Compression confusion

Hey guys doing my first nailhead its a 401 and was rebuilt around the 1998 era (bearing dates) and was never started up and installed in a 65 riv I got the motor for my project and took it apart to clean and inspect due to some water getting down a cylinder from sitting, crunching numbers trying to figure compression I ccd the pistons and came up with about 42 ccs bore is .030 over did the heads and came up with about 127 ccs block looks like it was decked and heads resurfaced nice machine work measuring to see how far the piston is from the deck I come up with .090-.095 below the deck at the 4 corners, using a .015 steel shim gasket Iam no where close to factory 10.25 compression is this common??? looking at an old 401 piston it seems like there is a little more thickness from the top ring to the top machined area of the old 401 piston, which would explain my piston being that far down in the bore, the pistons are cast stamped USA inside and E*E on the other side, just wondering if anyone is decking the blocks to help the compression and maybe Iam missing something, Thanks

 

Almost no Buick engine is as high in fact as the factory paper static specification. Even a 70 455 will be around 9.8.

 

Most Nailheads averaged .050"ish below deck from the factory. Yours seem a little low and more then expected even with a rod resize. It has to be the aftermarket pistons. You could pull one piston and check the compression height against stock (1.925"). You will pull the intake ports inward as you deck the block and shorter pushrods may be required depending. You also can't go back once material is removed (IE: break a had to find or one-off piston in an engine with abnormal dimensions).

Cheryl

 

is stock 1.930 or 1.925, i torn apart 5or6 401-425 they were never more than .060 in the hole. maybe[egge spelling?] pistons. Walt had one after market piston at 1.912. the 42cc seam a little low most stock and after markets were 45-46cc on Walts post.

 

I have no idea who made the pistons this motor was all rebuilt when I got it, free by the way from pan to carb....they look nice and measuring the bore sizes they dont seem like blems or the such, comparing the new pistons to the old ones these dont have a widow or an open area under the piston pin like the factory units, I will go to my shop and compare compression heights to see the differance, my plans are to run a fairly stock cam and run the roller rockers....just want to make sure I have enough compression to make it run respectable this is my first nailhead, just been looking at too many mopars, chevys, and fords in the past, I think if I keep it simple it should be a great street motor for my 63 skylark, thanks again...

 

to be that far in the hole your pistons would have a pin height of about 1.89

 

Thinking you had dish instead of dome!

 

did you minus the 42cc or added it. its a dome not dish

---------- Post added at 09:32 AM ---------- Previous post was at 09:27 AM ----------

Larry, i know your thinking 455 lol

 

Stock 65 401 piston, stock 64 425 piston, Badger .030 over 401 replacement all here: all 1.925" compression height. Your mileage may vary!

Cheryl

 

good one. lol

 

I get 8.63:1 SCR on my calculator using the above dimensions.

 

Silvolites are currently available, they're spec'd at 1.912"
Sealed Power also available (Summit Racing), they're even worse at 1.905"
My Ross Forgings say 1.930" on the box.
Not finding a spec on the Egge pistons.
It's a sin we can't get decent pistons for these engines without resorting to cu$tom made ones!

Stock CR was lower than the advertised 10.25:1, generally about 9.8:1

Don't think there is an easy solution here. We would have to crunch some numbers to see what kind of improvement could be made.

 

the 1.905 would add up pretty close to whats there, the very most i would deck it is .040 using the stock .015 head gasket and the heads already milled {they cant be milled much with 127cc} are they tulip valves., that would give him about 9.35-9.40 comp ratio. maybe the dome is a little bigger then first measured that would help too . guys run some numbers see if am close.

 

Well measured compression height from a used 401 piston and came up with 1.9287 and measured the current rebuilder pistons and got 1.886 hmmm well thats a .0427 differance now the numbers make sense I would hate to change the pistons cause the whole assy was balanced but I guess I could deck the block but my concern is with piston to valve clearance, I have to change blocks due to the rust issue so my thought was to machine one of my spare 401 blocks and just transplant the whole thing in a new block, has anyone ran a stock 425 cam lets say and got away with a 9 to 1 motor???or should I just scrap the pistons and start over??? If I do that I could just use one of my 425 blocks (I have been collecting blocks and parts leading up to this project)and just go that route, also the heads are nice and fresh looking at the intake valve it is a stock GM valve and has a hell of a dish which would raise the ccs of my heads do the aftermarket stainless valves have a flat surface like my exhaust valves which would decrease my ccs on the heads and raise the compression?Thanks again

 

now i know why some say stock are 1.925 or 1.930 your are just about in the middle. deck the block .040 would give about 9.3 comp if the pistons are now in the .095. the valves would never hit pistons, i think a stock cam would be alright. would not do the valves unless you need new ones or maybe valve job.that is the lowest cost way. you be losing a 1/2 point of comp so about 7 hp or 3/4 of a point with out decking and that would be 10hp if you can live with that. if you dont already have a stock cam i would run a good after market cam to make that up. maybe the guys will have some better ideals . also if you can measure the dome cc again might be more hopefully.

 

well I planned on running the 425 dual quad cam with toms roller rockers, I have read they give you a little more, I will be running the Hilborn EFI set up and the car is lighter than most check out my album to see pics of the car if your interested, I have had to deal with cars over cammed before and its a real pain dealing with that situation,just want to do it once right and if Iam a little conservative on the profile thats ok, looking at the numbers of the ccs on the pistons Iam conservative maybe off 1 or 2ccs at the most I need to really make a mold of the crown and then fill it to get a tru real world cc of them so if you think Iam in the right direction, I will deck the block like you suggested and go for it.

 

WOW lol. lets see what some of the guys think.

---------- Post added at 07:20 PM ---------- Previous post was at 07:05 PM ----------

Looked at your photos, great looking car really like that body style.is it going to hard to hook up the hilborn?

 

I have done a lot of fuel injected stuff on typical chevy stuff, hardest thing I feel is doing a crank trigger set up but whats pretty cool is the 6 bolt crank pattern and the register hole is pretty close to blower/supercharger stuff MSD has a universal trigger set up, that looks like it will work just put it between the balancer and the lower pulley, I have access to a cnc guy next door to me at my shop so thats the least of my worries.... also each bank of injector units isnt attached to the other side so decking the block and having manifold issues lining up shouldn't be of concern, the manifold gets here this week so i will see how it fits.