Eagler's Nest

Engines => 4 Cylinder => Volkswagen => Topic started by: Tom H on July 05, 2014, 06:27:26 AM

Title: VW rocker arm/pushrod contact when valve closed
Post by: Tom H on July 05, 2014, 06:27:26 AM

I'm going through "official" channels to get a recommendation on this, but am throwing it out to see if anyone has had this happen:

During the annual condition inspection of Stubby, I removed the rocker assembly from each head to check head nut torque.

I saw that several of the pushrods had been making contact with the rocker arm.  Of course, the ball end of the pushrod does normally contact the socket of the rocker arm, but this is not what I am talking about.

When the valve is in closed position, a spot on the cast/unmachined part of the rocker, surrounding the machined socket, contacts the side of the pushrod tube, not the ball itslef, but the side of the tube.  No contact is apparent when the engine is not running, but maybe the inertia of the moving parts causes some contact when running.  The rocker shows only one point of contact; the pushrod is damaged all around its circumference since it rotates during operation.

Some possible fixes that I have thought of:
1- longer pushrods
2- thinner pushrod mounting square spacers, current ones are 0.025"
3- use existing parts, de-burr the pushrods, grind a bit of un-machined casting from rocker arms at the contact area.

I've attached a pdf sketch showing the issue.

Title: Re: VW rocker arm/pushrod contact when valve closed
Post by: rockiedog2 on July 05, 2014, 08:20:09 AM

I'm sure not a vdub motor expert atall but is it possible the pushrod is the wrong diameter? I understand there are a couple sizes...

Title: Re: VW rocker arm/pushrod contact when valve closed
Post by: Steve on July 05, 2014, 11:51:30 AM

There are multiple Google search hits on Vw Type 1 pushrod failures from striking surrounds... My (guess) is the larger pushrods are for folks with heavier valve springs and other changes like hydraulic lifters and or high revs going down a drag strip or powering up a sand dune... I'd use OEM pushrods, but some input from Steve Bennett would be prudent ( I think this is a right sizing issue)... Almost universally the community that has valve train issues arrives at and departs from the field on trailers! - blowing up a motor is all in a day's work...

Title: Re: VW rocker arm/pushrod contact when valve closed
Post by: Tom H on July 06, 2014, 07:11:33 AM

Got a quick answer from Great Plains.  Steve is sending me a set of u-build pushrods and extra spacers so I can space the rocker assy out from the heads, and make the pushrods the best length.

Now I have to learn how to get the pushrods the correct length.  What is the best valve train geometry?  I'll have to do some research.

Title: Re: VW rocker arm/pushrod contact when valve closed
Post by: Steve on July 06, 2014, 01:07:39 PM

Quote from: Tom H on July 06, 2014, 07:11:33 AM

Now I have to learn how to get the pushrods the correct length.  What is the best valve train geometry?  I'll have to do some research.

Here are some geometry tips' attached:

Title: Re: VW rocker arm/pushrod contact when valve closed
Post by: Dan_ on July 06, 2014, 01:26:46 PM

Quote from: Tom H on July 06, 2014, 07:11:33 AM

What is the best valve train geometry?  I'll have to do some research.

Maybe put them on the outside... so you can keep an eye on them.

https://www.youtube.com/watch?v=SyRJeZ6s8uM&feature=player_detailpage#t=81

Start it over from the beginning to see how the VW heads were cut down.

Title: Re: VW rocker arm/pushrod contact when valve closed
Post by: Bob S. on July 06, 2014, 04:07:24 PM

MOST Vdub engine builders have one 'adjustable' pushrod used to get the length right then copy the length for the correct length x # of valves..

Title: Re: VW rocker arm/pushrod contact when valve closed
Post by: bill utt on July 06, 2014, 04:21:33 PM

Tom, I have yet to run into this problem. It almost sounds as if the rockers were machined to deep before the hardening process was performed.
another thing , if you are running HP springs, rods may be giving way on the ends. I got rid of the HP springs and went with stock. don't need Hp spings with low RPMs.
Bill

Title: Re: VW rocker arm/pushrod contact when valve closed
Post by: Tom H on July 07, 2014, 05:55:32 AM

Thanks for the replies.  Did some reviewing of the GP assembly manual and my procedure, did some reading on valve geometry, learned a lot.

First, the GP assembly manual, which is pretty good overall, does not go into geometry details.  It talks about clearance of the rocker assembly from the mounting studs.  Says for the 2180, install the finished pushrods, but does not talk about the resulting geometry.  I followed the manual, but, perhaps the manual could have included some more detail.

The pushrods are steel, 0.3120 diameter.  I have some old pushrods in the junk box that are aluminum and larger diameter.  I don't know if the steels ones are "normal", or if there are smaller diameter ones available.  But, I don't think this is the real problem.

The socket on the rocker castings may have been machined too deep, or, there was excess casting material on several of the rockers.  This allowed some contact at valve closed position.

Reading about valve geometry, including what Steve included above, shows that proper geometry should have the adjusting screw on the rockers parallel to the valve stem when the valve is half open.  Simple, makes sense, I probably knew intuitively that that is the way it should be, but did not look at it during assembly.

Our valve train geometry has the rocker opening the valve in the earlier phase of its rotation arc, instead of the middle of it.  So, I will probably have to increase the square shim thickness where the rocker assy mounts to the head, and lengthen the pushrod.  I'll have to make an adjustable pushrod to do this; probably use one of the damaged rods.

A friend has just assembled another GP kit and he says he may have the same issue.  I'm going try to look at it this week.

Title: Re: VW rocker arm/pushrod contact when valve closed
Post by: flydog on July 08, 2014, 05:51:45 PM

I think you are on the right track. Your sketch shows you are using swivel feet and you will find it takes a lot more than just one shim to compensate for them versus the stock adjusters.
I think I will be using stock on my build.
In your last post you mention valves opening sooner. I dont think the shims have any effect on that. Valve lash will.

Title: Re: VW rocker arm/pushrod contact when valve closed
Post by: Tom H on July 09, 2014, 05:38:42 AM

I visited my friend who built the GP 2276, but not yet run.  He had the same issue as me.  Figures, since he followed the same directions in the manual.  So, we shimmed and lengthened the pushrod and checked the results.  After several tries, we ended up with using four shims, 0.030 thick each, for a total thickness of 0.120, which for his engine is about the max that the hold down studs will handle with the nut fully engaged.  The pushrod ended up being about 3/16" longer than he originally had.  Good to know the technique works and comes out right.  We'll have to shorten the coupling nut that is used both to hold the rocker assy down and to secure the rocker covers (alum cast aftermarket type).

Flydog, I agree the valves don't open sooner, meant that the rocker is opening the valves earlier in its rotation arc.  With the shimming and longer pushrods, the rocker will be working in the middle of its possible rotation arc.

I made an adjustable pushrod from one of the damaged ones.  I plan to do the fit-up later today.  The parts have been shipped from GP, hope to get them this week.