Engine machining tolerances

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Migo
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Engine machining tolerances

Post by Migo » Wed Apr 24, 2019 7:33 am

Hi! Need help with my engine.
I need to find tolerances:
1. Cylinder diameter for piston +0.30 oversize - size +/-
2. Crankshaft bearings diameter tolerance +/-
3. Valve guide size and tolerance +/-

And one more: there is crack on the valve guide. How to fix it? I am going to drill the hole and install some bushing.

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John.Zibell
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Re: Engine machining tolerances

Post by John.Zibell » Wed Apr 24, 2019 9:06 am

For the valve guide, see if you can get K-line installed. http://www.sbintl.com/KLine.html There may be an engine machine shop in your area that installs them.
1926 Tudor


George Andreasen
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Re: Engine machining tolerances

Post by George Andreasen » Wed Apr 24, 2019 10:46 am

Cylinder bore depends on what type of piston you intend to use, aluminum requiring more clearance than cast iron. Any shop should have the clearance specs on hand. A caution here: bore and hone the cylinders FIRST, until they clean up to a standard oversize, THEN order the pistons. I did it backwards and ended up selling my replacement pistons in order to get the correct size.

The standard T crank is 1.248" diameter and the bearings are bored to 1.250", allowing .002" for oil clearance. Grinding the crank undersize follows the same pattern, i.e., a .020" undersize journal would be 1.228" and journals bored to 1.230".

My engine had a cracked valve guide also, which didn't show up until it was being bored for adjustable tappets. It split entirely in two, so the machinist bored it oversize enough to allow a large, steel valve guide to be pressed in. The tappet rides in it quite happily.


MWalker
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Re: Engine machining tolerances

Post by MWalker » Wed Apr 24, 2019 3:16 pm

George A. said, "A caution here: bore and hone the cylinders FIRST, until they clean up to a standard oversize, THEN order the pistons."

That is just backward. It assumes that you know what size the pistons will be before you ever get them. They might be the size you think they will, or they might be something else. A couple of thousandths makes a significant difference here.

The shop which bores and hones my T blocks won't bore them until they have a new piston in hand. When contemplating a rebore, I take the block to the machine shop and have them measure the cylinders, and they tell me what amount of overbore it will need to "clean up." Then I order the pistons accordingly. When I have the pistons to take them with the block, they bore/hone the cylinders to whatever spec they & I agree on, usually .004" more than the piston's measurement perpendicular to the wrist pin. These guys are about my age, and they grew up in this shop when their dad owned it. They've been doing this a long time, and I trust them to do it correctly. I'm certainly not going to argue with them about their procedures or tell them how to do their job. I haven't had a problem with one of their engine jobs yet, and I don't expect to.


Kohnke Rebabbitting
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Re: Engine machining tolerances

Post by Kohnke Rebabbitting » Wed Apr 24, 2019 6:37 pm

We always bore the block first a few thousandths under, as there is no way on some blocks, that you can guess the finished size, or the thought size, to see if it will clean all the, rust pits, wrist pin grooving, and previous boring that was done with a boring bar that was wore out, or a block surface that off before they bored, as there were many done that way.

Doing it that way, you never have to send a set of pistons back, that you thought would fit, and they wouldn't take back because they were slid up and down in the cylinders, a few times, with a feeler gauge.

We fit, on a Model T Driver never under .004, I like .004-50. and never over .005. If the engine runs at a higher R.P.M., we go more clearance.

Aluminum pistons will score at .003 very easy. Never go by the slip of paper in the box of pistons you are using.

Herm.


George Andreasen
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Re: Engine machining tolerances

Post by George Andreasen » Wed Apr 24, 2019 7:39 pm

MWalker wrote:
Wed Apr 24, 2019 3:16 pm
George A. said, "A caution here: bore and hone the cylinders FIRST, until they clean up to a standard oversize, THEN order the pistons."

That is just backward. It assumes that you know what size the pistons will be before you ever get them. They might be the size you think they will, or they might be something else. A couple of thousandths makes a significant difference here.

The shop which bores and hones my T blocks won't bore them until they have a new piston in hand. When contemplating a rebore, I take the block to the machine shop and have them measure the cylinders, and they tell me what amount of overbore it will need to "clean up." Then I order the pistons accordingly. When I have the pistons to take them with the block, they bore/hone the cylinders to whatever spec they & I agree on, usually .004" more than the piston's measurement perpendicular to the wrist pin. These guys are about my age, and they grew up in this shop when their dad owned it. They've been doing this a long time, and I trust them to do it correctly. I'm certainly not going to argue with them about their procedures or tell them how to do their job. I haven't had a problem with one of their engine jobs yet, and I don't expect to.
Good point........let me put it another way. Be SURE the pistons you order will be large enough that the boring/honing will clean up to that size. I ordered a set of .060 pistons after measuring the rough bores...."Yeah, those should clean up to .060 alright". Wrong. Ended up that they had to go to .080 oversize, so I had to sell that set of pistons (had them too long and vendor would not accept a return) and re-order the correct size. Lost money on that mistake.


Allan
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Re: Engine machining tolerances

Post by Allan » Wed Apr 24, 2019 8:27 pm

I certainly agree that the pistons need to be on hand before boring the cylinders. Herm's caution is also warranted if there is any doubt about the bores cleaning up at first.

There is some confusion re the cracked valve guide. The K line recommended will fix worn valve guides where the stem of the valve is worn in the block. I doubt it could be installed to the cam follower bore. These can be easily replaced with a valve guide from a modern engine. Any good machine shop can do this. Just because it is a valve guide when used, does not make it a valve guide in a model T engine. It is a tappet or cam follower bore.

Hope this clears up the confusion.

Allan from down under.

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