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[email protected]
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Joined: June 24, 2015
Posts: 719
Location: Rialto. CA
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 Posted: Mon Oct 25, 2021 5:45 pm Post subject: Rocker arm shaft size |
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| What would be a down size if ..I went with smaller rocker arm shafts size than the 36hp ..let's say 11.95 mm/0.470 in ... |
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Danwvw
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Joined: July 31, 2012
Posts: 8892
Location: Oregon Coast
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[email protected]
Samba Member
Joined: June 24, 2015
Posts: 719
Location: Rialto. CA
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| Posted: Mon Oct 25, 2021 8:27 pm Post subject: Re: Rocker arm shaft size |
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| I'm looking to install 5.5 valve stems maybe 4.5mm stems on okrasa heads ,custom made ratio rocker arms maybe 1.4 or 1.5 maybe 1.6 🤔.. angled valves so I can get a much bigger ratio rocker arms ...so I was thinking with smaller rocker shafts I would have room and with smaller rocker/shafts it would be lighter ... but was just wondering that 11.95mm shafts would wear out too fast ..it's because I've seen motorcycle rocker arms that size and thought ..hey if it works for that why not on a 36hp ...so it's got me thinking to go that rought "my plans in my head" but would like peoples thoughts...I get some are gonna nod there heads like this guy 🤦🤷 lol ..... |
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Danwvw
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Joined: July 31, 2012
Posts: 8892
Location: Oregon Coast
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| Posted: Tue Oct 26, 2021 7:13 pm Post subject: Re: Rocker arm shaft size |
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I wonder what size shafts the 356's had?
At least the later A heads had all of that. Not sure what the early 52 or was it 53 year 356 engine had, but it was a 36hp aluminum case.
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[email protected]
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Joined: June 24, 2015
Posts: 719
Location: Rialto. CA
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| Posted: Tue Oct 26, 2021 7:19 pm Post subject: Re: Rocker arm shaft size |
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| Porsche 356 shaft size are the same as 36hp |
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Danwvw
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TomSimon
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Joined: January 13, 2004
Posts: 751
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| Posted: Thu Oct 28, 2021 5:03 pm Post subject: Re: Rocker arm shaft size |
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| [email protected] wrote: |
| I'm looking to install 5.5 valve stems maybe 4.5mm stems on okrasa heads ,custom made ratio rocker arms maybe 1.4 or 1.5 maybe 1.6 🤔.. angled valves so I can get a much bigger ratio rocker arms ...so I was thinking with smaller rocker shafts I would have room and with smaller rocker/shafts it would be lighter ... but was just wondering that 11.95mm shafts would wear out too fast ..it's because I've seen motorcycle rocker arms that size and thought ..hey if it works for that why not on a 36hp ...so it's got me thinking to go that rought "my plans in my head" but would like peoples thoughts...I get some are gonna nod there heads like this guy 🤦🤷 lol ..... |
I don't think you are nuts, Jerry
My thoughts:
* The smaller diameter valve stems would lighten up the reciprocating mass, for better valvetrain control at higher rpms.
* I see no reason a guy couldn't machine his own ratio rockers, but pushrod cups and the valve tip contacter (pad or roller) both need to be hard and wear resistant. The body needs to be not-so-hard that it's brittle and breaks. Easy to talk about, harder to execute (ie AA 36hp ratio rockers, GBE T1 ratio rockers).
* Smaller rocker arm trunions (allowed by smaller dia shafts) reduces the mass close to shaft centerline. So close in fact, that reducing mass in the area has little effect on valve train inertia. Smaller shafts and trunions will however, allow you machine a more compact rocker arm, and have room for a larger spring. I've run Crower shaft mounted roller rockers that use 9/16" (14mm) dia shafts and 450lbs open pressure springs on a type 1 vw head (don't ask, it was a sh*t-ton of re-engineering and machine work).
* Ramblings; Remember that by increasing the ratio of the rocker arm, you multiply the valve travel, but you also multiply the force on the pushrod, lifter, and cam lobe. Lifters that don't spin, and cam lobes that have a small base circle, won't take much load before the metal starts to crumble from the contact pressure. How much is too much? I have no idea. Angling the valves in Okrasa heads is going to be a chore. I only know one guy with the skills and crazy/confident enough to try, but it might cost you a failed set of castings in the R&D effort.
* More ramblings; I can imagine a home built rocker arm with large contact area vw style solid bushings (instead of needle bearing on shaft) a guy could get away with much smaller shafts. You mention motorcycle stuff, that is a good comparison, except you'd have to support said shaft very close to the rocker. A vw has these shafts with a large cantalever or overhang, largely unsupported on the exhaust side. Find a motorcycle head that uses smaller shafts, measure the open pressure of their valve springs, and at least you'll know what Honda for instance, engineered as acceptable for a reliable machine.
