Checking Head Clearance on a Mac .67?

[Josh Stollfuss]

I picked up a used Mac .67 over the winter for my 1/8 Scale project. Over the weekend I disassembled and cleaned the engine, while I was reassembling the engine. I checked the head button to piston clearance. I used a cheap-o depth mic that I have to check how far the piston sits below the deck height of the (top edge of the sleeve?) at Top dead center. Then I measured how far the head button with shim protrudes down into the cylinder (Sleeve) the difference is the Piston to Head clearance, is that correct? I came up with only about .0035. The piston has a slight dish but I was checking on the non-dished portion of the piston. Do I need to take into consideration the dish of the piston in the piston to head clearance calculation? I noticed on the CMDI web site that the recommended piston to head clearance is .010-.013. I plan to run 60-65% nitro so I was thinking of increasing the clearance by sticking a .008 shim under the head button along with the shim that is already there to bring it up to around .011-.012 clearance. Does this sound reasonable?

Thanks!

Josh-

 

[waltr]

Josh,

It sounds like your on the right track.Don't worry about the dish.The piston is not measured in the dish.

One thing that I'm concerned with, was the sleeve in all the way when you measured it?

On the Macs,I bring the piston up to tdc before I take off the water jacket and button.That way I know that when I brought the piston up,the sleeve does not move with the piston.

The clearance range you stated is spot on too.

DANGER,DANGER,.0035 is WAY to tight for a 67 Mac.

I hope this helps.

 

[Josh Stollfuss]

Hey Walt, thanks for the reply. Yes I did make sure the sleeve was fully seated, I tightened the water jacket down on the head button/sleeve rotated it around to TDC then pulled the water jacket/head button back off. I was really surprised to find it at less then .004 clearance. I can't imagine that it would have run well at anything over 20% nitro the way it was.

Josh-

 

[Don Ferrette]

Josh I gotta go with Walt on this one, were you certain the sleeve way fully seated in the case? For the MAC67's in my scales I run 60% with .012" head clearance & they run great. B)

 

[Josh Stollfuss]

Hey Don, Yea I rechecked it 3 times, because I thought the same thing that maybe the sleeve was not seated fully. I checked the shim. It measured .0035, so I'm guessing it was a .004 shim originally that has flattened out some. I pulled the shim out and tightened everything back together and the piston did hit the head button. (Touched it) So the only clearance it had was from the .0035 shim. I'm glad I took the time to check it before I ever ran it.

Josh-

 

[waltr]

Josh,

Just FYI,motors rarely come with .000 head clearance.They all vary.Even when it's the same series motor from the same manufacturer.

Always check clearances before turning over a motor.

I went through a 84 Mac yesterday with -.003 clearance.

That would have been a disaster if it was ever run with no shims.

Bill over at Rossi sales has a really nice dial indicator already setup to measure from the glow plug hole.

Just a little tricky with dished pistons, but a nice tool.