Valve adjust - backside method questions

[Tom '74 911]

Hi -
I'm doing my first valve adjust and am using the backside method. Unfortunately, I don't have the "normal" .004 feeler gauge tool to help verify - should be here tomorrow. I've got some questions - some are specific to the backside method, some are general valve adjust questions. For reference, my engine is a '77 2.7.

1. At first check, valves 1-3 were all tight (.0025 wouldn't fit). Just to be sure - does the backside method work with ALL cams and rocker combos? I know my motor has been rebuilt and may not have the stock cams - will this method still work?

2. I adjusted valves 1-3 one at a time - each seemed to need 1/4"± turn to loosen a tad. When I cycled thru and went to recheck, they were all too loose. Now I can't seem to get consistent readings w/the feeler gauges. I've done 1001 sit-ups! Should I be wiggling the rockers back & forth and pressing on the valve adjustment screws in between trying the feeler gauges? Or should the rockers always be "at rest". I seem to get different results if I wiggle vs leave them alone - most often, they seem to be too loose after being wiggled.

3. Turning the adjustment screw 1/8 to 1/4 of a turn makes a big difference as far as the feeler gauges are concerned. How much of an actual difference would it make if the screw was 1/8 of a turn out of adjustment?

4. I'm sure I've got more questions, but I'll start w/that.

Thanks a lot,
Tom

[john walker's workshop]

it's hard enough for a beginner to get a perfect adjustment the regular way. if you need to get into alternate ways to do the same job, you should perfect the proper method first.

[Tom '74 911]

Originally posted by john walker's workshop
it's hard enough for a beginner to get a perfect adjustment the regular way. if you need to get into alternate ways to do the same job, you should perfect the proper method first.

JWW -
I appreciate your advice. The "proper tool" should arrive tomorrow. I started w/this alternative way, as it's been billed by others as fairly fool proof and easier than the traditional (proper) method. Aside from the 1001 situps, it does seem fairly easy - assuming the numbers are really correct etc... I'm just looking for some more guidence. I will follow yours and give the traditional method a try as well once the proper tool arrives at my doorstep.
Thanks,
Tom

[2.7RACER]

Hi Tom,
Here's a few answers.
First the type of cam you have makes no difference when setting the valve to rocker gap.
This is because you are measuring and setting the gap at the base circle of the cam. That is the low point of the cam.
Second, the 911 rockers are the same ratio 1.4:1 for your car.
Third, one full turn of the rocker screw is 1.0mm, that is right at 40 thousandths of an inch.
So even 1/4th turn is 10 thousandths.
You can see 1/10th of a turn moves the gap at the rocker/valve the full 4 thousandths of an inch.
So even 1/8th of a turn is 5 thousandths.
The typical feeler set comes with a 2,2.5,3 and 4 thousandths shims.
If the 2.5 thousandths feeler will not slide in, even after pressing the rocker against the valve, the adjustment is too tight.
To verify this try slipping the 2 thousandths feeler between the rocker and cam.
If the 2 slides in and the 2.5 does not, you then know the adjustment is SLIGHTLY tight.
To correct this, you would back off the adjustment screw very slightly. Remember 1/10th turn is 4 thousandths at the rocker valve gap.
So we are talking less than 1/10th turn of the rocker adjustment screw.
I would slip the 2.5 thousandths feeler between the rocker and cam, leave it there and tighten the adjustment screw until the feeler is snug. You should feel the screw meet the resistance when the rocker is pressing against the feeler.
Then slip the 2.5 feeler back and forth to make sure it isn't held tightly.
As you tighten the lock nut hold the screw so it doesn't tighten.
Now check the gap with the 2.5, it should slide in.
At this point the 3 thousandths feeler should not slip in.
When the 2.5 slips in and the 3 does not your adjustment is fine.
Take your time with the first adjustment until you feel comfortable with the procedure.
Always look at the cam and be sure the rocker is at the base circle, (low side), of the cam.
Take your time, this works.

[Tom '74 911]

Thanks for the response Doug and verifying:
1. that any cam and rockers installed should work w/this method.
2. that we're talking very slight adjustments of the screw between the "go" and "no go" gauges.
3. Also, as you suggested, I was sliding the 2.5 thousandths feeler between the rocker and cam, leaving it there and tightening the adjustment screw until the feeler was snug - I wanted to make sure that this was an OK method/procedure. This method would be so easy w/two people w/good communication skills!
4. That's a good tip too - to use the feelers on either side of the go/no-go feelers to see how close you are.
5. I will still try and verify w/the traditional tool and method tomorrow.

Thanks for the mental reinforcement. I think I'm basically there, just need a bit of practice to limit the number of back & forths (sit ups).

Thanks,
Tom

[Dennis Kalma]

OK, I have to ask.

I Have always wondered why the factory did not recommend the backside measurement technique, mainly as my previous sports car was a Datsun 240Z with L24 engine, and it used the backside technique. Completely different engine of course, SOHC, intake and exhaust valve on the same side of the head, used rockers against a fixed pivot to actuate the valve, the pivot being what was adjusted to set valve clearance.

Anyway, when I got my 911 back in 1985, I wondered why the heck it needed this goofy valve clearance measurement technique and was told that it was the only way to get a precise measurement in this rather exotic engine. I thought it was nuts, but hey, when in Rome, set off roman candles....who wants to take chances.

Now the guys at the factory are admittedly anal, but not stupid....why would they not be recommending this much more straightforward, accurate method? Can anyone think of any reason at all?

BTW - since reading the article on Pelican, my next valve adjustment session will be using the backside method, unless someone can offer proof positive that it will result in a bad setting or at least a reasonable risk thereof....

Dennis

[khamul02]

I used this method the first time out with great results thus far!
As far as why Porsche didn't suggest this method: They didn't think of it American ingenuity

[khamul02]

Oh and so they could sell you a specialty tool.

[livi]

For my first valve adjust I read up and found out about the 'alternative' method. Sounded great. However, for me, it turned out the good old factory method suited my hands and mind better. In fact, I found the actual adjustment pretty easy. Once all the stuff was removed to gain access.

Thats just me of course and the 2.7Racer method still sounds very attractive. Just did not work for me as well as the old way.

[berettafan]

Did my first valve adjust a month or so ago. I had the 'special tool' and it just isn't black and white like i had hoped. sooo much of it is 'feel'. So i tried the 'backside method' and liked it much better.

Of course on double check i STILL had to readjust several but that is my own fault and would happen using either method. The main difficulty for me was the 'go' gauge bent easily.

Given that a lot of the traditional method is 'feel' i would expect the backside method to be more accurate and, possibly just as important, consistant.

erik