Grinding a cam at home

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tundrabeagle3

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http://answers.yahoo.com/question/index?qid=20071019000014AA0T8uQ

I was hoping to apply this to a predator 212 by sanding the round part of cam maybe .20 on the intake side to get more valve lift.

" So when Cam turns around From REDUCED Base Circle Radius,,,and TO the LOBE PEAK,,,
The Rocker Arm has been RAISED a Relatively HIGHER Distance.
Result= Higher Valve Lift"

then it says to adjust valve lift for after you grind the cam. is this done by tightening down the rockers im guessing on a gx 200. Any one have any experience on this? I looking to get more torque from idle to around 6000 rpms.
 

rwd4evr

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it all seems about right in theory. yes tightening the rockers down takes up the slack left by removing base circle diameter. i'm not familiar with those motors though. don't get any metal bits in there. i'd like to try that but my four strokes are flatheads so valve adjustment means new parts.
 

fowler

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That is true in theory

But it is incredibly risky without the correct tools and training

If u were to get it wrong u will screw up your valve timing

Other risks invole kidding the valve on the piston

And punching the valve though the head

Once I work out how my scanner works and im able to bring up some diagrams
ill explain in more detail

EDIT
ok the scammer wont work but anyhow

the parts of a cam are as shown below
the base circle is what they are saying u must remove
as u can see the base circle must lead smoothly into the flank
the steeper the flank becomes then the faster the valve will open
and of course the faster it will close
this results in a slamming it open and slapping it closed
obviously this will cause abnormal wear

we recently had a document going around work
a truck had a worn inlet lob in one cylender
but this wasnt noticed
it snapped a valve and had a new piston, liner and head put on
went back to work and did 1 12 hr shift only to repeat the problem
this happened 4 times before somone else worked it out
only slightly out of spec but because the follower didnt have a nice even path it was slappping the valve around and breaking it

the other problem u have is if the base circle isnt round then u adjust the valve lash right up u may find the cam is opening the valves at the wrong times or not closing them at the wrong time

it is a doable progect but it with no training or a full understanding of what u are playing with it is risky at best
 

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tundrabeagle3

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It says to sand it in the same shape of the circle. I was going to use sand paper to remove a little bit of the round part of the cam. I wasnt looking to get tons of lift just a little more than stock. Aren't the risks the same as using any aftermarket cam?

it looks like nr racing also uses this technique on
this cam

http://www.nrracing.com/product-p/dynocm-200.htm
 

zbuck

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:wai:I like the statement in the NR ad. You get what you pay for. Meaning what? Back in the day on the old B&S 5 hp flattie stock cam we use to twist them. As they were very few preformance cams available and we could not afford them anyhow.
 

DeathStarr89

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I have done this on small engines with mixed results, like mentioned you have to make up for the base circle difference. It works IF and only if you do it in a well made jig and pay real close attention to the surface finish.

In a flathead i ended up using longer lifters, always wanted to try lash caps on the stock ones but never got around to it. On a OHV you'll need longer pushrods to keep the geometry correct. Use some dychem to check the pattern that the rocker leaves on the valve tip. It should be as centered as possible.


The cam I designed for my Camaro is actually a regrind on a stock core, all they did was take away quite a bit of base circle and re-profile the lobes.

Went from 112 LSA, .437 lift 196 advertised to 110 LSA, .549 lift, 293 advertised. $100 for the grind, $100 for new pushrods and just under 100 more HP. :thumbsup:
 

wobbulater

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No,you grinding the base circle down with sand paper is not the same as buying an aftermarket cam.
An aftermarket cam has been tested and manufactured to a certain spec,and will not cause valve-to-piston interference (wrecked engine pretty much.)
they have machines to grind cams that will make them with a PERFECTLY round base circle and a lobe that is symetrical with its self.

Sanding it down may work, but you risk one big factor,human error.....your hands won't provide you with a perfect base circle.
Not only that but if you dont have a micrometer,or digital caliper, or other tools for measuring microscopic clearances, you wont be able to measure how much material you are removing,and what part of the circle you are removing it from.

In theory doing it sounds like it would work,but I personally wouldn't risk it.To each their own though,if you do it let us know how it turns out!
 

tundrabeagle3

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I already did it last night. I got some 150, and 220 grit and carefully sanded the the round part of the intake side with a little motor oil on it . I measured it with a caliper about every 20 minutes, marked where i wanted to sand with a permanent marker and ended up taking about .020 off of it. I went really slow it took me about two hours, or so and to be honest you cant tell that its even been sanded down. I haven't gotten a chance to start the engine today but will do it tomorrow. It set my valve at .001 intake and .002 exhuast, will this be ok? What do you guys set the pilot screw on the carb at?
 

devino246

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I already did it last night. I got some 150, and 220 grit and carefully sanded the the round part of the intake side with a little motor oil on it . I measured it with a caliper about every 20 minutes, marked where i wanted to sand with a permanent marker and ended up taking about .20 off of it. I went really slow it took me about two hours, or so and to be honest you cant tell that its even been sanded down. I haven't gotten a chance to start the engine today but will do it tomorrow. It set my valve at .001 intake and .002 exhuast, will this be ok? What do you guys set the pilot screw on the carb at?

You took .20 off?
 

tundrabeagle3

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lol ... i set the valve lash again this time with the the lash cap :rolleyes: . while i was doing it i was spinning the flywheel with my hand and could see that the intake side did have more lift than the exhaust side. Should i sand the exhaust side .020 as well to get the lift more balanced? It started right up but i need to lean out the on the main jet more, to improve the throttle response . I did not get a chance to test it out and work on the jetting because it was past ten. But I will test it out tomorrow and post the results :stir:.
 

zbuck

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:wai: Sounds like you did a good job. Did you check valve to head clerance? I would not worry about the exhaust side right now. As some engine builders use an offset cam, with the more lift on either the intake/exhaust.
 

tundrabeagle3

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I tried using a 70 drill bit (i think)and it starts and idles fine. But it bogs out when i give it throttle with the choke off. When the choke is on it revs up like it normally would with the choke off. What do you guys think is wrong with it?
 

tundrabeagle3

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That would seem to indicate a lean condition. If performance improves with the choke on or partway on, it's not getting enough fuel.

i tried drilling the jet with a 68 drill bit and it would not start
I believe I need to clean the carb again. It gets dirt in it very quickly. I keep finding alot of dirt where the fuel shut of valve is . I am going to try a fuel filter, ngk plug and cleaning the carb again tomorrow.
 

gixxerkart

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Good lord... Gasp.... Why not buy a cam? I mean seriously its 50$-100$. That's the cost of a new motor! Why chance screwing it up?
 

rmm727

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It says to sand it in the same shape of the circle. I was going to use sand paper to remove a little bit of the round part of the cam. I wasnt looking to get tons of lift just a little more than stock. Aren't the risks the same as using any aftermarket cam?

it looks like nr racing also uses this technique on
this cam

http://www.nrracing.com/product-p/dynocm-200.htm

That cam is from DynoCams. NR Racing does nothing to it other than ship it to you. When it says that the base circle was reground, that means DynoCams (or whoever) has a pile of camshaft cores and puts then in a cam grinding machine and grinds a different profile on it. Since they are not adding material, they have to grind material off the base circle to get their profile. Some companies will weld on your cam (if blanks aren't available) and then regrind. Some are called soft regrinds meaning don't run more than 18lb springs if you want the cam to last or some are hard face welded or hardened and you can run pretty much whatever valve springs are needed.
 
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