Possible problems running oversize valves, 472 peanut relief pistons, MTS #15, Potter rockers.

DevilDeVille

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Hi,

Has anyone any ideas what might go wrong with such a combo? My main concern is valves interfering with pistons. Valve sizes are 2.25" and 1.88" and with 1.7:1 ratio, the valve lift should be .577"

Thanks in advance,
Martin
 

Ted in Olympia

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The peanut pistons are cut for high lift cams and big valves. Happened to me on my first build using 76cc heads and MT15 cam.

Hit just enough to bend the valve but did not ruin them, they were able to regrind them and reused them.
 

8ad-f85

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If you can turn the engine by hand with some clay on the piston, you'll see for yourself where if any interference is.
You'll need to rig up a couple of solid lifters and maybe some very light springs with Play Doh .

It's a pretty big diameter increase, so I'd for sure watch radial clearance too.
If it needs very minor work, a die grinder and some care will do the trick otherwise an old valve with a lathe bit brazed on will work if need be.
 

DevilDeVille

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Main downside for me is loss of CR, when adding valve reliefs to badger peanut pistons. I was wondering if i go with my standard size valved 76cc heads, i can dodge the interference issue albeit using MTS#15 and Potter rockers? Sorry for stupid questions, but better safe than sorry i guess...
 

8ad-f85

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Why worry about a tiny bit of compression loss from valve notches, if even needed?
Even if you were on the low side of dynamic compression, a degree of cam advance would make that up and I'm sure you'd never feel it running.
What would your compression be with the early heads?
What fuel available?
 

DevilDeVille

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10.5:1 as advertised, both engine block and the heads will be skimmed a bit in the machine shop. Best available fuel on the pump here is 98 RON which equals to 93 PON or 93 PUMP in U.S. In order to get two rigid lifters, i should probably let them be fabricated here - can anyone provide me with accurate measurements?
 

8ad-f85

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10.5 as measured?
I'd be nervous to do that with such a small cam unless the vehicle only weighs 1800lbs.
I'd suspect 9.5 would be much safer without any penalty.
Is that with large chamber heads?
What does it calculate to with 76cc heads if considered?
What is your intake closing point (in degrees) and trapped or dynamic compression ratio?

A solid lifter could be made from an old hydraulic pulled apart or any other way that comes to mind.
Any old chevy solid lifters in your area floating around?
Sometimes really light springs one might obtain from a hardware store used as valve springs are sufficient.
 

DevilDeVille

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10.5 is a number factory gave for peanuts with 76cc heads for 1968 if i am not mistaken too badly. Thats the engine i am building. 472 +.040 over. Previous owner planted it in my 1978 DeVille but as it seems, at some point it has been "rebuilt" by bakers or gardeners or kindergarten teachers or shepherds so that one bank had original peanuts in it and the other one had one peanut and three brand new soap dishes. No wonder i had difficulties of obtaining smooth idle :D

Only information about the cam is a card that came with it:
 

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8ad-f85

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If your engine measures an actual 10.5 (and it would be imperative to really check) the following numbers are from a website used to calculate the trapped compression from when the valve actually closes.
I used sea level for altitude.

Static compression ratio of 10.5:1.
Effective stroke is 3.62 inches.
Your dynamic compression ratio is 9.47:1 .
Your dynamic cranking pressure is 198.69 PSI.

This is probably too high for your fuel.

If 9.5 is truly measured...

Static compression ratio of 9.5:1.
Effective stroke is 3.62 inches.
Your dynamic compression ratio is 8.58:1 .
Your dynamic cranking pressure is 175.04 PSI.

This is more reasonable.

 

8ad-f85

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I guess I didn't realize that you weren't decided on that yet.
Hopefully there's some porting on the heads to justify the bigger valves.
 

DevilDeVille

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Yup, there is some evidence of homegrown porting. Since there is no hurry, i am still concidering going with original size valves on original heads and MTS #5 or #10 with the small displacement boat anchor and saving the big valve heads and #15 for 512 cid to come, with proper h-beams and forged pistons.
 

DevilDeVille

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Happy New Year and for those who still care or dare to care - Merry (past) Christmas. I've been messing with my remnants of brain and decided to go with big valve heads and (probably shitty) KB 472 pistons so i can use my MTS#15 (for a second or two, at least :))

Have not ordered the pistons yet, so any input is welcome. Intend to use them wiht PEP rods i already procured.
 

Darius

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Happy New Year and for those who still care or dare to care - Merry (past) Christmas. I've been messing with my remnants of brain and decided to go with big valve heads and (probably shitty) KB 472 pistons so i can use my MTS#15 (for a second or two, at least :))

Have not ordered the pistons yet, so any input is welcome. Intend to use them wiht PEP rods i already procured.
Sorry, but KB pistons don't give me warm fuzzies.

bro. d
 

Red98422

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I’m with bro D on this one. There have been quite a few problems with the KB pistons...and forged pistons aren’t that much more expensive
 

Cad Man

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For the difference in price is it really worth the chance to use those pistons ??

So if this is .06 over with cometic gasket I get 9.7 to 1 that's with

4.360 bore
4.062 stroke
76cc chamber
26cc peanut
Zero deck
.049 thick 4.385 fire ring
 
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Cad Man

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If your at -.01 deck height its
9.5 to 1..

I have not yet seen a 10.5 to 1 piston that's close to its rated compression without decking the heads !

I would run from those pistons ...
Supposedly if the ring gap is right they will hold up but cheap cast pistons cost me 1 motor and thats more than enough for me. Never ever again
 
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Roy Heisenberg

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What vehicle(s) do you drive?
472 Caddy in a ‘75 Jeep J-20 TH-400 Dana-60 100LL
Martin,

Forged pistons are just the best way to go, and they will take more Abuse than the cast or cast hyper-utetic with high silicon content. Never have seen a forged piston fail of any of my engines, and did lose the top ring land on a cast piston in my old 505 Caddy. My favorite combo now is cast rings on forged pistons. The cast rings are softer than Moly rings, and the rings wear faster than the cylinder bores. This allows you to refresh the rings, hone the cylinders, and bore the block less often. Save wear on your Caddy Block, and always run Forged pistons with cast rings if you can.

Regards,
Roy

PS One more thought... A few years ago, I had to dodge a clearance issue, and the quick solution that worked was to Retard the Cam by 4 degrees. It worked, and that 472 is still running today with extra top end punch !!!
 
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