Conflicting piston clearance info?

[mike657894]

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.

[Brian T]

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.

[mike657894]

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

[1931 flamingo]

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

If yes, forget about it..................drive it.
Paul in CT

[mike657894]

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.

[George Miller]

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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.

[Dodge]

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.

[Ronnie Lawson]

Piston Measurement.jpg
This is how to measure a piston

[mike657894]

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.

[George Miller]

Why take it out you are measuring it wrong.

[mike657894]

Originally Posted by George Miller

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.

[Railcarmover]

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..

[mike657894]

Originally Posted by Railcarmover

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.

[J Franklin]

Bore it out and fit new pistons and rings.

[Railcarmover]

Originally Posted by mike657894

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

[PC/SR]

The piston, if correctly made, is not round. Agree with George, the pic shown is measuring it wrong.

[Dodge]

Ok, stop now and let some one that knows what their doing look at it.

[Tom Wesenberg]

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.

[Railcarmover]

Originally Posted by Dodge

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.

[mike657894]

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.

[BILL WILLIAMSON]

Leave piston clearance measuring up to a QUALIFIED, Engine Machinest!!!!!! There's an ART to it----
Bill W.

[mike657894]

Also that picture was not to show measurement but to show that something fit in there when it shouldn't. And that the gap moved around to where you put it.

[mike657894]

Originally Posted by BILL WILLIAMSON

Leave piston clearance measuring up to a QUALIFIED, Engine Machinest!!!!!! There's an ART to it----
Bill W.

I could have the trained machinist measure it but his numbers have not been different than mine yet. he did tell me that is not where to measure the piston but it didnt matter because it was so sloppy. my mentor is a retired machinest machine repairman uaw skilled trades for GM saginaw steering gear/delphi/nexteer. So when I do get over my head help is available.

but i have to disagree every two bit lawnmower mechanic can measure a clearance.

[George Miller]

Go ahead and set it up at .002 clearance if you like. It will not be to loose then.
Good luck and by.

[mike657894]

I plan to set it up .004 to.007 depending on what the numbers look like when I get the bores properly measured. As I can do that with just a piston kit. Unless I find numbers beyond what Im expecting. which is possible but unlikely.

[100IH]

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Changing piston sizes also requires rings at the same oversize. Cylinders can be honed to open up the bottom of the bore to remove taper but not recommended for getting to the next bore increment. Cylinder bores are always measured at the bottom, below ring travel. Even then, if someone did a lot of honeing to remove taper, the readings would be somewhat out of whack- bigger. Pistons must be measured at the very bottom of the skirt, 90 deg, to the pin. Micrometer readings are all about an experienced touch and feel. Good luck, the measuring is generally free at the machine shop if they supply the parts and labor.

[Tom Wesenberg]

Don't set it up to .007" or you may have a ticking engine. My 28 had crap (rust) build up in the rear of the block, and it used to ping badly due to the hot spot caused by this blockage. Anyway I got it all flushed out, but have a light tick near the rear, which I'm sure is from an overheat collapsing the skirt of a piston. Hopefully I'll have time next summer to swap engines and tear this one down to verify what the tick is. Shoot for .0035" clearance and break it in easy.

[Railcarmover]

Originally Posted by mike657894

I could have the trained machinist measure it but his numbers have not been different than mine yet. he did tell me that is not where to measure the piston but it didnt matter because it was so sloppy. my mentor is a retired machinest machine repairman uaw skilled trades for GM saginaw steering gear/delphi/nexteer. So when I do get over my head help is available.

but i have to disagree every two bit lawnmower mechanic can measure a clearance.

The worst thing about inexperience is fear, especially when those who fear spread it.
The difference between a professional machinist and a car hobbyist is the machinist measures it once, correctly. The hobbyist measures it four times till he's satisfied its correct. Either way the results are the same. Attention to detail, patience and cleanliness is the skills all engine builders use,newbie or journeyman.

[mike657894]

I just measured each bore 5 times. You are right. I got 3.890 or near there on all 4 bores the pistons did grow down the skirt to 3.870 and under. The pistons under closer inspection are old. I can make out front-0 on one and that half worn off another and couldn't find it on the other two. the numbers with the piston out make more sense for heavy equal wear between cylinder and bore. I am going to get the machinist's opinion and have him measure it up a second time before I order anything. Im thinking a 20 over kit.

[Railcarmover]

Sounds like your headed to the machine shop.Before you go, measure out the entire engine, the last thing your wallet needs is a surprise from those guys. Next is to formulate a clear vision of your completed engine, that's where I'm at on my build. I got lucky, I dodged the machine shop according to the calipers and mic, now I set a horsepower target of 50 to 55hp figure a mechanical advance distributor, dual small carbs and a header, might rock a b cam too..already found a mitchell OD and a great chassis the vision for the car is a smooth 65 on the freeway.Heres some dyno sheets,help you figure what you want.

http://www.modelaparts.net/dynosheet...ynosheets.html

[mike657894]

Called the machine shop and they said they will bore it 20 over for $85. I asked them if they were sure and that it was for all four not 85 a hole. they said it was. That seams down right cheap. I am going to keep it stock for now. A head would probably be my first power adder but I want to replace all the worn parts before then.

[Tom Wesenberg]

It used to be $10 a hole in the 70's, so $85 for four holes is a good deal, and now is the time to do it if the taper or clearance is too much. If you just give it a light hone and install new rings, you might wind up with a slightly better worn out engine.

[Railcarmover]

85 bucks for a bore job?jump on it,thats cheep...did you pull the oil pan yet?

[100IH]

Your goal right now would be to determine how much taper is in the largest (most worn) cylinder. Original specs from Ford were .007 I think. Certainly many kept going long after they reached .007 wear. Patching up or ring and valve job often meant honing to open up the bottom of the cylinder to reduce taper but increased clearance and piston slap. Doing this a second time: not recommended. $80 well spent as long as it includes the final honing to clearance the new,larger pistons.

[mike657894]

Its all over the place. Needs to be bored. the biggest number I got out was 3.893 in the middle and smallest was 3.880 at the bottom. May need .030 over. babbit feels tight. I didnt have platigauge on hand but no shake by hand. I will gauge those before I put it together. If I get the time im going to pull the engine out this weekend and clean it up to take it in.

macs says there pistons are made to 3.8745 20 over that would be 3.8945 with .0035 for clearance the bore should be cut to 3.898 ? does honeing remove any or just smooth it out and crosshatch? Im thinking I need to order the pistons first so that I can know exactly what size they are. but im not sure if I should get 20s or 30s

[V4F]

did you by chance do a compression test before you took it apart ?

[Chuck Sea/Tac]

Don’t buy pistons until your machine shop cleans the bore. He’ll tell you what it cleaned up at.

[100IH]

And so much about measuring and that is good - but don't forget - don't pour $ into a bad block. Have the machine shop clean, then magnaflux. Then he will mic the crank and the bores. At that point your ready to get out the calculator.