does anyone have some info on these pistons....need to refresh my memory....i believe they was last made in 1991 and i thought they was rated at 12.9 to 1 compression with a 122cc head. but not for sure what the dome cc was worth.

figuring for my number 2 cylinder

.010 down the hole
.039 felpro head gasket
108cc '148 chambered head (just got done cc'ing it)

just trying to see where my compression is at.

so far i got number 3 hole only .001" down the hole and number 1 is at .010" if i have to many pistons jumping around i might tear down the short block to correct these.

just as note i had a set of 049 heads that was stock and with carbon built up in them i cc'ed one chamber at 120cc .......mortec shows them to be 122cc

i thought those '148 chambered heads was around 118cc but they have them down as 112cc so i guess my bigger valves offset the difference.

im sure i cant be to far off with my cc'ing.

7040's are 50cc's, rated at 13.0 or a tick over as I recall with a 118cc head.
They were rated at .008 down & a factory type .028 gasket.
Pretty sure this is correct but I can look it up tomorrow if someone doesn't post. ;)

Same piston as the L2308. I have a set of +0.060" Speed Pros still in the box with both 7040P and L2308 on the box. Pistons are all marked 7040! Speed Pro information I have on them is as Follows:

P/N: 7040PA60
Bore: 4.310"
Type: Forged
Application: LS& 454 Open Chamber Heads
Compression Height: 1.645"
Nominal Deck: 0.025"
Skirt clearance: 0.006
Dome Height: 0.580"
Dome Volume: 50cc
Compression ratio:
105 cc chamber: 15.28
106.9cc chamber: 14.89
108cc chamber: 14.67
109cc chamber: 14.47
110cc chamber: 14.29
113cc chamber: 13.76
114.8cc chamber: 13.46
118cc chamber: 12.96
119cc chamber: 12.81
124cc chamber: 12.12

ddeennis: With your figures of 0.010" deck, 0.039" gasket and a 108cc chamber I come up with 14.67 static compression, dynamic depending on your cam's intake closing number. With Mike's 0.008"/0.028"/50cc/118cc figures the compression ratio is 15.32:1.

Hey Mike, Just got my 69 L88 '512 block, 7115 crank and '804 rods back from my local (75 miles away local) machine shop and will be assembling it soon along with the GM aluminum heads I sent to you.

Hi Dave,
Great, glad I could help with your project & you didn't bolt the heads on as they were :rolleyes:

I tend to think the seat work should help a "little" over the way they came,,,, :D

thanks for the info.

i'm coming up with different compression ratio

using the formula

.7854 x (bore [to the second power]) x stroke
plus
.7854 x (bore [to the second power]) x deck
plus
.7854 x (bore [to the second power]) x gasket thickness
plus
head cc minus dome cc

divide all of this by deck,gasket,chamber

inches converted to cc by 16.4

957.077= bore and stroke volume in cc
2.392=piston down the hole .010 in cc
10.500= gasket volume per print from felpro .039"
58.000= 108 cc - 50cc for the dome on piston
-------------
1027.969 is total volume in cc's

2.392
10.500
58.000
-------------
70.892 = total chamber volume left

1027.969 divided by 70.892 = 14.50 to 1 compression.

is there something im missing here?

using prestage.com i put in the info as requested and it shows even higher compression ratio around the 14.7 to 1 mark.

so i looked up the formula to caculate long hand and this is what i come up with.

any comments on this?

ddeennis: Sorry for the misinformation I previously posted above (which is corrected now). When converting my old BBC Speed Pro and TRW piston printed information from from a catalog to a MS Excel spreadsheet I transposed the 7040PA (454) and 7041PA (427) information. My old piston box is labeled 7041PA60/L2308AP60 which is for a 427, yours are 454. graemlins/clonk.gif

Originally posted by David_Bates:
You are using 4.00" for a stroke! 7040PA pistons are 427 pistons ( 3.76" stroke ). Using those pistons with a stock length 6.135" rod and a 4" stroke (454) crank, your pistons will be 0.115" out of the hole assuming a deck of 0.010".

Single cylinder volume is 969.837cc using 4.31" bore, 3.76" stroke, 108cc chamber, 50cc dome, 10.5cc gasket and 2.392cc deck. well if these are 427 pistons then they fit real nice in a 454 with a 4" stroke and stock lighten rods ( i thought)..i even went back out to measure stroke and with the piston at bdc it 4" on the mark to the top......this has been a running motor signiture below.....i'm just going thru it to take some time to correct a few things and take a better compression ratio check. this is a 454 right along with the 454 flexplate and hormonic balancer.

now it could be possable the rods i got maybe shorten if these are really 427 pistons. i bought the pistons and rods together from a 454 alcohol motor. and i picked the 454 core out to use these in.

but in anycase the measurements still stand

4.310 bore
4.000 stroke
.010 deck
10.5 cc gaskets
108cc chambers
50 cc dome on piston

so whats the compression is 14.50 to 1 right?

Originally posted by David_Bates:
Same piston as the L2308. I have a set of +0.060" Speed Pros still in the box with both 7040P and L2308 on the box. Pistons are all marked 7040! Speed Pro information I have on them is as Follows:

well i just looked up your 2308 pistons and they do show for 427 ci......and you say they are the same as mine. ---7040PA----other markings on pistons------251-----and .060

my 454 has these in there in anycase.

has it been common to use these pistons with shorter rods in 454?. the rods are the only thing i didnt measure putting this together. besides a better accurate compression ratio.

the crank is a 4" stroke there no doubt about that......and i even second guessed myself and went back out to measure and yes the flat top of the piston is 4" down the hole at bdc.

the pistons where never modified "to work" or cut down for there was no marking on them.

so if these are truly 427 piston i would have to assume the guy i bought them from used shorter rods to make it work on a 454 crank.

anything else might be missing.

