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Is the 1.57:1 Rocker Arm Ratio another Corvair Myth?
Posted by: Andrew ()
Date: April 09, 2015 11:39PM

I was looking for a bolt torque specification in the '65 service manual tonight and happened to notice that it listed the rocker arm ratio as 1.5:1. I've read that the ratio is 1.57:1 time and time again on this forum and in other books. It seems like I've also read that maybe Chevrolet used both ratios. So I looked in the other manuals I have: '61 manual, 62-3 supplement, and '66 supplement. Here's what they say:

80hp
Tappet lift
Intake 0.20926"
Exhaust 0.22935"
Valve Lift
Intake 0.314"
Exhaust 0.344"

98/102hp
Tappet lift
Intake & Exhaust 0.252"
Valve Lift
Intake & Exhaust 0.380"

95hp
Tappet lift
Intake & Exhaust 0.257"
Valve Lift
Intake & Exhaust 0.385"

110/140hp
Tappet lift
Intake & Exhaust 0.260"
Valve Lift
Intake & Exhaust 0.390"

All of these work out to 1.5:1 or very close. Where is this oft- repeated 1.57:1 ratio coming from?

P.S. Does anyone else find it odd that Chevrolet specs the lowly 80hp lobe lift with a hundred thousandths spec?

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Re: Is the 1.57:1 Rocker Arm Ratio another Corvair Myth?
Posted by: larry202br ()
Date: April 10, 2015 06:25AM

Check out the specs on the reverse cam in the Crown 5050d thread!

1961 rampside with a 65 / 67 110 engine, car four speed.
1965 convertible (SOLD)
1966 Custom two door/four door 180 Turbo.

KC0SKX
Olathe, ks
HACOA

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Re: Is the 1.57:1 Rocker Arm Ratio another Corvair Myth?
Posted by: JimBrandberg ()
Date: April 10, 2015 07:19AM

With roller rockers on my race engine 1.57 and 1.65 have been verified due to concerns with coil bind and such. I guess I never verified stock rocker arms. Proper roller rockers are a Ford thing. Is Andrew a mythbuster?
Jim Brandberg
Isanti, MN
CorvairRepair.com

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Re: Is the 1.57:1 Rocker Arm Ratio another Corvair Myth?
Posted by: Andrew ()
Date: April 10, 2015 02:49PM

There's an article in the CORSA Tech Guide titled "Camshaft Usage" by Bob Helt. In his camshaft spec chart there's a footnote to the lift figures:

"Lifts so noted and all specs for the 1966 and later, use a rocker arm ratio of 1.57 to 1. 1965 and earlier used the 1.5 rocker ratio. All lifts are measured in inches at the valves."

Maybe Bob will weigh in on where this increased ratio rocker information came from. Is there a new part number for this rocker? Maybe some other GM insider records? I have seen pictures of "LM" rockers that appear to be shaped differently than earlier ones. The fact still remains that the supplement says it's still 1.5 to one. Of course the manuals have errors in them, so they're not gospel. Will someone check the specs in the 67-8 and 69 supplements to see what they list?

Another thing to consider is the 66-69 cars only account for approximately 150k cars of the 1.8 million total production. So if the 1.57 ratio is a reality (and I don't really doubt that it is) then less than 10% of Corvairs had them.

I wonder if the new higher ratio was a result of the valve bounce tests results with the 891 camshaft. The timeline of that test and a new rocker would fit. They could tame the lobes and retain the lift (like the 304 camshaft?) What's the possibility that a higher ratio rocker was an easy way to get more lift (and air) for the smog or AIR engines coming out in '66? Whatever it was they must have had a good reason to invest in new tooling.

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Re: Is the 1.57:1 Rocker Arm Ratio another Corvair Myth?
Posted by: jjohnsonjo ()
Date: April 10, 2015 03:34PM

Measure your rockers and see if they are even 1:5, mine have always been less.

J.O.

