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Old 07-01-2016, 09:53 PM   #1
Bluebell
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Default How do I measure a small piston to head clearance

Hi guys I have a block with Ross 3 5/16" pistons. I have a pair of C7RA alluminium heads, which have been faced to get a good surface. If I put a head on loose with no gasket it rocks on the piston. If I put a used gasket on with the head and just hand nipped down on 4 studs, the engine turns without any noticeable contact. I'm guessing I have about )0.020" P. to H. clearance on this.
What is the best way to check a small clearance like this? and how do I remove the small amount of metal from the heads? Details are good.
I was hoping not have to remove metal from the heads.
Thanks in advance.
Red.
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Old 07-01-2016, 09:59 PM   #2
Mark's 37
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Default Re: How do I measure a small piston to head clearance

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Originally Posted by Bluebell View Post
Hi guys I have a block with Ross 3 5/16" pistons. I have a pair of C7RA alluminium heads, which have been faced to get a good surface. If I put a head on loose with no gasket it rocks on the piston. If I put a used gasket on with the head and just hand nipped down on 4 studs, the engine turns without any noticeable contact. I'm guessing I have about )0.020" P. to H. clearance on this.
What is the best way to check a small clearance like this? and how do I remove the small amount of metal from the heads? Details are good.
I was hoping not have to remove metal from the heads.
Thanks in advance.
Red.
I would get some Playdough from the toy store and put a layer over the piston then install the head with a good gasket and slowly rotate the engine a few revolutions by hand, not with the starter. If the engine comes to a halt when turning it...stop. Don't force it as you don't have enough clearance. Then remove the head and peel the Playdough off and measure the thinest parts with a digital micrometer. Then you will be sure of the clearance. Removing metal (milling) from the heads will decrease the piston to head clearance. Using a thicker head gasket could help.

http://www.flatheadv8.org/clayhead.htm

Last edited by Mark's 37; 07-01-2016 at 10:27 PM.
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Old 07-01-2016, 10:29 PM   #3
RalphM
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Default Re: How do I measure a small piston to head clearance

You can also use balls made of aluminum foil, place them in different spots on top of the piston. Then turn over and measure
I believe you should have about .050 clearance to be safe. The only way to increase clearance is by thicker gasket as you said,
Or have the chambers remachined.
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Old 07-01-2016, 10:39 PM   #4
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Default Re: How do I measure a small piston to head clearance

Actually you can clearance the head with a Dremmel tool by just removing the material where the clearance is tight. I usually take off .030" off the heads and then clearance them this way, only I like a tighter clearance closer to.040" I use the aluminum balls their more accurate. also use an old compressed gasket. or a .050 shim gasket.
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Old 07-01-2016, 10:41 PM   #5
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Default Re: How do I measure a small piston to head clearance

Another way to do this is to use small (pea-sized) balls of aluminum file stuck to the pistons and valves with a dab of grease. I just did this to my engine last week an it worked slick. I just picked up my heads today from the machine shop after being milled .025 on one side and .010 on the other. I now have .045 over 5 pistons, and .043, .047, and .055 on the other 3, which I am satisfied with. (I don't want to be grinding on my heads either.) These measurements were obtained with the heads snugged down over the old gaskets that were in excellent shape.

I had aluminum foil at home, but no Play-Dough, so I went that way. I had no problems with little balls slipping or changing shape when I removed them from the surfaces. I was always was kind of worried about a clay-type medium stretching or deforming when I removed it.
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Old 07-01-2016, 11:51 PM   #6
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Default Re: How do I measure a small piston to head clearance

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Mate...ring me. Sorry those heads I sold you are giving you grief. I have the tooling to sort it.
Your technique of fitting heads sans gaskets and checking clearance is spot on! It's what I do. I have struck your problem before, I made a tool that exactly replicated the radius of a Ross piston, [I have another that matches Ford piston] and have a fixture to hold head in milling machine in order I can machine whatever needs to come out.
You need to determine this amount for each cylinder. Don't bother with clay etc, you already know you have contact, so that ain't gonna show you f@%$ all! Use either feeler gauges or a DTI to determine how much the head is lifting as each piston comes up and passes TDC. Write these amounts down. Also, take note of which head fits which side, although, in theory it shouldn't matter.
Send the heads and your information to me and I'll machine to suit.
You can do it like others have described above, but by using a form tool with the correct radius, the contact points are the bits that get machined off, no need to use playdough etc to show where.
Now....if its the valves that are hitting, its a different story. I have no fancy tooling for that, I just use a die grinder and carbide burrs.
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Last edited by Brian; 07-02-2016 at 12:12 AM.
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Old 07-02-2016, 02:57 AM   #7
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Default Re: How do I measure a small piston to head clearance

Well there is a stand up offer right there , top man Brian.

I bought some edelbrock heads on our local auction site , my bad never looked closely at them till a few months later and the seller was an outright liar, someone had used a hole saw to cut around several studs to attempt to remove them, result un useable. No they were not a bargain price.

