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Cylinder Barrel Shims
Posted by: wittsend ()
Date: March 16, 2022 06:15PM

I'm putting together a "Ring & Bearing" '61 80 HP engine. While it will never be a powerhouse if there are things that might add a little to the output I put them under consideration.

The heads (#588) I had to cut .017 to clean the surface. Even with the .017 removed the step is .090 (and that is not accounting for what the piston might be in the hole, much less the shim). I have .032 gaskets. I believe it all totals up to around .135-ish. With these already undesirable quench numbers would there be any benefit to not using the .012 barrel shims? I'm thinking No, but thought I'd ask.

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Re: Cylinder Barrel Shims
Posted by: JimBrandberg ()
Date: March 17, 2022 04:54AM

I don't know what you mean by barrel shims. In my mind "barrel shims" are for when you put LM head on EM cylinders.
If you're talking about omitting the cylinder base gaskets to gain compression ratio I would not do that.

Jim Brandberg
Isanti, MN
CorvairRepair.com



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Re: Cylinder Barrel Shims
Posted by: vairmech ()
Date: March 17, 2022 04:58AM

If you started with a .100 gasket step are you sure you have 80 HP heads? I didn't think the 80 HP heads had that big of a step.
Whatever you have you would be better off taking another .013 off the gasket. If you have the tools take another .040 - .050 off the gasket surface and then take the appropriate amount off the flat surface. Before yo do that make sure you level the rocker gasket first if you use a mill. I posted about this a long time ago with pictures.

Ken Hand
Handy Car Care
248 613 8586

Vairmech@aol.com

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Re: Cylinder Barrel Shims
Posted by: wittsend ()
Date: March 17, 2022 10:11AM

Jim B. Yes, thank you. I did mean the cylinder base gasket. I couldn't remember the common term (64 year old brain stuck in neutral) and..., well..., they do go on the cylinder barrel and the end effect is they do shim up the barrel. At least that part of my brain had some "gear mesh" to create a descriptive term.

Ken H Yes, 80HP heads for sure. That is why I mentioned they were #588. I had posted a question about this about a year back. I went and got the exact numbers for this post.

The cut to clean the gasket surface was .016 from where I started. And, that start measurement was within the .570-.580 nominal depth from the outer surface others had reported. The distance from the newly cut surface to the quench area step was .085. So, that math says the original gasket surface to quench area was .101. If I recall correctly did the Corvair came with .042 gaskets? If so the quench area would have been .143. I don't know how far the piston may be in the hole (I haven't assembled that far year) but if there is a nominal amount add that to the .143 and the quench is a far cry from the desirable .040.

That is why I was asking if the .015 of the (want to say it correctly...) Base Cylinder Gasket would make any difference to remove as it is roughly only 10% of the quench area. If it is an "anything helps" then I will. Otherwise I will assume GM put them there for a reason. For sure I won't be welding/putting the heads in a mill. This is as I mentioned a "Ring & Bearing" job for a very low use engine.

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Re: Cylinder Barrel Shims
Posted by: cnicol ()
Date: March 17, 2022 10:36AM

I wouldn't worry about compensation for material removed from the head gasket register.

While compression may go up a tad, actual combustion chamber volume "as built" typically falls short of the advertised compression ratio so the minor amount removed probably brings it closer to the advertised compression ratio.

The only way to know for sure is to measure your combustion chamber volume and do the math. FYI, in my experience compressed head gaskets measure .030" (5cc), deck height (at least on 164 engines) is often -.010"

Craig N. Coeur d'Alene ID.
66 Black Monza 4dr, 4.2L V8 49k
61 Seamist Jade Rampside 140 PG
65 Canadian Monza 4dr 110/PG 40k
66 Sprint Corsa convt - First car! Re-purchased 43 years later
2+2 gnatsuM 5691

+17 Tons of parts

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Re: Cylinder Barrel Shims
Posted by: JimBrandberg ()
Date: March 18, 2022 05:51AM

It's a little late but I don't know if I'd put much effort into 61 heads with the weak valve springs.
I'm also not a big fan of the hand chokes. They're great when they're working but the splitter at the front of the engine compartment, the long cable to the driver and the mechanisms on the carburetors can be troublesome.
Just because I wouldn't put much effort into them doesn't mean that they're bad. A good running 61 80 HP can be sweet. I'm only saying if I was putting some money into heads I might start with dual springs and automatic chokes.

I usually figure the stock steel head gaskets at .032. I think what Ken is alluding to is if you're removing material from the head gasket surface you need to make sure the flat portion of the head doesn't bottom out against the cylinder before the head gasket seals up. That can be accomplished by removing material or using a thicker head gasket.

Sometimes racers will try to get by without the cylinder base gasket to get all the compression they can but since they are a gasket I wouldn't leave them out of a regular engine.

The 84 HP engine is the 80 HP bottom with 102 heads for more compression so it's certainly been done. However, with rising gas prices getting by on 87 octane is looking more attractive if one is going to be driving it much.

If just doing a rings and valve job sort of a thing with what I have on hand but finding that the head gasket surfaces are needing flycutting I would be inclined to just use thicker copper head gaskets. If .032 is stock, there's also .042 and .052 or you can get more by using two. I like to use crushing stock steel head gaskets when I can but some folks actually prefer copper. I was surprised to find that the earliest engines had copper head gaskets.

Just my opinions on getting by with what you have and I could be wrong. A Machine Shop can do anything but the cost goes up.

Jim Brandberg
Isanti, MN
CorvairRepair.com



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