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914World.com _ 914World Garage _ How to cut steel pushrods
Posted by: Highland Jan 23 2014, 09:25 AM
How to most people get their pushrods cut? Should I take the pushrods to the same machinest that worked on my engine parts (crank, cylinders, rods,...) or to a regular machine shop?
Also, what tolerance should I give them? +/-0.001"?
Below are my numbers for a Raby 9590 cam with 0.426" lift for exhaust and intake. Are these numbers typical?
Cylinder.....Intake Lift....Intake Rod Length
1...............0.432................10.567"
2...............0.433................10.607
3...............0.431................10.640 (required 0.03 shim)
4...............0.424................10.578
Cylinder.....Exhaust Lift....Exhaust Rod Length
1...............0.410...............10.623"
2...............0.409...............10.618
3...............0.412...............10.663 (required 0.03 shim)
4...............0.410...............10.612
Posted by: stugray Jan 23 2014, 10:09 AM
I believe I cut mine with a hacksaw ~1/16" long and then ground them down with a bench belt sander.
I was able to get them very accurate.
I was also able to find a length that worked fine for all valves.
I did not have to make custom lengths for each valve.
The number I used is written down at home but was very close to what Jake reccommends starting with.
I also shimmed my rockers up by a small amount (.049 if I recall correctly I have to look). But I have pretty big valve lift (.505 & .475 ish)
We sanded the shims to match exactly between the 8 locations.
Posted by: '73-914kid Jan 23 2014, 10:34 AM
I think there was someone who took them to a machine shop to have them cut on a lathe, and there was some misunderstanding as to what length they were supposed to be cut at, and ALL ended up too short. ie. ruined
Any machine shop can do it, but a hacksaw and a file to get the surface nice and flat would probably do just fine.
If you take it to a machine shop though, make sure you talk to the guy who will be doing the cutting and explain explicitly how they are to be cut....
Oh, a chop saw with a locking jaw to stabilize the pushrods and make a perpendicular cut also works well.
Posted by: nathansnathan Jan 23 2014, 10:37 AM
I had a place in long beach cut pushrods once. They said they'd use a lathe, but when I picked them up they said they used a hacksaw and you could tell by the gaps. 
The ends only seat on the longest part if they're not faced. Personally I would go to +/- 5 tenths - keep them within a thousandths of an inch and do them per valve - not gonna happen with a hacksaw. 
I recall reading a post by Jake on thesamba about the 9590 cam, you would see about .426 intake and .415 exhaust on account of the different ratios of the rocker arms.
Posted by: worn Jan 23 2014, 10:46 AM
How to most people get their pushrods cut? Should I take the pushrods to the same machinest that worked on my engine parts (crank, cylinders, rods,...) or to a regular machine shop?
Also, what tolerance should I give them? +/-0.001"?
Below are my numbers for a Raby 9590 cam with 0.426" lift for exhaust and intake. Are these numbers typical?
Cylinder.....Intake Lift....Intake Rod Length
1...............0.432................10.567"
2...............0.433................10.607
3...............0.431................10.640 (required 0.03 shim)
4...............0.424................10.578
Cylinder.....Exhaust Lift....Exhaust Rod Length
1...............0.410...............10.623"
2...............0.409...............10.618
3...............0.412...............10.663 (required 0.03 shim)
4...............0.410...............10.612
I used a lathe. I also used a home made adjustable rod to get the length correct and the angles right. Centering on mid lift is a good approximation to optimum valve train, but the angles are what you are really going for.
If you use a saw, you can true the ends by putting them in a drill press and running them down on an abrasive to grind away the high spots. If you are careful you can even chuck into an electric drill and go against a grinding wheel.
I took a machining class in college. The first assignment was to make a 1 inch steel cube with a file, flat within 0.001, square all around. A LOT harder than it sounds, and worth the effort put in.
Posted by: stugray Jan 23 2014, 11:08 AM
Oh, but I thought the rocker ratios were supposed to be exactly the same.....
Posted by: nathansnathan Jan 23 2014, 12:22 PM
Oh, but I thought the rocker ratios were supposed to be exactly the same.....
Since it's not a split lift cam, there's nothing else but the rocker ratio that could account for the difference everyone is seeing, right? That's definitely what he said, anyway, 1.25 vs 1.3 to 1. I found the link.
http://www.thesamba.com/vw/forum/viewtopic.php?t=236531
Posted by: 74914LE Jan 23 2014, 12:40 PM
Take them to a quality machine shop, and discuss in detail what you want. Call out the exact length for each, when finished, with ends attached. Make sure that they measure, and mark each pushrod with the exact meansurements you want. A quality machinist will get it right.
Lots of great info on this topic from Jake Raby.
Posted by: HAM Inc Jan 23 2014, 12:59 PM
http://www.amazon.com/Weston-52-0501-W-Arrow-Saw-8000/dp/B001B7DWJW/ref=sr_1_1?ie=UTF8&qid=1390503553&sr=8-1&keywords=arrow+cut+off+saw
Posted by: aircooledtechguy Jan 23 2014, 01:15 PM
This is how I do mine and they come out spot-on every time.
http://aircooledtech.com/custom_push_rods/
Posted by: Jake Raby Jan 23 2014, 06:15 PM
An arrow cutter (archery) is the absolute best way. I can cut 8 pushrods to an accuracy of .003" in less than 90 seconds.
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