Valve spring/keeper clearance question.

Beck · April 26, 2011, 10:39:31 PM

I'm thinking about one of the Comp Cam Thumper cams for my new toy. I know I should just build a motor that really makes the power, then the sound it makes will be right too. I don't really know what internals are in my small block. It was there when I bought the car. Externally I see it has an Edelbrock Performer RPM intake and an Edelbrock carb. It has a set of freshened old double hump GM heads. It currently has GM exhaust manifolds but that may change to headers. The compression ratio is unknown. Ignition is currently a GM HEI but that will probably change to a MSD with a 6AL2 box.
HRM did an motor build article in this months edition. They used the Thumper cam. In the article it states they didn't have enough clearance between the valve guide and the spring keeper to use there stock 91 Chevy PU heads. They bought a set of World Products heads to solve the problem.
Will I have the same clearance problem with my old double hump heads? If so I will definitely not use that cam. What and how do I need to measure?

enjenjo · April 26, 2011, 11:12:43 PM

You need to check the clearance between the bottom of the spring retainer and the top of the valve guide at maximum lift. You should also check the springs for coil bind at full lift.

Beck · April 26, 2011, 11:57:51 PM

Quote from: "enjenjo"You need to check the clearance between the bottom of the spring retainer and the top of the valve guide at maximum lift. You should also check the springs for coil bind at full lift.

How does this sound. Air up a cylinder. Remove the retainers and springs from that cylinders valves. Measure the distance between the retainer and valve guide. I then have to divide this by 1.5 since I am using standard rocker arms right? How much extra clearance do I need?

OR I could devise a method to depress the valves to the advertised lift of the cam to check for coil bind and retainer/guide clearance.

I don't want to buy the cam and install it to find it doesn't work. I want to do the measurements first.

enjenjo · April 27, 2011, 12:14:07 AM

That would work as a ball park figure, But I would still chevck it with the cam installed just to be sure. Plus you may have trouble with piston to valve clearance.

chimp koose · April 27, 2011, 12:28:46 AM

Measure the guide to retainer clearance .subtract the valve lift from the guide to retainer clearance. You now have the clearance at maximum lift. You should have .050 clearance at a minimum. Make sure you have the right valve springs for the cam. Double hump or camel hump heads can vary from 57cc up to about 64cc. I am building a .040 over 350 w/flat tops @.040 deck height 60cc chamber volume and .015 head gasket. It works out to 10.26:1 CR.I have read good things about the thumpr cam. It has surprized some respected engine builders.Check out speedtalk.com and do a search for thumper cams. If the lift is over.480 or the duration @ .050 is over 240 it would be wise to check piston to valve clearance. Valve to piston of .060 in,.080 ex, are generally accepted ,with less you better have GOOD valve springs. Also make sure to check rocker to stud clearance,there should be enough to put a paper clip between the slot on the rocker and the stud at max lift. If you require 300# or higher open spring pressure ,you should get screw in studs.

chimp koose · April 27, 2011, 12:48:10 AM

Thumpr cam specs out at 243in 257exdur @.050 with .487 lift. it is ground 4 degrees advanced on a 107 lobe separation angle.It recommends at least 9.5:1 CR which you should likely have with those heads .I built a 400sbc years ago with a stock short block ,.025 deck height(typical stock),291 camel hump heads @ 62cc, .021 head gasket and a .480 lift/230@.050 109 lobe separation angle cam advanced 4 degrees.I measured piston to valve clearance and had lots.I had a lot of ex.valve clearance but you have 27 degrees more ex duration,I would want to check up on that clearance for sure.

chimp koose · April 30, 2011, 02:41:19 AM

Beck, I just measured piston to valve clearance on my motor today.Flat top 350/ .040 deck ,camel hump heads milled .035". Cam has .525lift 253@.050 dur,installed 4degrees advanced and I have more than enough exh. clearance. I think that thumper cam would work for you.

Beck · May 02, 2011, 01:25:55 PM

I talked with a tech guy at Comp. He said I would have no problems with my heads. Everything would clear according to him.

UGLY OLDS · May 02, 2011, 06:38:11 PM

QuoteHe said I would have no problems with my heads. Everything would clear according to him.

[/quote]

I would STILL check during assembly ....."Him" ain't going to help if you start bending valves ...

Trust Me .... :oops:

Bob.......

river1 · May 02, 2011, 10:01:15 PM

Quote from: "UGLY OLDS"
QuoteHe said I would have no problems with my heads. Everything would clear according to him.

I would STILL check during assembly ....."Him" ain't going to help if you start bending valves ...

Trust Me .... :oops:

Bob.......

yup as they say trust BUT verify

later jim

chimp koose · May 03, 2011, 02:20:18 AM

Beck,which part number of thumpr are you considering?The last cam specs I posted are for the mutha thumpr . There is a smaller thumpr cam I just saw the specs on, .479lift /227 dur @.050 intake, .465lift /241 dur@.050 exhaust.That cam is not much different than the old 350horse /350 cam GM offered.

Beck · May 03, 2011, 11:48:13 AM

I was looking at the smaller cam
Thumpr Hydraulic Flat Tappet Cam
Adv/Dur: 279/297
RPM Range: 2000 to 5800
Intake/Exhaust Lift: .479/.465

chimp koose · May 04, 2011, 03:07:20 AM

I do not think you will have any problem with valve to piston clearance with that cam unless the heads or block have been milled considerably.I ran a .480 lift 230 duration cam in my bone stock 400sbc short block with 291 casting# camel hump heads and steel shim .021" head gaskets and had more than enough clearance.GM ran the .465 lift 350/350 cam in production engines with camel hump heads and a steel shim head gasket. I doubt .014" more lift on the intake side will get you in trouble as factory V/P clearances are generous (they had to warranty their stuff). When you have the intake off, use a mirror to look at the under side of the heads below the intake ports. Factory machining marks will run parallel for the entire length of the head as they were surfaced in a planer. Radiused machining marks will indicate that the heads have been milled.A thoughtful machinist will stamp the amount milled from the heads at either end of the head or in the portion that overhangs the lifter valley allowing you to find out how much has been removed without having to take the heads off.