HEAD CLEARANCE

What is the best method to determine the proper amount of shims for  piston to head clearance of a nitro engine

CMB/ Rossi   .67

Several ways to measure. Most require a mic. Some a depth gauge.

The quick and easy method is to use a piece of thin solder. Put it thru the plug hole and bend it to the egde of sleeve. Turn over the top. Use your mic to measure the squish of the solder. 

Another is to use your mic to measure the depth of the head button. Also measure your piston at top dead center. It helps to have a bolt and a washer to hold the sleeve down while turning over the piston. Subtract the 2 and thats your head clearance. 

You can use a depth mic. Pull head button off and set depth mic in plug hole. You need special adapter for this. Measure it on a flat surface. Now remount the button and measure the distance after you turn over the top. Subtract the 2. 

Others use a mic gauge. These slip into the plug hole and can slide to the sleeve edge. If you cant turn it over, it is the next size down. Repeat until you can. It will be around those sizes.

Now the best is subject to opinion. They all get you to the right place. So go with what you have or can buy. BTW your looking to be around .012 on that motor. 

Mike

I used a depth mic for almost 40 years. Due to some manufacturers moving to dished pistons,I recently moved to a dial gauge. The dial gauge is way more accurate,and left me wondering why i took so long to convert.

Mike Hughes said:

Several ways to measure. Most require a mic. Some a depth gauge.

The quick and easy method is to use a piece of thin solder. Put it thru the plug hole and bend it to the egde of sleeve. Turn over the top. Use your mic to measure the squish of the solder. 

Another is to use your mic to measure the depth of the head button. Also measure your piston at top dead center. It helps to have a bolt and a washer to hold the sleeve down while turning over the piston. Subtract the 2 and thats your head clearance. 

You can use a depth mic. Pull head button off and set depth mic in plug hole. You need special adapter for this. Measure it on a flat surface. Now remount the button and measure the distance after you turn over the top. Subtract the 2. 

Others use a mic gauge. These slip into the plug hole and can slide to the sleeve edge. If you cant turn it over, it is the next size down. Repeat until you can. It will be around those sizes.

Now the best is subject to opinion. They all get you to the right place. So go with what you have or can buy. BTW your looking to be around .012 on that motor. 

Mike

Click to expand...

TNX  Mike retired CNC machinist  precision  measuring  tools not a problem. Has  been by far to may years didn't want to trust my ole memory.  I use to use t to clay the piston to head then adjust the shim for .010 to ,014 clearance dependent on the % of nitro

This subject comes up ever so often and needs to be revisited for the newer boaters. It is like everything else, there are many opinions and many different ways to do the same thing. When this happens, I try to keep the system as simple and reliable as possible for me.

I have all of the different mics and measuring tools to do this job, still I use a product called "Plastigage". It can be purchased at a very reasonable price at most automotive parts stores.

It is simply a very precise soft plastic thread that you cut off a short piece, place it on top of the piston between the head and piston. Torque the head down to your specs. Then rotate the engine over one time, remove the head and measure the width of the flattened plastic thread and compare it to the scale printed on the paper that the plastigage came in. This is a very exact measurement that can be used with any piston crown shape whether tapered or dished and any squish shape on the head. This to will help if you are using graduated squish angles. It can also give you the exact squish angle between the two.

The Plastigage comes in different sizes to measure different clearances. These different sizes are color coded as to which thickness range that you want to measure.

Just my way of doing this procedure,

Charles

TomGracey said:

What is the best method to determine the proper amount of shims for  piston to head clearance of a nitro engine

CMB/ Rossi   .67

Click to expand...

Tom,

You will want to measure at the maximum crunch point (TDC) of the piston in the cylinder. This becomes easy to do once the cylinder has been raised to eliminate the sticking of the piston at TDC. The posted photos show what is done to totally eliminate any sticking of the piston at TDC. Raising the cylinder a known amount, exactly .200" in this case, eliminates the sticking problem. Sometimes we use a gage block (shown in the photo) & sometimes we use a precisely machined ring that sits under the liners lip. We also check the measurement after any shims that may be necessary are included in the measurement. We use .001", .003", .005" & .008" shims to set the deck clearance & timing to exact numbers. Every part must be super clean to do this!!

Jim Allen

Jim Allen said:

Tom,

You will want to measure at the maximum crunch point (TDC) of the piston in the cylinder. This becomes easy to do once the cylinder has been raised to eliminate the sticking of the piston at TDC...

Click to expand...

Jim- I must be missing something here... regardless of pinch, will not the rod still push the piston all the way to TDC?... we understand that

whatever clearance is taken up at the w/pin and at the crank pin/ rod bushing interface will lead to a bit of error, but the majority seems

like it would be taken up while running considering compression event, no?... like I said, we're probably missing something here.

I've always just used a depth mic at TDC and at squish deck to button touch down surface and did the arithmetic to determine how much

shimming it needs- adding .001" as insurance, so to speak.

Steve mentioned these dished out pistons...I just consider that volume + bowl/ squish angle volume to determine trapped CR... it seems like you

can offset the piston dish volume by going with less bowl volume, if fabricating a button... maybe the OEM's take that into consideration with

that design when designing their bowl volume in the button.

My NR46DD has NO squish band on the standard AND 'turbo' button... they angle upward right from the cylinder wall and intersect the bowl

wall, flat deck piston, no dish.

Thanks-

Tim

"Jim- I must be missing something here... regardless of pinch, will not the rod still push the piston all the way to TDC?" I do not know what you're missing except that you DO NOT want to take any measurements at the point where the piston is stuck in the bore (pinch point). In the photo, the depth micrometer is reading .394" with the .200" gage block in place. .394" minus .200" gives .194". This number tells me the cylinder is at the correct height for the desired timing. .194" minus .185" (measured head button depth) gives .009" clearance. .009" plus .007" gives a .016" total deck clearance.

Jim Allen

Stick some solder in the plug hole and be done with it.

It ain't rocket science it's a toy boat..............................................

DEEP BREATHS Terry DEEP BREATHS>>>>>>>>>

dwilfong said:

Stick some solder in the plug hole and be done with it.

It ain't rocket science it's a toy boat..............................................

DEEP BREATHS Terry DEEP BREATHS>>>>>>>>>

Click to expand...

That's exactly what I do.  You have to use a thin solder tho to get a good reading (0.016" for me) and be sure to cut the little tang left from the champher at the piston crown. 

No mixing up the numbers, accounts for all the play in the rod/wrist pin etc.  

Terry Keeley said:

That's exactly what I do.  You have to use a thin solder tho to get a good reading (0.016" for me) and be sure to cut the little tang left from the champher at the piston crown. 

No mixing up the numbers, accounts for all the play in the rod/wrist pin etc.  

Click to expand...

I like to see the tang that way I can check right to the edge and know I am there.

With a tapered squash and no flat spot it is nice to know you are at the edge.

You can also check the squash angle knowing the edge and chamber ends thickness no matter what type of crown or dish you have on the piston.

O and you don't have to tear it down to check it.

Cheep as heck to do also.

Just get a dial caliper off EBAY for a few bucks.