Elimination of all cylinder base shims - yes/no?

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C J Murray
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Re: Elimination of all cylinder base shims - yes/no?

#16 Post by C J Murray » Wed Aug 07, 2019 5:21 pm

84cc, really? It looks like they were trying to unshroud the intake valve. 24cc removed is a lot. A skilled machinist could weld up the squish across from the plug and make those heads stellar. Is it possible to get a 30* squish without welding? That would allow taller piston crowns to bump CR. As long as the ports are not hogged out those heads can be salvaged.
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Re: Elimination of all cylinder base shims - yes/no?

#17 Post by Steve Hatfield » Wed Aug 07, 2019 5:41 pm

CJ
Yes using solder, I believe I have 30* squish now.
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Re: Elimination of all cylinder base shims - yes/no?

#18 Post by Mike Horton » Wed Aug 07, 2019 5:51 pm

+1, to CJ's assessment.

The best detail in your pic, is no cracks between the spark plug hole, and the intake valve seat hole, the very reason I chose to use my S90 heads. My spare 912 heads, acquired from a vintage racer, selling out, had cracks in 3 of the 4 chambers. (simple math, for a spare engine; weld repair of 912 heads, $2,000 or up, 22* AA P/C, just under $500, delivered to my door, on sale, from AA = more parts for the overhaul,

Good luck, Steve, good C/SC/912 heads, are getting harder to find, Are those ex. valves stock size?

I use Loctite 515, as a sealant on base shims, easy to clean up, next time. I don't see a problem with no base shims... Have FUN!
Mike

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Re: Elimination of all cylinder base shims - yes/no?

#19 Post by Steve Hatfield » Wed Aug 07, 2019 5:56 pm

Yes Mike the ex valves are stock sodium filled
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Re: Elimination of all cylinder base shims - yes/no?

#20 Post by Mike Horton » Wed Aug 07, 2019 6:02 pm

Steve Hatfield wrote:
Wed Aug 07, 2019 5:56 pm
Yes Mike the ex valves are stock sodium filled
Thanks
My S90 heads, have the 31mm ex. size, 40mm Int., and those appear to be the C/SC/912 34mm perhaps? That was once thought to be a "speed secret",
Mike

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Re: Elimination of all cylinder base shims - yes/no?

#21 Post by Steve Hatfield » Wed Aug 07, 2019 6:48 pm

Mike
Their 34mm ATE 3048 and pass the magnet test proving their the early sodium’s. The original owner had a plan.
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Re: Elimination of all cylinder base shims - yes/no?

#22 Post by Mike Horton » Wed Aug 07, 2019 6:58 pm

Steve Hatfield wrote:
Wed Aug 07, 2019 6:48 pm
Mike
Their 34mm ATE 3048 and pass the magnet test proving their the early sodium’s. The original owner had a plan.
Steve, I suspected as much... Might run fine, as they are... ,
Mike

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Ron LaDow
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Re: Elimination of all cylinder base shims - yes/no?

#23 Post by Ron LaDow » Wed Aug 07, 2019 10:16 pm

1) Don't see any problem leaving the shims out, and perhaps a help; it removes one thermal 'stop', equalizing temps.
2) Really don't like sticking big domes in a 'loose' head to get the CR back. They need more ignition advance to get the flame to crawl over Mt. Everest, meaning you have to back the CR down, meaning looser quench clearances, meaning more spark lead to fire that 'big' chamber, meaning a lower CR, etc. Fix the heads (or find some good ones) and use the smallest dome with the tightest quench you can to get the CR you want.
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Re: Elimination of all cylinder base shims - yes/no?

#24 Post by Vic Skirmants » Thu Aug 08, 2019 7:53 am

"1) Don't see any problem leaving the shims out, and perhaps a help; it removes one thermal 'stop', equalizing temps."
Ron,
Technically, the copper shims would not be a thermal barrier. But the sealer could be; hm?

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Re: Elimination of all cylinder base shims - yes/no?

#25 Post by Steve Hatfield » Thu Aug 08, 2019 8:17 am

Shasta has a set of “Mt. Everest” 34.5 dome pistons that will offer a 8.71:1 cr and CJ has said that “that may work”. So if Vic, you will please weigh in?
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Re: Elimination of all cylinder base shims - yes/no?

#26 Post by C J Murray » Thu Aug 08, 2019 8:25 am

It's not very hot at the bottom of the cylinder anyway but sealer only fills the voids that don't transfer heat I'm guessing.

30 years ago when I started racing motorcycles base gaskets presented a big problem. Many stock engines used paper base gaskets that are perfectly adequate for the rpm and compression ratio they used on the street. When rpm and CR were increased dramatically the cylinders would start dancing on the case and beat the gasket to a pulp and make it expand outwards quite a bit. The solution was to ditch the base gasket and have copper head gaskets special made by Cometic to replace the factory composite head gaskets. This stabilized the cylinder clamping and if you needed to adjust squish you could order a different thickness copper head gasket.

I have never had any problem when eliminating a base gasket.
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Re: Elimination of all cylinder base shims - yes/no?

#27 Post by C J Murray » Thu Aug 08, 2019 8:33 am

Piston domes that stay below the spark plugs and have some reasonable degree of squish usually work. Lower domes with a lot of squish at an equal CR are a safer bet.
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Re: Elimination of all cylinder base shims - yes/no?

#28 Post by Ron LaDow » Mon Aug 12, 2019 12:25 am

Vic Skirmants wrote:
Thu Aug 08, 2019 7:53 am
"1) Don't see any problem leaving the shims out, and perhaps a help; it removes one thermal 'stop', equalizing temps."
Ron,
Technically, the copper shims would not be a thermal barrier. But the sealer could be; hm?
Vic, I'm concerned with the joints rather than the thermal conductivity of the material. Pretty sure the sealer won't cause a thermal break, since it'll be squeezed out of the metal-to-metal contacts, but those are going to be far from uniform, thereby constituting the break.
So that will be one thermal break instead of two, IMO.
Ron LaDow
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Re: Elimination of all cylinder base shims - yes/no?

#29 Post by Ken Tuvman » Mon Aug 12, 2019 9:00 am

Thank You!

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