65 Corsa Turbo Vert
79 Honda XL 500S
69 Honda CL 160 D
2010 BMW F 650 GS
2003 Bounder 36D
2013 KIA Optima SX turbo-AKA ZIPPY (wife,s car)
69 Newport Holiday Sailboat
Baja 150 dune buggy cart
Coleman HS 500 UTV
2016 KIA Sorento SXL Turbo

Bethlehem,Pa


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Re: Is the 1.57:1 Rocker Arm Ratio another Corvair Myth?
Posted by: medurham ()
Date: April 10, 2015 05:16PM

My Clarks Roller Rockers are 1.58.

Mark Durham
Hauser, Idaho
Currently Corvairless

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Re: Is the 1.57:1 Rocker Arm Ratio another Corvair Myth?
Posted by: jjohnsonjo ()
Date: April 11, 2015 05:01AM

I would believe that about Clarks, I was speaking of GM, some racing teams would measure bunches of them searching for a set of full 1:5s

J.O.

65 Corsa Turbo Vert
79 Honda XL 500S
69 Honda CL 160 D
2010 BMW F 650 GS
2003 Bounder 36D
2013 KIA Optima SX turbo-AKA ZIPPY (wife,s car)
69 Newport Holiday Sailboat
Baja 150 dune buggy cart
Coleman HS 500 UTV
2016 KIA Sorento SXL Turbo

Bethlehem,Pa


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Re: Is the 1.57:1 Rocker Arm Ratio another Corvair Myth?
Posted by: Bob Helt ()
Date: April 11, 2015 11:48AM

This story was investigated and reported on on my book, CORVAIR SECRETS, years ago. Here it is.


CORVAIR VALVE ROCKER RATIOS



THE CORVAIR LITERATURE

You may have noticed that various Chevrolet documents list two different valve rocker ratios as being in production. Often the 164 CID 95-hp engine is listed as having a 1.50:1 ratio while the other 164 cid engines use a 1.57:1 ratio. Sometimes all the 1966 and later engine are credited with a 1.57:1 ratio while the 1965 and earlier engines got a 1.50:1 ratio. Yet the Parts Manuals only show one rocker for replacement for all years and engines. So what gives here?

A CORVAIR ENGINEER INVESTIGATES

Well known and respected Chevrolet Corvair engineer, Bob Kirkman, decided to investigate this apparent discrepancy while still working at Chevrolet and determine the real situation. Bob is also the author of the comprehensive analysis of all the different Corvair cylinder heads that Chevrolet documented in their drawing structure. This detailed head analysis was published in both the Sept. 1993 CORSA Communiqué and the CORSA Technical Guide Supplement (Also recently revised as Volume 2).

WHAT BOB FOUND

All Corvair camshaft drawings— 1960 thru 1969—except one, reference the valve rockers as having a ratio of 1.57:1. The one exception was the 1962 drawing (long before production was planned) for the 164 CID 95-hp camshaft, 3839889, that apparently mis-stated the rocker ratio as 1.50 instead of 1.57. This error was corrected later, but apparently it was picked up by the tech writers and promulgated throughout much of the subsequent Corvair literature. All other Corvair camshaft drawings for all cams list the rocker ratio as 1.57:1.

So Bob concluded the following:

“No physical changes [to the rocker ratio] were made in any year. All years list [the] 1.57 ratio. The only reference to a ratio of 1.50:1 is on the 95-hp cam drawing in a time period before the part appeared on production cars off the line. It is very likely that someone finally noted a mistake on the drawing and fixed it. Although [all of these Corvair] cams have slightly varying base circles and lifts, they all end up with a reference to 1.57:1.”

THE FINAL WORD

So now we know that all Corvair engines for all years used the design rocker ratio of 1.57:1 that multiplies the cam lift to that of the valve lift. We don’t know just why this particular rocker ratio was selected, but it must have been considered optimal in regard to valve lift and dynamics.

