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Old 11-06-2017, 05:20 PM   #1
mike657894
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Default Conflicting piston clearance info?

On Macs spec page it lists bore as 3.876 and piston as 3.8745. I measured my bore and it was 3.880 and the piston 3.843. that says to me my clearance is .037 pistons appear to be aluminum and only have slight carbon on 1/3 of the piston top. Suggesting it was a recent rebuild before the 30 years of storage.

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Old 11-06-2017, 05:37 PM   #2
Brian T
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Default Re: Conflicting piston clearance info?

Hi Mike,
You maybe measuring the piston below the piston pin, try measuring 90 degrees across from the pin, pistons are normally egg shaped on the thrust side.
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Old 11-06-2017, 05:45 PM   #3
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Default Re: Conflicting piston clearance info?

It seems I was misreading the Macs numbers. I measured at the top of the piston. I have the head off to do some valve work. I can move the piston side to side in the cylinder a lot. I got the feeler gauge out and the largest feeler would fit between piston and bore stopping on the ring. Even though it's a stiff feeler
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Old 11-06-2017, 05:56 PM   #4
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Default Re: Conflicting piston clearance info?

Does it run OK?? Does it burn oil?? Was/is compression good?

If yes, forget about it..................drive it.
Paul in CT
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Old 11-06-2017, 06:12 PM   #5
mike657894
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Default Re: Conflicting piston clearance info?

That's where I'm at with it. 67# or near there on all cyls stock head. Doesn't burn oil. Runs starts well. Cylinders are smooth with no ridge. I plan to finish my valve job up when my parts come in. Just wondering if someone could shed some light on why someone would assemble it like this. Or where the small pistons were sourced from? I know from doing Honda motorcycle valves that .002 has no feel to it and .007 has slight feel. It would seem that the 037 goes down the piston as I'm moving the piston in the bore inline with the crank.
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Old 11-06-2017, 07:59 PM   #6
George Miller
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Default Re: Conflicting piston clearance info?

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measuring from the top will not give you a accurate measurement. You have to measure the bottom of the piston 90 degrees from the wrist pin. If you had that kind of clearance it would rattle real bad.
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Old 11-07-2017, 04:23 AM   #7
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Default Re: Conflicting piston clearance info?

The top of the piston above the rings is quite a bit smaller than the piston skirt. It is no surprise you got a reading of .037.
The feeler gauge needs to be against the piston skirt and the bore. It is normally done with the piston inverted down the bore.
If it was quiet, just put it back together and run it.
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Old 11-07-2017, 10:29 AM   #8
Ronnie Lawson
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Default Re: Conflicting piston clearance info?

Piston Measurement.jpg
This is how to measure a piston
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Old 11-07-2017, 07:08 PM   #9
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Default Re: Conflicting piston clearance info?

here are the photos of pushing the piston to one side and the largest feeler in there .034 then pushing the piston to the other side and putting in the feeler. the videos show the shake in the bore. SO im going to take it apart and measure the bores with the proper tools at multiple locations in the bore and order a piston kit. or consider having it bored. The engine was noisey when reved but I thought that was just the lifters talking.



https://www.youtube.com/watch?v=YnXK...ature=youtu.be

https://www.youtube.com/watch?v=qKbI...ature=youtu.be


when I get a piston out ill take the proper measurements and post it on the thread along with piston pictures. I am assuming these are turned down A pistons or from something else.

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Old 11-07-2017, 07:29 PM   #10
George Miller
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Default Re: Conflicting piston clearance info?

Why take it out you are measuring it wrong.
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Old 11-07-2017, 07:37 PM   #11
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Default Re: Conflicting piston clearance info?

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Originally Posted by George Miller View Post
Why take it out you are measuring it wrong.
did you hear the knocking I created with my hand in the video? also how would you measure the piston correctly while in the bore??? I have a xl600r it has .002 on the intake valve clearance. setting a clearance that tight is near maddening with that .002 feeler being near foil. this aint no .002 anywhere. it aint no ten. It could be less than 37 but not by a whole lot. watch the videos.

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Old 11-07-2017, 09:48 PM   #12
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Default Re: Conflicting piston clearance info?

Those slugs are as loose as the knob on a shit house door,you dont need to measure anymore brother,you need to yard'em out...bet that sucker sounds good with the spark retarded..

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Old 11-07-2017, 10:32 PM   #13
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Default Re: Conflicting piston clearance info?

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Originally Posted by Railcarmover View Post
Those slugs are as loose as the knob on a shit house door,you dont need to measure anymore brother,you need to yard'em out...bet that sucker sounds good with the spark retarded..
I like the way you put that. You dont need a number to know thats not how it should be. This is the only model A i have been around other than at a show or in a museum. So I just figured it was an unbalanced one and it didnt like high speeds. But its got some real shaking going on at 45 which is the highest speed I had it to so far. Well its best to catch this stuff early. Before it gets pricey. I have put less than ten miles on the car. And have had my hands in a lot places now on it.
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Old 11-07-2017, 10:40 PM   #14
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Default Re: Conflicting piston clearance info?

Bore it out and fit new pistons and rings.
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Old 11-07-2017, 10:51 PM   #15
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Default Re: Conflicting piston clearance info?

Quote:
Originally Posted by mike657894 View Post
I like the way you put that. You dont need a number to know thats not how it should be. This is the only model A i have been around other than at a show or in a museum. So I just figured it was an unbalanced one and it didnt like high speeds. But its got some real shaking going on at 45 which is the highest speed I had it to so far. Well its best to catch this stuff early. Before it gets pricey. I have put less than ten miles on the car. And have had my hands in a lot places now on it.

Here's your prayer since the 'recent rebuild' was done by a hack who didnt measure his work..that you can save the bores by honing get the right sized pistons and be done with it. If you open it up and she needs the full monty do some homework before diving in, it gets expensive.I got lucky, I bought a burnt chassis that's been a gift that keeps on giving..the engine is nice
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Old 11-08-2017, 12:51 AM   #16
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Default Re: Conflicting piston clearance info?

The piston, if correctly made, is not round. Agree with George, the pic shown is measuring it wrong.
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Old 11-08-2017, 12:53 AM   #17
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Default Re: Conflicting piston clearance info?

Ok, stop now and let some one that knows what their doing look at it.
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Old 11-08-2017, 03:00 AM   #18
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Default Re: Conflicting piston clearance info?

You need the rings off the piston to measure the clearance and you should use long narrow feeler gauges rather than the standard wide short ones. As George said, you are 90* off on where you measure the skirt clearance.

Bad engine vibration can be caused by a worn out main bearing, and the center one usually goes first because Ford forgot to use counterweights on the crankshaft.
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Old 11-08-2017, 09:08 AM   #19
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Default Re: Conflicting piston clearance info?

Quote:
Originally Posted by Dodge View Post
Ok, stop now and let some one that knows what their doing look at it.

There are no 'wizards' in a model a engine. Basic mechanical skill, a strict attention to detail and cleanliness is all you need to tear one down or assemble it.You don't need thirty years fixing engines to see piston slap, just watch his videos.


Tom, I agree with the lack of counterbalance. the lack of a simple mechanical advance distributor added to that flaw.
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Old 11-08-2017, 10:37 AM   #20
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Default Re: Conflicting piston clearance info?

Im not in over my head here. I measured the way I did just to ball park what was going on. just so I didnt start the post with no numbers for those that like math. I do worry if this is how the top end was put together I may find similar problems down low. But I wont let it get me down. my mechanical mentor had mentioned doing his own babbit on his john deere 730 20 years ago and he is still using that tractor so thats no fear for me.
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