I have a 72 olds 455 that i put a larger cam in and if i tighten the rockers down , i loose compression. Cam is 474 lift i was told the cam would fit with no modifications. Any help please?

 

I moved your post to your own thread instead of resurrecting the old one. Was a valve job done on your heads, were they milled?

 

Yes a valve job was done. No milling

 

Somethings too long or too short depending on how you look at it. When the heads were remove did you use the same thickness head gasket to replace it with? If its losing compressing across all cylinders I'd start with pushrod length and check lifters. I have no idea if the cam is too big or not, but I'm sure someone will chime in.

 

With out seeing I can only speculate that the new cam has a larger base circle. Thus bottoming out the lifters when everything is tight. The solution is to shim the rocker arms. Several companies make a shim kit to do this with. To do so bring #1 cylinder to top dead center compression. Loose the two rocker bolts. Now, while rotating the intake pushrod between your fingers tighten that bolt until you feel light drag on the pushrod. Now do same for exhaust rocker. Use a feeler gauge and measure the gap between rocker stand and cylinder head with feeler gauges. Take the measurement you have and subtract .010 and add that much shim under the stand and tighten to 25 ft/lbs. Rotate the engine 1/4 turn and go to the next cylinder in the firing order. Repeat until you come back to #1 on top.
I realize that you may not bevabke to get the shim stack perfect based on the shine they give you. The allowable gap after shimming can be as small as .006 or as large as .030. So if you can't get the shim stack to come out right at the number using the .010 then use the shim pack that gets close without being too tall.

 

Seems unlikely, however. A larger base circle would mean that a taller lobe peak is required to achieve a net lift. At some point you can't get the lobes through the cam bearings, which is why high lift cams often use a REDUCED base circle.

More likely is that the valve tip height was not set properly due to a Chevy-centric machine shop who did the valve job.

 

Either way the solution is the same.


Had I not just scrapped about 30 old cams I could have shown you the massive variation in base circles that were on factory and aftermarket cams.

 

X2.
Smitty, yes I would have loved to see an aftermarket cam with a larger base circle than stock too. Not sure why they would do that and still achieve their goals. In the dozens of cams I've sold or checked myself in just the last few years, none had larger than stock base circles.


Just sayin.

 

Mark,

A little off topic, but for my education, I assume you would like to see a larger base circle to let you run a more aggressive ramp on the lobe for a given lift? I'm guessing this is effectively the same effect you get by running mushroom lifters?

 

Smaller base circle allows more lift, as the lobe can only be so big then it hits on things.

This shows why you need to check lifter preload during assembly.

I have free shims for under the pedestals if you wish to use that method to proceed.

 

Thankyou everyone for your help. Octania i would like to try the shims, if you could tell me where i could buy them.

 

summit shows some IDK if these are correct for your application

http://www.summitracing.com/search/m...r%20shims&ar=1

 

What I would recommend is tighten everything (rockers/stands) down and wait an hour or two. You may find you have no issue at all?

Don't be concerned right away about "no compression", as the lifters "bleed-out" the compression will return!

This may not be your particular problem but I would give it a try first.

Thanks, Gary in N.Y.

P.S. Occasionally on these flat-tappet hyd builds it takes some time for the spring pressure to overcome the excess oil in the lifter bodies, especially if there are close tolerances between the lifters and the plungers. We've seen this with the hyd-roller lifter platform also.

 

With the lift being only 474 even if its an aftermarket cam that lift is not too high and Smitty is right about the larger base lobes I have one of the W30 after market cams from the late 70s and it has larger lobes! The cam ran great however! but I had HS Rollers so no problems with adjustments. Its the only one I have like that also! I compared all the other cams I have and none of them have larger lobes! The manufacturer was Mellings.

 

It could have a larger base circle, but as mentioned that's rare and I've never seen or heard of one like that prior to this.
And fwiw, Melling, Edelbrock, etc buy those regrinds from other companies. They don't grind them themselves.

 

What I have is 100's of 0.020" shims like

http://www.summitracing.com/parts/cr...ake/oldsmobile

Just get me an address and I will drop some in the mail if you want to go that route.

PM here maybe a few days
text or call 517.449.oh 43 two for faster action.

I assume you know how to verify proper lifter preload or can search the interwebs and find out. Basically, you want oh 1/2 to 3/4 turn on the screw from when the lifter plunger just starts to move until pedestal screw comes tight.

 

Sounds like Chris has a good deal.

Just an FYI regarding the Summit Racing information: some of it is INCORRECT . I think they mistakenly took Crane's description, i.e. the shims will decrease lifter preload by approximately .030", .060" or .090" depending on the shims used, to mean that the shims are .030" and .060" thick. The shims are NOT .030" and .060" thick; instead they are .018" and .043" thick.

 

While I am the first to point out errors in published catalogs, in this case you need to do the math. Putting a 0.018" shim under the rocker pedestal increases the gap (or decreases preload) at BOTH the pushrod and the valve tip. If you press the rocker down onto the valve tip, the resulting gap at the pushrod end of the rocker is 0.029, which is close enough to 0.030 for me.

Math is your friend (as is a CAD package for the computer, as it was faster to draw this up than to use trig... )

 

Thanks Mark Good info, I did not know back in the 70s who ground the cams for Mellings but they had really good cams and I never had a problem with them. The two I have are still good one was the 308 auto cam and the other the 328 cam and the 328 has the larger base circle. All my Erson, Comp, Lunati and Crane dont.

 

Yeah and it's a shame to see a year later that after telling the Summit tech about their error that they still list the shim thicknesses. Crane only identifies the two as "thin" and "thick" .

 

Listen to Gary before seeking a more complicated solution.

Obviously this only works if the valves are just barely cracked open -- less than the normal lifter preload. If the lifters are bottomed out and the valves are still open, then you need shims, shorter pushrods, and/or adjustable rockers.