Head Clearance for Mac .21

[Plan D]

On the Cmd website they say .008 for clearance on their .21. I'm at .015 with a .002 head shim. Am I stuck trying to find a lathe to fix this or does this make enough of a noticeable difference in performance to even worry about it. Thanks for any and all help. Dave

 

[topfuel443]

Dave send it to me, and Ill get the H.C. down to .008 thousandths. No charge.

~James

send me a PM if you want to.

 

[Plan D]

Topfuel you've got a PM Thanks

 

[Norm Doerr]

Top Fuel;

Take it down enough so he has the option to go to .006" if he needs to.

Norm

 

[Terry Flynn]

I had the same problem. I forgot that the piston is dished. the head clearance was right after I plastic gauged it. and mine also had a .002 shim in it.

Terry

 

[waterbug]

I started to say something earlier, but decided to wait and see if anyone would catch it..... Terry wins...1/2 :lol: :lol: He came back and added to his post...

But wait !!!!!! There's more to do !!!!!!!

 

[Don Ferrette]

The dished pistons found in alot of motors these days makes the head guage set from Terry Keeley real nice. The new ones go down to .006"

 

[Plan D]

Are you guys thinking that I was measuring in the center of the piston? I took my measurements on the outside edges of the piston.

 

[Don Ferrette]

Are you guys thinking that I was measuring in the center of the piston?  I took my measurements on the outside edges of the piston.

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Guess we weren't sure as it's easy to forget about that dish. If you've measured (& re-measured) and truly find yourself at .015" at the squishband then have James take it down .009" to get .006" as previously mentioned so you can go back up to .008" with your .002" shim. B)

 

[waterbug]

Dave

No... Just the volumes of the piston dish and head button.. On my OB the head button volume was changed.... On the IB, .008-.006 is the norm.... Two totally different engines..

 

[waterbug]

Also head buttons are cheaper than a sleeve !!! When it comes time to rebuild, all you will have to do is drop in a P/S and slap the head back on and still be close to .008-.006..

 

[Norm Doerr]

Also another thing I learned to gain bottom end is to machine the outside edge of the Sband flat. Then cut the inside edge at a 1 to 1.5 deg. angle towards the bowl. Of course make sure you take the burr off the bowl area or it will act like a glow plug. You will have to cut the out side edge of the entire head to get back to .006"

 

[AndyBrown]

Norm,

The squish is already flat. I've tied the 1.5 degree angle at the inner half. Does not add anymore torque.

If the MAC was stock it should get down to about .005" with no shim. Tell me what you found for the deck height.

 

[ClayGlover]

The dished pistons found in alot of motors these days makes the head guage set from Terry Keeley real nice. The new ones go down to .006" 

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Are you getting little scores in the piston crown and button from the feeler guages after turning the motor over Don? I see the outline of the guage on my pistons :huh: , no big deal, just wondering if I was doing it wrong or being to rough forcing it over

 

[Don Ferrette]

The dished pistons found in alot of motors these days makes the head guage set from Terry Keeley real nice. The new ones go down to .006" 

87514[/snapback]

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Are you getting little scores in the piston crown and button from the feeler guages after turning the motor over Don? I see the outline of the guage on my pistons :huh: , no big deal, just wondering if I was doing it wrong or being to rough forcing it over

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Ummm, might be a tad on the rough side. Haven't seen an "outline" on any of mine, I just "bump" it to & over it. :unsure:

 

[Norm Doerr]

Norm,The squish is already flat.  I've tied the 1.5 degree angle at the inner half.  Does not add anymore torque.

If the MAC was stock it should get down to about .005" with no shim.  Tell me what you found for the deck height.

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I cut the Squish flat all the way across, and played with different bowl shapes. I forgot that the headbutton came out of the bag ready to go OOOPS LOL. Clearance is .007. The deck is .0716". A few things have been done to it. The Mac 21 is the best 21 out out of the box....strong as heck plus I LOVE the rear exhaust.

 

[AndyBrown]

Deck .0716". I guess you droped the liner down .022".

Or you have Marty's Engine. B)

 

[Buckshot]

Deck .0716".    I guess you droped the liner down .022".
Or you have Marty's Engine.  B)

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Dropping the liner down? More speed secrets?

-Buck-

 

[Norm Doerr]

Deck .0716".    I guess you droped the liner down .022".
Or you have Marty's Engine.  B)

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I have all of Marty's Engines. He had a bunch of MAC 21s he was working on, I also built two new engines based on all the R&D Marty did with the MAC 21. I still have a grocery list of things to try on the dyno,(things that Marty wanted to pursue but never had time). That little motor is a VERY good engine and is strong as hell. Yes we dropped the liner and did some combustion chamber work. LOL can't get much past you. LOL. The MAC 21 produced the highest BMEP of any of the other two jillion dyno pulls of ANY engine.

 

[Norm Doerr]

Deck .0716".     I guess you droped the liner down .022".
Or you have Marty's Engine.  B)

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Dropping the liner down? More speed secrets?

-Buck-

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More like a preference thing. The MAC 21 out of the box runs VERY well, you can run the thing and win races with the stock numbers. Some of us just like to tinker too much LOL.