Size of 3/8 pistons

[Kurt in NJ]

I have a 59 block that is bored 3/8, it is one of the most perfect blocks I have seen, doesn't even have the "factory" cracks, looks to of never had plain water in the water jacket,never rusty inside or outside, the valve seats are high and narrow---just a minor problem, it was fitted too tight and scuffed the pistons and slight scoring of cylinder walls----the worst one cleaned up at 3.377, is there a particular piston that runs larger (for4" stroke)----what do the bore sizes generally end up at to fit pistons available today----
I know it is possible to get 3/8 +.010 or something like that but i wanted to keep as much bore as I can to save the "one more time"

[flatheadmurre]

Usually the boresize is what it is and the pistons are shaped and sized to expand and fit in the bore.
Didnt the manufacturer of the pistons supply the measurements you needed to bore it ?

[flatjack9]

What do the pistons measure?

[Kurt in NJ]

The block came with scored collapsed Pistons for standard stroke, they fit the bores .012 loose
I want to get new Pistons
There is a interesting story, I bought the block without main caps but was told that he would have them "next year" for 3 years I stopped and asked about them (at Hershey) -- no caps, then about 7 years later I was walking by a booth and the guy came running after me "I have the caps for that block you bought
What I am asking is if some brands run larger in size

[Bored&Stroked]

Tell you what - I'll measure a couple of new ROSS 3 3/8 pistons - just to see what they come at "off the shelf".

I will say that .012 is too dang much in my eyes (even for forged pistons) . . . this sucker will be "slapping" around and this isn't good for sealing the rings - or for additional skirt wear.

[rotorwrench]

Sponsored Links (Register now to hide all advertisements)

A machine shop will generally not finish hone a cylinder wall until they have the piston in hand. Forged pistons require more clearance than stock ones due to the expansion rate of the forged aluminum. Every manufacturer should supply information about the recommended clearance for their pistons. 3.375" is about as large as most flathead Ford V8s can go with some exceptions. Pistons can be custom made but a person has to be worried about how thick the remaining cylinder wall is.

[JWL]

Yes, the Badger pistons are larger for a given bore size than the Ross pistons. I encountered a situation where the Badgers were to be replaced with Ross pistons of the same advertised bore size(3-5/16) but the correct Badger bores were already too big for the Ross pistons even allowing extra clearance for the forgings. I do not remember the exact dimensions at this time.

[flatjack9]

That's why I asked what the pistons measured. Some manufacturers make them for a 3 3/8 finished bore size and some make the actual piston 3 3/8.

[Ol' Ron]

Plus the Badger pistons are the heaviest pistons I'vbe ever seen, I think they are made of lead.

[GOSFAST]

The piston size, generally speaking, has the desired p/w built in to the piston according to the finished bore size when they are mfd. Also usually determined by the piston mfr. There are some exceptions but it's been this way for years now.

It really all comes down to the cam-grind (on the skirts) used by the mfr.

I can give you some accurate numbers from the pistons we have on hand, from Badger's, Nylen's, and Ross. Remember Ross's are all forged though.

With the Badger's and the Nylen's, both cast, and based on a +.060" over piston (I just happen to have these in front of me at the moment) both brands measure 3.2465" and have a recommended clearance of .0015". This is fairly standard on these style pistons.

The Ross pistons I have here, based on a 3.312" finished bore, measure 3.308". The same size carries over to the 3.375" pistons, they measure 3.371". So Ross is recommending .004" P/W. This is a "nominal" number from them, it is NOT "etched-in-stone", each individual unit MAY vary. It's up the builder/installer.

We just delivered a twin-turbo BBC 1800+ HP unit with .0045" P/W. Personally I didn't think it should be that tight but that's where C-P piston's wanted it.

Thanks, Gary in N.Y.

P.S. The actual P/W we choose depends entirely on the environment where the unit finds itself in the end. I would just add, even at the .004" P/W clearance on the Ross pieces we've never had single complaint about "piston-slap", not once!

[Frank Miller]

Any thoughts on knurling the pistons? I think the rings will be ok.

