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I put up the tools against$300
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689 Posts
Discussion Starter #1
I was curious to know if anyone has used the speedpro brand hyper pistons. I was looking at a set of the .340 dome to get my compression up on my 781 heads. Just curious about clearance issues with stock block and head measurements. They seem kinda cheap but I'm not sure if thats a good indicator of quality. Egay has a seller with new Lunati forged pistons that seem to get my compression where I want to be. $350 for the Lunati w/o rings or $250 for the speed pro's w/ rings.
 

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I put up the tools against$300
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689 Posts
Discussion Starter #2
oh yeah gross valve lift is .552 intake and .555 exhaust 274 intake duration at .006
 

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Hammer and a torch
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1,254 Posts
Forged would be a better choice but just adding rings will be another $100... I like Lunati stuff myself and have always ran their camshafts... I know its a Holley brand but I switched a cam and lifters in a SBC 4 different times in 4 different engines for a friend of mine and never had a problem... (till he hand windowed it so bad the cam was in 4 pieces and 6 lifters were found in the pan)

Oh and just another note with the 781`s be sure to have your machinest take .125 off the valve guides if they are stock... Most retainer/lock combo`s will tend to get very close to the top of the guide and smash the umbrella seals and possibly bend a pushrod or two... :eek:
 

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I put up the tools against$300
Joined
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689 Posts
Discussion Starter #4
it appears that he did, on the intake side only though. the tops of the intake guides are flat and kinda tapered on the exhaust guides. Not sure where I would find a spec for this to check........ Also, does the X in the casting mean anything? It seems like I read somewhere that this meant experimental. Just curious. It's funny you mention bent pushrod. thats the only thing I found wrong with this motor when I disassembled it. 1 bent pushrod
 

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Hammer and a torch
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1,254 Posts
I too have 781X heads and not sure if they are any different but assume they may be like the SBC X heads which most say have a 5cc larger intake runner...?

As for the guides you can take the installed height and figure from there or just assemble a couple of valves with the inner springs and measure the amount of travel that it will do... Kinda like checking clearance for the valve to piston test... :thumbup:
 

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I put up the tools against$300
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689 Posts
Discussion Starter #6
just kinda going out on a limb here and asking a theoretical question Bumpstick. Could this be remedied with a longer valve stem, spring shims, and reduced ratio rockers? Much easier to just machine the guides but I was just wondering if this would be a different approach. :)
 

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Hammer and a torch
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1,254 Posts
Yes of course it could be done that way but also in most cases a cam gross lift of .550 with stock ratio rockers 1.70 should be ok but will still be close just be sure to not float the lifters...

But also on your idea you would need longer pushrods and taller rocker studs and really no need then for reduced ratio rockers... Using .100 longer valves and pushrods and rockers studs would for sure get the clearance but a little machine work is alot less trouble and parts...

Oh and just one more little bit of info... Using that lift cam and those heads you really should be needing longer rockers studs... Cheapest I found were some 1.9`s from Summit in the Trickflow brand which came as ARP`s which was on the bag and the studs... :thumbup:

Need the length for roller rockers and poly locks...
 

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I put up the tools against$300
Joined
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689 Posts
Discussion Starter #8
I was just kinda concerned about clearance. The deck is off 5 thousandths on one side and 4 on the other, so its going back to the machine shop. I haven't bought rocker arms yet so I really appreciate the info. If you had not yet purchased rockers, pistons, & rods what approach would you take? This is going to be a street motor so I would like to get a good bit of durability out of it. I called Lunati and asked about the pistons I found and they said with 118cc heads I would be around 14:1 as they were. He did say that I could have the tops of these pistons shaved to leave a .125 dome and that would put me around 10:1. I'm sure I'm showing my ignorance but exactly what is a hollow dome piston? If the dome is hollow, how do you mill them and not have a big hole in them. Once again I'm not really familiar with "hollow" domes, please direct me as necessary.
 

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Hammer and a torch
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1,254 Posts
I would for sure keep the compression down to atleast 10 to 1 for a true street driven setup... As for milling down hollow dome pistons I would not go that route at all no matter hollow or not mainly because of the weight change would be pretty drastic in the balance (hence lot of change in the overall bob weight)...

For rockers for a BBC I would use nothing but the Comp pro magum full roller rockers with the lift your wanting... Alum is ok but with valve train issues of BBC`s I tend to go a little over kill for only a few bucks more (NO off shore cheap crap for sure...) Pistons I would opt for something either like Speed pro forged or another well known brand (stay away from off shoots again) and keep the dome to .125 or less even with the open chamber heads, also try for full floating pins they see to work better in a BBC... Rods... Well the stock BBC rods are really nice stuff its just they need good bolts and of course the 3/8ths versions are lighter which is also a good thing with the pistons... Stock 3/8ths truck rods can be had pretty cheap just make sure to have them checked and if possible again go with the full floating pins and have the rods bushed or you may even find some that way... There were dimple 3/8ths rods in a few apps...

Just a note on the hollow dome pistons too... Yes they are hollow but not quite like you think... The hollow area in some designs is above the deck of the piston but there is still alot of material there which has to be to take the combustion pressure... Also the valve reliefs are below the deck line of piston and have material reinforcing them for sure...

The basic plan I gave is a good idea on a street/strip engine because you know your gonna take it to the track to test it... :thumbup: If you want longer lasting durability then go with much less cam and hyper type pistons but of course suffer in performance... Kinda a trade off no matter what the build is...
 

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I put up the tools against$300
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689 Posts
Discussion Starter #10
Thanks, this board is great. I think I'll bite the bullet and get the proper Lunati forged ones not the ones I would have to mill. That way if I want to go with a little cheater later on I can be confident that they are not a weak link. Boy it sure gets expensive going the big block route the right way. I told myself when I started this that there was no reason to gamble on a compromise for cheap stuff. Good stuff fails too but I would be a lot more ticked off if it was a sweatshop rod or rocker that I saved $50 bucks on.
 
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