:48 AM

You have to open the valve pocket in the piston crown because milling the head will change the location where the valve is when open. Out from center line as Jay stated is only 1 part of custom fitting a modified engine

Cam timing will not be an issue if you use cam MFG. spec L/C#'s. but yes you will have to measure everything that moves to get low L/C #'s for more midrange.

deck height + sling out needs to be considered , you will have to calculate your base/head combo.

a manual camchain tensioner will be needed to take up the slack from the milling.

Modeling clay is a thing of the past.. thick solder wire is much easier and clenaer to measure valve to piston . right thru the plug hole and then measure the squished part of the solder. a steel wire to measure between valve to valve. certain paper clips are the right thickness.

and as you said, INTERFERENCE engine. so the valves will smack in stock configuration...

the real answer is == it is not safe to mill a head -unless you are prepared to hand fit every part inside of it.

**koolaid_kid** · 10-23-2011, 09:35 AM

I am a bit puzzled about a couple of statements you made, trippivot.
If the head is milled, won't it change cam timing?
Why would hand fitting be necessary?
Is this specific to the 550?
I ask because I just finished my GPz build. I installed Wiseco 10.25 (807cc) pistons and milled the head 0.030". I had the cam sprockets slotted and degreed the cams. They ended up in a different place than OEM. No hand fitting was required. I did all this on the advice of those who had a lot more experience than me, and it worked out perfectly.
Is this different on GS engines? I really don't know why it would be, but again, all I did was what the "experts" told me.

**trippivot** · 10-24-2011, 10:27 AM

cam timing only has to change if there is a mechanical obstruction around 10 degrees before and after TDC. (piston to valve) Changing opening/closing degrees is NOT absolutely necessary in all situations as safe as stock cam timing numbers are.

Valve angle into the comb chamber of a head has a lot to do with where the poppet valve is located at full lift after milling. The engine design from a GS 550 to a KZ to a XJ will all have different modified valve locations (at full lift) given an equal amount of head material removal. more angle away from perpendictular means more change

were the pistons you bought already cut for bigger valves? The angle of the floor of the valve pocket will be off as well OR your valves might not even get that close to the piston for it to matter.

I'm sure your experts had your best interest in mind so you would not have to measure and re-check everything.

**Big Jay** · 11-03-2011, 01:21 AM

Anytime you lower the camshaft location in an engine, you also retard it.

**Zafe** · 12-17-2011, 11:59 PM

Hello,

Not wanting to thread jack, but didn't want to start a new thread this one seemed similar enough

Seems to be the 77 gs550 motor has 8.6:1 Compression

*correct me if im wrong,

I would like to gain some performance by a exhaust, jetting, and milling the head a tad. however I do not see much about peoples success or documentation about their experiences.

Are the gains worth it? are they reliable ?

**Nessism** · 12-18-2011, 12:51 AM

Originally posted by Zafe View Post

Hello,

Not wanting to thread jack, but didn't want to start a new thread this one seemed similar enough

Seems to be the 77 gs550 motor has 8.6:1 Compression

*correct me if im wrong,

I would like to gain some performance by a exhaust, jetting, and milling the head a tad. however I do not see much about peoples success or documentation about their experiences.

Are the gains worth it? are they reliable ?

Your best bet to increase performance noticeably is to install a 650 top end. There are several good threads in the archives detailing the process. The 550 bottom end seems to hold up will to this mod, and the parts are not overly difficult to find.

Good luck

**Zafe** · 12-18-2011, 12:59 AM

Originally posted by Nessism View Post

Your best bet to increase performance noticeably is to install a 650 top end. There are several good threads in the archives detailing the process. The 550 bottom end seems to hold up will to this mod, and the parts are not overly difficult to find.

Good luck

sounds like its time to source a 650 top end, any performance 650 bolt up kits? maybe higher compression, lighter components?

Thanks again