I just got my car (68 coupe) on the road recently. (after a 3 yr frame off) It made it about 200 miles before the intake pushrod on #1 broke and fell into the galley. Also the exhaust valve looks as though someone beat the end of it with a hammer lodging the Ball end of the pushrod into the rocker. I pulled the engine and took it apart, but the only damage I found was just the two damaged pushrods.
I am curious if anyone might have any ideas what may have caused this. I am using this as an opportunity / excuse to change the cam and was wondering if someone could give a little advice... I have looked for hours and hours and still have no idea which way to go.
This car is going to be used as a weekend / cruise car that will see some limited track use. I dont want to turn the engine any more that about 6200 rpm tops.
My setup is:
SBC 383 Eagle Steel crank, GM Forged pink rods, 10.5:1 hyper pistons, .030 over bore not a OE roller block
Edelbrock performer RPM heads w/64cc chambers, 2.20 / 1.6 valves 120# seat / 320# at .500 lift, max valve lift 0.575 per spec sheet, 190 cc intake ports? (I think)
RPM air gap intake, 750 edelbrock performer carb
2004r trans with 3000 stall lock up converter
4:10 gears

Existing cam is a hyd flat 226/228 dur at .050 cam 488 - 490 lift 1.6 roller rockers (I would like a little hotter cam than this)
Questions:
Should I step up to roller? (will I have to change springs if I do?) Or should I stay with a flat hyd cam? Would a roller be a lot better if I choose that route? I have heard both ggod and bad things about the "Thumper " cams... would that be a good choice? since I will be replacing rockers, should I stay with 1:6 or should I go to 1:5? (I was considering Comp cams ultra pro magnums)

Thank you in advance for any help that anyone could offer.

Brent

It's the edelbrock carb that is causing all the problems... just kidding.

That is a pretty mild cam, especially for a 383 and especially for 10.5:1, bet it runs really well.

When switching to 1.6 rockers sometimes you have to open up the push rod holes in the heads, I am not sure if you have to do this with edelbrock heads, but if the p/r contacts the head it will bend.

A roller will be a TON better if you go that route, it is expensive, but the gains are worth it. If you go with a roller you will most likely have to change springs, I would go with a Comp .530" lift cam or higher with 290+* of duration, hydraulic roller.

As for the thumpr cam, if I had the option of taking a bullet to the foot or having one installed in my car against my will, I would take the bullet. They are not made for power, they are made for sound, sound does not win races.

Also, did you check your p/r lengths? How does the valvetrane geometry look just eyeballing it? You can tell if something doesn't look right.

Thanks for the reply Ace!! If I could fix the problem with just a quick carb swap I would toss the edelbrock across the street! I think it might be a good idea to just stay with 1:5 rockers to avoid future probs.
I think I am pretty clear on your opinion of the thumpers )

I just went out and looked at the top of the valve. It seems that the wear is right in the middle of the valve , so it appears the geometry was acceptable.

This cam should give you what you're looking for if you want to go roller:
http://www.summitracing.com/parts/CCA-12-468-8/

I don't have any experience with Thumpr cams, but I have listened to the online recordings and they do sound pretty wicked, especially the biggest ones "Mega" Thumprs I think. I'm sure they're no slouch in performance as well.

Bret ,we have very simular combinations except I'm running a solid cam. I looked at switching to a roller cam but it was too expensive for me at this time. I would estimate it would run you about $1,000.00 to switch to a roller set up. So my solid cam I'm keeping for a few more years. I got rid of my Edel 750 and went to a Holley 700 double pumper, that was a good switch for my car.

Do you have the heads off?What brand of pistons do you have?

Yes, I do have the heads off, I was expecting to find some obvious contact between valves and piston.... There is no evidence of that at all. The pistons are speed pro flat top with valve reliefs hypereutectic part number H860CP.

You knew what I was thinking.The other thing to look for is coil bind.Even though the head spec. sheet says .575 lift it could be wrong.Is your .488 and .490 lift with the 1.6 rockers or 1.5s? If it is with 1.5s then you have .518 and .522 with the 1.6s.I don't like 1.6 rockers.They change valve train geometry by moving the pushrod cup closer to the rocker stud and put more pressure on the rocker arm and stud.Get a cam with the lift you want and use 1.5 rockers.JMO

With those pistons and assuming a good quench, I would bet that you are closer to 10.8 to 10.9 on your compression. Similar pistons and a .046 quench I am at 10.9 to 1.

Do you have any detonation now?

Chad, you are exactly right the cam numbers I gave you are for 1.5 rockers. With the 1.6 rockers .518/.522. ( the rockers were already installed in the engine when I purchased it. ) The edelbrock spec sheet says I should be able to run 5.75 lift which should give me over .030 clearance. I have not removed the springs as I dont have the tools to do so, but I was thinking of installing beehive springs just to make sure. I understand they allow more clearance. BTW I have decided that 1.5 rockers are in my future here since I have to replace them anyway I will get the 1.5 Thanks for your help

Eric, you may be right about the compression I dont know, I was told it was a 10.5:1 . I never heard and detonation as I was driving it. but I didnt get to drive it much, I was driving it down to get the 4.10 gears installed when it broke.

I came up with 10.9to1 like Eric. If you go with a hyd. cam look at this kit cam , lifters , springs and locks .
Elgin, Hyd. Flat Tappet Complete Cam Kit, Chev SB, .488/.510
Chev SB
Lift: .488/.510
Duration: 303/313
Duration @ .050": 234/244
Lobe Separation: 112 LC
Good idle, Great street/strip, 2500+ stall, Headers, OK with NOS
$154.95


Part mumber E1067K http://www.competitionproducts.com/

Chad, there is no product on that link. Which cam are you recommending?

Oh yeah, look what is finally home........

edit........I see your last post now ^........

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Glad to see it home.I see you copied my corner TV shelf.

I really like the Voodoo cam in my '67. The springs that come with the RPM heads work with the cam and there is no valve train noise with 1.5 rockers. Nice lumpy idle and pulls past 6,000 (altough I haven't found out how far yet ).