ADJUSTING THE VALVE ROCKER PRELOAD

The threads on the rocker studs are 3/8-24 so each turn of the adjustment nut moves the rocker pivot up or down by 0.0417 inches. When adjusting the pre-load at the rockers, the valves stay stationary with the rocker pivoting at the valve tip. So then the internal piston in the lifter is repositioned via the pushrod. Thus, the movement ratio is now 5/3 or 1.667 at the lifter. This now produces 0.0695 inches (call it 0.070") of internal piston movement in the lifter for each turn of the adjustment nut.
Since most original Corvair lifters allow a range of internal piston movement of about 0.120" and most currently available replacement lifters also allow a movement range of about 0.140", this will provide about a full two turns of the adjustment nut to cover the allowable lifter range. Thus, setting the adjustment at anywhere from ¼ turn-to ½ turn-to one full turn inward from zero lash will allow a safe and proper setting.

Bob Helt

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Re: Is the 1.57:1 Rocker Arm Ratio another Corvair Myth?
Posted by: Andrew ()
Date: April 11, 2015 01:32PM

Thanks for your comments and research Bob. If the "bad" 1962 drawing is the source of the wrong ratio, can you speculate as to why the 1960 and '61 manuals have the spec wrong too? I mean, weren't these originally published concurrently with the cars? Granted, most of us have reprinted manuals but there have to be "first editions" out there, right? I wonder what they have to tell us.

If the 1.5 ratio is wrong (and again, I'm just playing devil's advocate) how do we reconcile the published lobe lift versus valve lift specifications? Both can't be right. It would be easy to measure some NOS camshafts for lobe lift if someone has some new in the tube (Motohead?) I would assume the valve lift figures are low as the writers probably just multiplied lobe lift by rocker ratio. An 891 cam would have 0.018" more lift at the valve than the published spec...provided that the rockers were made to spec.

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Re: Is the 1.57:1 Rocker Arm Ratio another Corvair Myth?
Posted by: Bob Helt ()
Date: April 11, 2015 04:01PM

If the 1.5 ratio is wrong (and again, I'm just playing devil's advocate) how do we reconcile the published lobe lift versus valve lift specifications? Both can't be right. It would be easy to measure some NOS camshafts for lobe lift if someone has some new in the tube (Motohead?) I would assume the valve lift figures are low as the writers probably just multiplied lobe lift by rocker ratio. An 891 cam would have 0.018" more lift at the valve than the published spec...provided that the rockers were made to spec.



Andrew,
I believe that all lobe lift specs are correct. I would think that the valve lift data would be suspect. The only data that I know that would be applicable for comparison would be the four engine test reports that Clark's sells. The test engineers measured the lift and rocker ratios. Hopefully they knew what they were doing and that their data wasn't fudged. But it agrees with little else.

But even then if you might think that the gear lube data represents a can of worms, the lobe, lift and ratio data are a much worse mess IMO. No two pieces of info agree with themselves or with any other sampling.
We might never make much sense of it.

The rocker ratio might even turn out to be 1.50. That is what Chevrolet used I am told. There must have been a reason why the 1966 95 and 110 engines were said to have different rocker ratios using the same rocker arms (and different cams). Maybe the angles or effective valve stem lengths affected this ratio?????

Stay tuined

Bob Helt

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Re: Is the 1.57:1 Rocker Arm Ratio another Corvair Myth?
Posted by: davemotohead ()
Date: April 11, 2015 04:11PM

I do have some NOS cam shafts new in the Tube smoking smiley I even have a Junky mesmerized by one!







1960 4dr sedan caveman car
1961 Rampside (Chetside)
1962 Rampside (Barnside)
1962 Short Rampside (Shortside)
1962 Monza 700 Wagon
1963 Monza 900 coup (General Nader)

-----------------------------------
Rust Free Lancaster Ca

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Re: Is the 1.57:1 Rocker Arm Ratio another Corvair Myth?
Posted by: jjohnsonjo ()
Date: April 11, 2015 04:58PM

I recently installed a Clarks 304 repro, the lift at the cam lobe was within .001 of specs

J.O.