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[email protected]
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Joined: June 24, 2015
Posts: 719
Location: Rialto. CA
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| Posted: Thu Oct 28, 2021 11:37 pm Post subject: Re: Rocker arm shaft size |
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| TomSimon wrote: |
| [email protected] wrote: |
| I'm looking to install 5.5 valve stems maybe 4.5mm stems on okrasa heads ,custom made ratio rocker arms maybe 1.4 or 1.5 maybe 1.6 🤔.. angled valves so I can get a much bigger ratio rocker arms ...so I was thinking with smaller rocker shafts I would have room and with smaller rocker/shafts it would be lighter ... but was just wondering that 11.95mm shafts would wear out too fast ..it's because I've seen motorcycle rocker arms that size and thought ..hey if it works for that why not on a 36hp ...so it's got me thinking to go that rought "my plans in my head" but would like peoples thoughts...I get some are gonna nod there heads like this guy 🤦🤷 lol ..... |
I don't think you are nuts, Jerry
My thoughts:
* The smaller diameter valve stems would lighten up the reciprocating mass, for better valvetrain control at higher rpms.
* I see no reason a guy couldn't machine his own ratio rockers, but pushrod cups and the valve tip contacter (pad or roller) both need to be hard and wear resistant. The body needs to be not-so-hard that it's brittle and breaks. Easy to talk about, harder to execute (ie AA 36hp ratio rockers, GBE T1 ratio rockers).
* Smaller rocker arm trunions (allowed by smaller dia shafts) reduces the mass close to shaft centerline. So close in fact, that reducing mass in the area has little effect on valve train inertia. Smaller shafts and trunions will however, allow you machine a more compact rocker arm, and have room for a larger spring. I've run Crower shaft mounted roller rockers that use 9/16" (14mm) dia shafts and 450lbs open pressure springs on a type 1 vw head (don't ask, it was a sh*t-ton of re-engineering and machine work).
* Ramblings; Remember that by increasing the ratio of the rocker arm, you multiply the valve travel, but you also multiply the force on the pushrod, lifter, and cam lobe. Lifters that don't spin, and cam lobes that have a small base circle, won't take much load before the metal starts to crumble from the contact pressure. How much is too much? I have no idea. Angling the valves in Okrasa heads is going to be a chore. I only know one guy with the skills and crazy/confident enough to try, but it might cost you a failed set of castings in the R&D effort.
* More ramblings; I can imagine a home built rocker arm with large contact area vw style solid bushings (instead of needle bearing on shaft) a guy could get away with much smaller shafts. You mention motorcycle stuff, that is a good comparison, except you'd have to support said shaft very close to the rocker. A vw has these shafts with a large cantalever or overhang, largely unsupported on the exhaust side. Find a motorcycle head that uses smaller shafts, measure the open pressure of their valve springs, and at least you'll know what Honda for instance, engineered as acceptable for a reliable machine. |
This rambling talk/info is why I love thesamba.. it has others also give insight on things they may not have said or answer or had the nerve to say...thanks for the replies ..I'm gathering all the insight given to me and I know some will also give me insight on a whole lot of things that I'm over looking ...maybe to much time oh my hands these past months 🙄 to much/many dreams 😴 .... |
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TomSimon
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| Posted: Thu Nov 04, 2021 5:37 pm Post subject: Re: Rocker arm shaft size |
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I'm no Porsche expert, but from the reading I've done, it's my understanding that the early 2-piece Porsche case, which I think is referred to as the 'Pre-A- engine, is nothing more than an align bored 36HP VW case with Porsche serial number stamped into it. I surmise that Porsche didn't have their own in-house engine program up and running yet, so they started off buying parts from VW, align boring cases and using their own Porsche cylinder head.
In addition, the early 2-piece Porsche/36hp VW case have the same head stud pattern. It turns out Porsche started with designing and building their own semi-hemi head, the pre-A version uses a head stud pattern identical to 36hp VW, making them a very rare cylinder head indeed. The 356 A, B, and C heads look the same as a pre-A head, same canted exhaust valve and 356 valve cover, but on A, B, and C engines the stud pattern is unique to 356 Porsche, and only fits the 356 Porsche 3-piece engine cases, not a 36hp VW. I'm not positive, but I think the rocker assemblies are different, and don't interchange, as well.
In theory at least, a guy could locate a set of rare and valuable Porsche Pre-A cylinder heads and matching rocker assemblies, and bolt them onto a 36hp VW case. I believe Pre-A Porsche used a 36hp VW crank, non-rotating radius nosed lifters. my guess is the entire lower end is VW. With Porsche heads, you'd essentially have a Pre-A Porsche engine at that point.
Not allowed to compete in the 36hp Challenge at Bonneville, but a cool engine for sure.
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