Bluebell tell us more about your flathead, it sounds cool.

Last edited by my4dv8; 07-02-2016 at 03:17 AM.
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Old 07-02-2016, 03:47 AM   #8
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Default Re: How do I measure a small piston to head clearance

Brian [my4dv8], sounds like they'd be repairable...use a slot drill in milling m/c to open up the holes to a suitable size to be able to press in appropriate sized aluminum tube. Thing is to not go right through, you need a shoulder to press up to. Restores your stud holes back to std.
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Old 07-02-2016, 07:27 AM   #9
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Default Re: How do I measure a small piston to head clearance

Hi Brian, the heads you sold me are fine. The "grief" is not un-expected. I will do some measurements over the next few days and get back to you.
Your offer is great and I will probably take you up on it. I will give you a call tomorrow.
My 4dv8, there's not much to tell. It is a C69A block with new French rods and the Ross pistons. other than that it is standard. With the heads from Brian and the 3 5/16" bore it should have a C.R. of about 7.5 or 7.6:1.
I am doing it to go in a Bren carrier.
The idea was to increase the cubes a bit, so as to get a bit more torque down low. a carrier weighs about 3 1/2 ton, so the FHV8 works hard. Especially while turning.
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Old 07-02-2016, 07:35 AM   #10
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Default Re: How do I measure a small piston to head clearance

Hi Brian, the heads you sold me are fine. The "grief" is not un-expected. I will do some measurements over the next few days and get back to you.
Your offer is great and I will probably take you up on it. I will give you a call tomorrow.

My 4dv8, there's not much to tell. It is a C69A block with new French rods and the Ross pistons. other than that it is standard. With the heads from Brian and the 3 5/16" pistons, it should have a C.R. of about 7.5 or 7.6:1.
I am doing it to go in a Bren carrier.
The idea was to increase the cubes a bit, so as to get a bit more torque down low. a carrier weighs about 3 1/2 ton, so the FHV8 works hard. Especially while turning.
I have enjoyed doing this motor, (not finished yet) and can see how one can develop an affection toward these things.
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Old 07-13-2016, 12:37 AM   #11
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Default Re: How do I measure a small piston to head clearance

I just want to say a big public "thank you" to Brian (who I have never met)
My two C7RA heads arrived back today, all nicely machined to give 0.00" clearance (without a gasket) in each cylinder.
Brian wouldn't take any money for doing the job (not even the courier cost) He said these flatty's are his passion.
All the best, to you, Brian.
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Old 07-13-2016, 01:38 AM   #12
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Default Re: How do I measure a small piston to head clearance

Thank you, and you're most welcome my friend.
And thank you for the money you've put in my bank, you did not have to do that, but, thanks, I will accept it with much gratitude. But I still want a ride in your 'tank' lol.
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Old 07-13-2016, 08:11 AM   #13
Ol' Ron
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Default Re: How do I measure a small piston to head clearance

I calculated your CR as 8.8:1.This is alittle hi for 87 Oct gas. However, using a latr ignition system you might get away with it. The advance curve is critical.
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Old 07-13-2016, 01:58 PM   #14
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Default Re: How do I measure a small piston to head clearance

Hello Ron, I thought about 7.5:1 C.R. I'll have to work it out now that you've said. Our gas here is either 91 or 95 with 98 available (98 is ethanol)
I will be running a crab dizzy.
Thanks for the warning Ron. I was just going to run it.
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Old 07-13-2016, 02:26 PM   #15
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Default Re: How do I measure a small piston to head clearance

Years ago I got a set of steel shim gasket, I cain't remember where I got them, I think it was in Texas, I think you could get them .010--.020 thick. Maybe someone remembers where they are. Walt
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Old 07-13-2016, 03:23 PM   #16
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Default Re: How do I measure a small piston to head clearance

When you calculate the CR you have to add 15cc to the chamber volume for the gasket. This is just a gustamate, so the actual CR maybe a few tenths off.
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Old 07-13-2016, 04:46 PM   #17
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Default Re: How do I measure a small piston to head clearance

Fel-Pro flathead head gaskets have a 0.062" compressed thickness. I'd say that alone provides plenty of clearance.
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Old 07-13-2016, 05:09 PM   #18
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Default Re: How do I measure a small piston to head clearance

Brian is indeed a great ford guy,he helped me with a set of bearings for the rear axle mod I,m doing,same thing ,would not take any $$,
Thank you Brian,and the mod has worked out great and I,m just waiting for one on my 33.34 fords to need a rear end swap to test it out.
Lawrie
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Old 07-13-2016, 07:57 PM   #19
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Default Re: How do I measure a small piston to head clearance

I'll second that. Brian is a top guy, incredibly knowledgeable about what works and what doesn't. He has a "Chevy-eater" flathead engine in his '35 tudor that is a killer.
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