Factory relieved 80" flathead cylinder

theory behind the gas travel paths

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thepigstye
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Factory relieved 80" flathead cylinder

#1

Post by thepigstye » Sat Sep 08, 2018 5:00 pm

any one got one handy? I'm just wondering what the distance is, from the original head gasket surface to the new relieved area in the bore. How deep does the relieving go in relationship to the piston.
Top edge of piston? Edge of top ring? Maybe pics of various home machined paths,etc.



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Re: Factory relieved 80" flathead cylinder

#2

Post by kitabel » Sat Sep 08, 2018 5:28 pm

Depth equal to machined valve relief on the deep side, flush with the deck on the shallow side.
Probably deeper than the piston dome @ TDC. Never exposes the top ring land.

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Re: Factory relieved 80" flathead cylinder

#3

Post by thepigstye » Sat Sep 08, 2018 8:42 pm

machined valve relief ?

Ya lost me there

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Re: Factory relieved 80" flathead cylinder

#4

Post by kitabel » Sun Sep 09, 2018 2:41 am

There's a larger circle machined in to the deck above the seat at a 30 degree (?) angle. The idea is to expose the seat opening to the surface immediately the valve cracks open. This is deepened with a factory cutter after every valve job. It's usually not exact, with a "step" between the top of the seat and the bottom of the relief.
If the cylinder has been hand relieved, this will be blended with the deck and the top of the seat on the deep end.

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Re: Factory relieved 80" flathead cylinder

#5

Post by Frankenstein » Sun Sep 09, 2018 11:26 am

It's been years since I've seen the factory relief job. I had a set of bone stock 80 jugs in the 70's, the reliefs were very shallow, less than what Kitabel hints at. I'd say maybe 1/8" inch at max depth? As I say, it's been a long time and perhaps my memory could be faulty??
DD

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Re: Factory relieved 80" flathead cylinder

#6

Post by 1950Panhead » Sun Sep 09, 2018 2:30 pm

Original 80" cylinder

Image

thepigstye
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Re: Factory relieved 80" flathead cylinder

#7

Post by thepigstye » Sun Sep 09, 2018 2:35 pm

could ya lay a rule across the gasket surface and see what the depth of the relief is , at the bore?

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Re: Factory relieved 80" flathead cylinder

#8

Post by kitabel » Sun Sep 09, 2018 2:46 pm

It varies continuously across the bore.

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Re: Factory relieved 80" flathead cylinder

#9

Post by thepigstye » Sun Sep 09, 2018 2:54 pm

so all the Factory relieving what totally inconsistent on these cylinders? Is there not some kind of formula that determines each path (intake to bore,exhaust to bore)?

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Re: Factory relieved 80" flathead cylinder

#10

Post by chuckthebeatertruck » Sun Sep 09, 2018 3:04 pm

I have a set of '39 80" cylinders. Before the were rebuilt, I was curious about the rather pathetic factory reliefs.

At the deepest point, right at the bore, I measured 7/64ths on the front and 1/8th on the rear.

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Re: Factory relieved 80" flathead cylinder

#11

Post by chuckthebeatertruck » Sun Sep 09, 2018 3:24 pm

thepigstye wrote:
Sun Sep 09, 2018 2:54 pm
so all the Factory relieving what totally inconsistent on these cylinders? Is there not some kind of formula that determines each path (intake to bore,exhaust to bore)?
Yes and no.

sure all sorts of formulas exist, but they go out the window if you have sunk seats, pop up, etc. all of that forces you to adjust your reliefs to the actual cylinder and the position of the top ring land compared to the relief depth at the bore.

The only cardinal rules I know of are: 1) don't let the top ring come closer than .100-.125 of the relief, 2) don't remove more material than necessary, and 3) don't do any relief work until you are 100 percent on your valve guides and seats - it's generally best to start the relief by unshrouding the valve after cutting new seats and guides. Fresh parts make it all so much easier.

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Re: Factory relieved 80" flathead cylinder

#12

Post by thepigstye » Sun Sep 09, 2018 3:32 pm

not a problem there. New oversized intakes, new exhausts, new guides installed, valves reshaped, intake seats cut at 15/30/45º, exhaust seats nut 30/45/60º
Thanks for the measurements, that's what I needed to know. What's the recommended tool to cut the valve reliefs on their back /sides?

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Re: Factory relieved 80" flathead cylinder

#13

Post by kitabel » Sun Sep 09, 2018 6:08 pm

Despite all the work done in the last 80+ years, I'm not convinced that the shapes used are optimal.
The flow leaving the intake seat ATDC is rising, and passes across the chamber following the "scallop" (roof), then the curved transition to the quench surface, and down into the bore. The exhaust flow BBDC follows the same path in reverse. In neither case does any relief at the bore edge serve a purpose, I think it's one of those "that looks right" intuitive opinions. Each relief would then be full seat depth only at the seat itself, and taper upward to become flush with the deck at the bore edge.
I can see, however, a small constant radius across the entire bore edge (1/16"?) to reduce pumping loss.

NOT advice, use your own judgment!

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Re: Factory relieved 80" flathead cylinder

#14

Post by thepigstye » Sun Sep 09, 2018 8:09 pm

so do ya think a WR would run the same if one filled in the paths, so it was like a WL cylinder?

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Re: Factory relieved 80" flathead cylinder

#15

Post by kitabel » Mon Sep 10, 2018 3:34 am

No. The relief aids flow from the seat, the question is how big it it, and where.

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