Dennis,
The 7040 is a 454 piston with a 1.645 c/h & a 50cc dome.

Some of the books are screwed up in their interchange info I think as I have run into the same deal as Dave before somewhere.

Just for info, it would be virtually impossible to use anything shorter then the stock 6.135 (actual 6.136 by the book ;) ) rod without major surgery as the piston would smack the crank counterweights at BDC.
Also, the piston would be .019-.020 in the bore depending on whose rod length you want to use with the nominal 9.800 deck block.

The compression will be 14.56 with your above figures from your first post assuming a 4.280 head dia & rings .250 down.
This is also not taking into account what you may have taken off the dome to fit your head.

well thanks wolfplace ....i tell ya there for alittle while wasnt sure of what i had going on.

it was a lot easier just when i built this motor in the begining. called trw tech line they said 12.9 to 1 bla bla bla in a 454 and said they stoped making them in 91.....so for me i knew the compression was high enough for this motor and didnt bother to check into anymore. and called it 13 to 1.........

anyway i didnt really remove anything from the pistons. i took sanding roll and rounded all the edges and worked the spark plug area just a tade.

i worked all the pistons the same.....then i grinded on the cylinder heads to make room........and thats what i'm doing now just going back and clean up some of my roughed in work. i found just a few spots where the pistons hit the heads (from racing and driving the car) nothing seriuos just enough to keep a shinny spot on the pistons.

with this much compression i think i will lower it some. because really 14.5 would be my lowest cylinder. i have 3 holes that the pistons sit at .001 down so that means i'll be pushing 15 to 1 compression. all other cylinders fall inbetween........

i won a set of copper heads gaskets off ebay for like 47 bucks shipped to me, that have a round bore of......4.620 and .065" thick......i was thinking of using these......

not only would this give more alittle more head room but lower the compression ratio to around 13.25 i believe.

let me know what you think.

lower the compression?, keep it jacked up there?, i really dont think im going to loose anything by bringing it down.i have never ran copper gasket before whats your thoughts on them? i have ran steel shim on about every motor with out problems figured it might be similar..

cc the piston domes to double check your compression numbers.

i'm heading out right now i was going to bolt a head on with gasket ......bring pston up to tdc with a small film of bearing grease and just cc the entire combination. this should get me dead nuts on i think.

I don't think lowering the compression a little is going to hurt you at all.
Not a big fan of copper gaskets on a street/strip deal but they are great when you are taking the heads off every round :D
I really don't like them without "O" rings as you don't get good clamping pressure around the cylinders.
If you are going to use them buy some spray Hylomar & put a couple of good coats on both sides & let them sit for a half hour or so before installing them.
Permatex & Valco (my choice) sell it in spray cans
Check here, click on the Valco & then Sealants.
They also have excellent Silicone.
Valco Silicone & Hylomar (http://www.berrydistribution.com/cgi-local/shop.pl/page=roy.html)

My preference would be to get rid of some of the dome but then you may get into balancing issues although lighter pistons will not hurt you compared to heavier & you get to pull the short block. apart :rolleyes:

well i got interupted before i could cc the combo with the head on (went with a buddy to look at a 51 chevy car.....he bought it lol 200 bucks)

this motor is strictly strip.... it wont see any street duty....i'll use those copper gaskets if they will hold up for the strip......

i should have a better idea in a few hours what my worse compression ratio will be using a felpro .039 gasket will be.

There is also the .051 FelPro 1017-2 ;)

just got done cc'ing one chamber with it on the block.

#2 cylinder

TDC .010 down the hole
used gasket .043" measured

it took 70cc to fill up to the second spark plug thread.

so i'm comming up with 14.66 to 1 for compression.

now knowing this is the worst hole i have all the others would read higher. so the holes with .001" deck height would be 15.09 to 1 in the best holes

so in anycase this is alot more compression then i thought in the begining.


i think i will try the .062" copper gasket since i bought it and hope it dont blow out with this kinda compression. i'm comming up with a compression ratio of 13.57 to 1 in number 2 cylinder if i change the gasket out from what i use now to copper and my best holes with .001 will be around the 13.94 to 1 range.

so i guess the copper will go on and now i kinda have an idea were the compression is at or it could be if i need more.

Maybe i'd better point out the obvious here, those pistons are OPEN chambered and the heads are listed in my interchange book as oval port, CLOSE chambered. Something is going to collide here, like the open chambered dome and the closed chambered head. I've saw a guy grind the chamber open once but it didn't work real well.

ddeennis: See my edited posts on page 1.

Dave

just read the updated post thanks david that helps ease my mind there for awhile i thought maybe i had some screwy combo. but this just helps confirm my findings. the bottom line its alot of compression.

at the time i was building this motor the only open chambered heads i had laying around was these '148 truck casting heads and they sure looked like open chambered heads compared to the 390 and other closed chambered 90 some cc heads i had. there was only two spots to deal with to make clearence when i put these heads on. and really i put one of the pistons into the chamber at first to see it it would fit and it did. so i figure what the heck. but once the piston was in the block and the head to go on some areas touched.

the engine has run time on it and it had just a few spot that was kissing but im sure that was from the 7000 rpms blast down the road and track. so with alittle more polishing with sand rolls and going to a .062 gasket any clearence issues will be gone. heads bolted to the block with no gasket the engine spins over freely.......


I would like to thank everyone for there input on this and helping me out. is always nice to get this kinda reponse.