[Bored&Stroked]

I have no problem with .004 to .005 on a forged piston - it will expand and you need some room for it. Like Gary, I've never had a piston slap issue at the recommended clearances - but if it was three times the clearance (for forged) at .012 . . . I wouldn't be running it. Sounds like you're going to need 3 3/8 + .015 or so pistons. Before I headed this custom route, I'd definitely Sonic and Pressure test the block - as custom pistons and rings will probably be over $100 a hole.

Update: Just measured a new Ross forged, comes in at about 3.370 or so. I think .007 is a bit much for clearance - especially if it is for a street engine.

[Kurt in NJ]

Some of the holes are 3.774, I think the pistons were used when they were put in and they were put in too small of a bore for used pistons---there is metal transfer on the pistons, yes I do have a piston knurler, could probably make them work, but why, i want 4" stroke, would like to use the thin rings, don't want heavy pistons, was reading some about coatings ---it's a possible option----

[GOSFAST]

Originally Posted by Kurt in NJ

Some of the holes are 3.774, I think the pistons were used when they were put in and they were put in too small of a bore for used pistons---there is metal transfer on the pistons, yes I do have a piston knurler, could probably make them work, but why, i want 4" stroke, would like to use the thin rings, don't want heavy pistons, was reading some about coatings ---it's a possible option----

Hi Kurt, what size do you mean above (in red)??

Just a guess, I would doubt the pistons were in too tight, but you never know with these things. The "coatings" you mention are going to add minimally to the end size?

Thanks, Gary in N.Y.

P.S. Just a tip but if you have "metal-transfer" from the bores to the pistons it is usually caused by "water/water temps", as in continual overheating OR running the unit very low on oil. The latter being somewhat rare, usually other damage comes along with low oil.

[Kurt in NJ]

I think the pistons were used, some of the marks didn't match the marks on the bores, perhaps it is just the grey burned piston, I will look closer tomorrow, i am moving the pile the block is in, the other bores are turned away in the pile and real good measuring/inspection(the whole bank(5-8) blindly measures 3.774, as do #1,2,4--#3 is at 3.377) is hard---all I have is the note on the tag from when i put it away some 20 years ago, i will pull it out and clean the grease out of the bores, come up with some wall thickness figures, I don't have a sonic tester, but I have some homemade measuring tongs that will give me an indication----as i said in the beginning this block doesn't have faking waterjacket rust---looks like always clean rust inhibiter or antifreeze----all these old blocks have some problem, this one just seems to have a bore size problem, when I was in high school i drove a 46, every week or 2 I would buy a flathead core at the junkyard, sold the car, kept the spare good blocks/engines---at that time I rejected quite a few blocks that would be fixed today

The 3.774 is what some of the other bores measure---didn't go into .0005, just that the bores are tight on mike at 3.774,no way mike fits at 3.775 but I don't know if that is a cleaned up size---using inside mike,checked with outside mike,checked with standard

[JWL]

I think there is just a "3" missing in the dimensions--for example 3.377.

[Kurt in NJ]

Ok yes 3.377 for the larges bore, 3.374 for the smaller bores

[oldfordman33]

The solution is simple. Sent the pistons you want to use to the machine shop that you want to bore the block.
That way he can bore the block to fit the pistons. Saves a lot of headaches.

Oldfordman33

[rotorwrench]

There is no substitute for ultrasonic thickness test. The corrosion pitting is usually where you can't even see it down in the bottom or even a slight core shift during manufacture so dividers won't help there. Machine shops that have ultrasonic testing equipment usually don't charge all that much for a check. A person would know right away if there was a core shift. Corrosion pitting is harder to find but there are areas that are more susceptible to check. A good pressure test is also an idea. You won't want to spend a bunch of bucks on parts unless your foundation is good.

A motor bored all the way out is good for a good time but maybe not for a long time. I hope it works out for you.

[Kahuna]

As said, do the sonic testing first.
I would then call Ross and inquire about the piston cost.
I would also call Egge and ask the same thing.
I really can't imagine that it would add more than a hundred bucks to the piston cost as a set, but B&S maybe right.
My last engine had cast Egge pistons: 3.330.
There was no extra charge for them with a 4.25 stroke.
Just my experience.
I forgot to add:
I had my current engine block Sonic tested, Pressure tested & Magged.
I really believe that the cost ($250.00) is worth every cent.
It gives me great peace of mind, knowing I had a great foundation to
begin with