Conflicting piston clearance info?

[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]

Sponsored Links (Register now to hide all advertisements)

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.