Flathead bore, taper and roundness questions

T

thehazmatguy

Junior Member
I'm a flathead newb. I've never done a complete engine rebuild on anything, including a flathead. I'm double and triple checking stuff that I do as well as what I'm having done.

I got my 1950 8CM engine back from the machine shop but I had some doubts. So, I bought a cheap Fowler dial cylinder bore gage and micrometers on Amazon, cued up some YouTube videos and took a swing at measuring it. I wasn't happy with the numbers I was getting so I bought a second HF digital indicator to use with the lower half of the Fowler cylinder bore gage (I'm very sure they are made by the same company). All of the measurements are within decent range of each other considering my amateur status as an engine builder, cheap equipment used and variation in cylinder taper.

The machine shop guy told me many times that this is one of the best flatheads he's seen. He told me that he was only going to lightly hone the cylinders, I should get some new standard sized rings for the stock pistons and I was off and running. It seems that is exactly what happened. Nearly all of my measurements are in the range of 3.1875 to 3.1899 inches specified by Ford for standard sized cylinders. He gave it a light hone with a few un-honed smooth spots showing in a few places in the cylinders.

You're saying, "Dude, get to the point."

Every cylinder shows a significant open bell shape at the top of the cylinder. At the very top, within an inch of the top of the bore, each bore opens up a lot. I have no idea if this is stock (doubt it) or it's on purpose (rebuilding trick used by an old school guy that knows more about flatheads than I ever will?) or if this is a simple mistake by someone when honing the cylinders (kind of my thought).

I'd like some opinions but I'm leaning towards having the engine bored ten over. Here's my numbers:

20160703_112117_zpsiqzuiywg.jpg
 
I'd bore it and install new Pistons then all those out of round and taper concernes go away. But that's me, I'm sure they're others with different opinions.
 
The wear and tear happens. This sounds normal plus someone usually removes any ridge that might be there. As the cylinder warms up, this area will choke back in since most of the heat generated is at the top of the cylinder. If you have too much taper, it would be wise to go ahead and bore to the next oversize available. These old engines can take a lot of bore jobs and up top is where you want your best seal.
 
Back in the day, I would hone this out to straighten the cylinder. Then Knurl the pistons and re-assy the engine. Unfortunately this didn't last all that long for a number of reasons, one being the condition of the ring groves. You could also buy .002/3" oversize piston which eliminated the re-bore. If it were mine, I'd bore 125" oversize, get a set of Egge 3 ring pistons, mill the heads and have a nice strong engine.
 
how much do you intend to drive the car if it,s a little like on a week end or car show may be 3,000 miles a year. then just hone it to brake the glaze get std rings with the money you save you could do a lot on the car to make it look & drive better. if you intend to drive it a lot then follow the advise given here
 
Even .020" oversize pistons might be hard to get. They might offer .010" oversize but that doesn't mean they have them in stock. Boring to .010" OS isn't as easy as a person might think. It's only .005" skim on either side. I'm not saying it can't be done but it doesn't leave a lot to work with. The pistons will have to be sourced first then bore the cylinders to get the proper clearance.
 
Go to .030" over min
Pistons and rings aren't a problem .030" and up. everything is the same price regardless of size.
Unless you jump in with 3 ring pistons and the appropriate ring set then things can go north quickly.
R
 
A tip when checking the overall condition of bores for size/wear and assuming you have a fairly accurate way to measure? Bore gauge with a "mike" is the best option for this.

First you need to determine what the actual bore size is now. The easiest way to determine this is by cleaning the very top of the one bore where the ring doesn't travel. This will be the best starting point. It can be done with a little carb-cleaner and some Scotch-Brite pads. Clean all the carbon away and set the gauge to "Zero". You'll have between 1/4" and 3/8" of unused bore at the top to get the original size.

If your looking for an accurate size you would use the "mike" in conjunction with the bore gauge. If you're only looking for the basic "condition" of the bores simply measure up/down and front-to-rear with the gauge still at "Zero" and you'll see the variations. Simple really.

Thanks, Gary in N.Y.

P.S. If you decide to bore it to any o'size I would recommend boring it first (before ordering any new pistons) and see where they clean up, just to be on the safe side. There's been many a time on these Flatheads .030" wouldn't even work? It needs to be bored on ALL 8 cylinders first.
 
If that was mine I would not consider anything but boring it. Do it once, do it correctly.
 
Another way to measure taper in the cylinders is to take a used ring, square it in the cylinder using a piston. Start at the bottom, use a feeler gauge to find ring gap. Now, put the ring at the top of the cylinder-where the ring comes to- measure that gap.

Take that difference and divide by 3 (3.1416, pi). This will be the taper.

And, yes- I would bore it. No machinist I ever met would bore an engine without having the pistons in hand.

Karl
 
Karl Wolf said:
Another way to measure taper in the cylinders is to take a used ring, square it in the cylinder using a piston. Start at the bottom, use a feeler gauge to find ring gap. Now, put the ring at the top of the cylinder-where the ring comes to- measure that gap.

Take that difference and divide by 3 (3.1416, pi). This will be the taper.

And, yes- I would bore it. No machinist I ever met would bore an engine without having the pistons in hand.

Karl
Click to expand...

Hi Karl, you bore the block first, before you order ANY pistons. Any decent machinist will know how much to leave for finish honing. We leave .006" to hone And we use a block-plate, even on these Flatheads.

If you don't bore it first you have no way of knowing what size pistons will work, this is how it's done! If you believe it will go .030" you bore it .024" over, if it doesn't clean at .024" you change the plan to go .040", boring it .034" over, etc until all the bores are "clean". You want to minimize the amount of material removed on a "conventional" rebuild.

Thanks, Gary in N.Y.

P.S. The bore gauge will not find every low/high spot in bad bores, only the boring bar will show this. You will also find the wear is not necessarily centered in the bore, in other words if the bore is "worn" .010" it doesn't mean there is .005" missing from each side? Most times it really isn't due to piston thrust!
 
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