65 Corsa Turbo Vert
79 Honda XL 500S
69 Honda CL 160 D
2010 BMW F 650 GS
2003 Bounder 36D
2013 KIA Optima SX turbo-AKA ZIPPY (wife,s car)
69 Newport Holiday Sailboat
Baja 150 dune buggy cart
Coleman HS 500 UTV
2016 KIA Sorento SXL Turbo

Bethlehem,Pa


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Re: Is the 1.57:1 Rocker Arm Ratio another Corvair Myth?
Posted by: jjohnsonjo ()
Date: April 11, 2015 05:00PM

While we are at it, what is the differance in thousanths between cold lash and hot lash lash. I read somewhere just how much it grows, but can no longer find it.

J.O.

65 Corsa Turbo Vert
79 Honda XL 500S
69 Honda CL 160 D
2010 BMW F 650 GS
2003 Bounder 36D
2013 KIA Optima SX turbo-AKA ZIPPY (wife,s car)
69 Newport Holiday Sailboat
Baja 150 dune buggy cart
Coleman HS 500 UTV
2016 KIA Sorento SXL Turbo

Bethlehem,Pa


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Re: Is the 1.57:1 Rocker Arm Ratio another Corvair Myth?
Posted by: Bob Helt ()
Date: April 13, 2015 09:44AM

Hi everybody,
My posted analysis of the Corvair's true rocker ratio was mainly based on Bob Kirkman's investigation and report that I cited.

But Andrew questioned how that could be and challenged my thinking very subtly and got me thinking about this subject again. Last time was probably 15 years ago. Andrew was right. Things just didn't add up if we accept Bob Kirkman's report.
So I did a memory clear and started thinking from scratch again. And thanks to Andrew's questioning my previous analysis, I now think that I have the final answer to the question of the Corvair's design based rocker ratio.
So please see the posting below.





The Small Block Chevrolet V-8 engine (SBC) used a nominal rocker ratio of 1.5:1, although actual measurements were said to be closer to 1.46:1. So it just seems natural that the Corvair’s valve system that was designed by Al Kolbe who also designed the SBC system would also have the same 1.5:1 rocker ratio. And it did. The Corvair rocker ratio started off the same with a 1.5:1 ratio. And Chevrolet’s documents show that it remained at 1.5:1 from the 1960 model thru the 1963 model year for all Corvair engines.

But for the 1964-65 model year things changed. The base, 95 HP engine specs stated that a new ratio of 1.57 was used on this engine. All other 1964-65 Corvair engines continued at 1.5:1. Why only this engine? And why was it changed?

Why was this done? Chevrolet engineer Bob Kirkman wondered too and did some research while he still worked for Chevrolet with the results reported in a letter by him. He stated in the letter that all of the Corvair camshaft drawings, except one in 1962 for the future 164 cid 95 hp engine, #3839889, stated a ratio of 1.57. And that this one camshaft likely missed a last digit in the ratio and was mis-identified as having 1.5:1 ratio………A misprint if you will.

But here is a twist. Despite Bob’s honesty and diligence, it appears that he got the data mixed up in his head. He really meant to say that this one camshaft drawing was mis-marked as having a ratio of 1.57:1 instead of 1.5:1. Where the 7 came from, no one knows. Was it just a typo? Or was there some change or discovery behind the added 7? We probably will never know. But it being a typo is most likely.

Yes, this is just speculation about Bob’s mixing up the data. But it just could not have been as he stated. There are just too many things that would conflict and disagree with that statement. But everything falls into place if you assume that Bob mixed things up.

So as things worked out, the drawing was released with the 1.57:1 ratio for the 1964-65 95 HP cam and thus it got reported as such while all other ratios remained at 1.5:1. And as additional information: only one type of valve rocker arm was ever used in production engines. So there was little possibility of the rocker arm changing the ratio.

Now the mystery deepens.

The rocker ratio specifications for all engines were revised to 1.57:1 for model years 1967-69!!!!!. Why? No one knows. Could it be that the 164 cid engine cams actually resulted in a change in rocker ratios? Not likely. Could it be that management wanted to hide the error of the additional 7 and just decided to make them all the same? Or, maybe, there was an engineering reason here. Could the ratio actually have been 1.57:1 all along and was the correct ratio after all? That doesn’t seem to be the case either. First, there were no changes ever reported that would do this. Also, the four Chevrolet Engine Test Reports being sold by Clarks show the engineers actually measuring the rocker ratios and all of their measurements seem to fall much nearer the 1.5:1 spec than 1.57:1.

So for now, with the current state of discovery, we must conclude the rocker ratios for all Corvair engines were all at, or very near, 1.5:1 and that the 1.57 spec was identified in error.
Bob Helt

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Re: Is the 1.57:1 Rocker Arm Ratio another Corvair Myth?
Posted by: strangecars ()
Date: April 13, 2015 09:57AM

I would like to know how to physically measure the rocker arms and come up with a conclusion as to what the ratio is in our engines??

Steve Morton
1961 Rampside,
63 vert,
64 coupe,
65 EO vert,
67,
#0003 1969

Morton's Classic Garage
Parts and Service
Nampa, Idaho 83686
60+ Corvairs and 4 semi trailers full
of parts

Mortonsclassicgarage@gmail.com
[www.facebook.com]

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Re: Is the 1.57:1 Rocker Arm Ratio another Corvair Myth?
Posted by: Andrew ()
Date: April 13, 2015 10:37AM

Steve, the easiest way would be to measure a camshaft for lobe lift with a micrometer, then assemble it into an engine with an unworn (NOS) rocker arm, then use a dial indicator to measure lift at the valve retainer. The second number divided by the first would be the ratio. You'd have to use a solid lifter, or a very light spring in place of the real valve spring. You could do the same to all valves to check the arms against each other for consistency if you're really bored.

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Re: Is the 1.57:1 Rocker Arm Ratio another Corvair Myth?
Posted by: jjohnsonjo ()
Date: April 14, 2015 05:25AM

Use a dial indicator on both ends of the rocker, can't mic the cam when its in the engine.

I have read a lot about this 1:57 issue and often wondered if it has something to do with the fact that Vairs have angled valves. So the lifter will display slightly different ratios when everything is dead square as opposed to the measuring at the slant it normally runs at.

J.O.

65 Corsa Turbo Vert
79 Honda XL 500S
69 Honda CL 160 D
2010 BMW F 650 GS
2003 Bounder 36D
2013 KIA Optima SX turbo-AKA ZIPPY (wife,s car)
69 Newport Holiday Sailboat
Baja 150 dune buggy cart
Coleman HS 500 UTV
2016 KIA Sorento SXL Turbo

Bethlehem,Pa


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Re: Is the 1.57:1 Rocker Arm Ratio another Corvair Myth?
Posted by: caroseiii ()
Date: April 14, 2015 07:17AM

Its not an early v. late thing, as the same part number is in use for all years - 6255648 in group 0.333. The other part number, 5723559 is the set of rocker, nut and ball together. All the early literature indicated 1.5 ratio and all the late literature indicated 1.57 but the rocker part itself never changed.
Crawford Rose

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Re: Is the 1.57:1 Rocker Arm Ratio another Corvair Myth?
Posted by: strangecars ()
Date: May 05, 2015 04:32PM

For no other practical purpose but just to ask the question. Could a center trunnion of a small block Chevy roller rocker be remade to fit correctly in a 140 hp.

Steve Morton
1961 Rampside,
63 vert,
64 coupe,
65 EO vert,
67,
#0003 1969

Morton's Classic Garage
Parts and Service
Nampa, Idaho 83686
60+ Corvairs and 4 semi trailers full
of parts

Mortonsclassicgarage@gmail.com
[www.facebook.com]

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Re: Is the 1.57:1 Rocker Arm Ratio another Corvair Myth?
Posted by: MattNall ()
Date: May 05, 2015 05:26PM

Most use Ford rockers closer to begin with.... a few topics here covering it.

MODERATOR
Sea Mountain, between Charleston Harbor and Coos Bay! SW Oregon Coast
Click HERE for My Website...Click HERE for My TechPages!
...............110-PG.................................................Webered-Turbo

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