Thread:

BBC over bore question

I have a 4 bolt block casting number 3969854. The book calls it a 69/72 396/402/454 2 or 4 bolt. Its already @ 4.250. What are the chances of it taking another .030..or is it something that will need to be sonic checked first?

It just seems odd that GM would cast a block that they
would ship as a 396 @ 4.094 and would be thick enough to bore to 4.250....or more.



The bore is dimensionally good where it's at but with the oem type 11.0-1 trw (427)pistons i am about out of valve clearance with a .620 lift cam. I am planning on going to a cam that has .670 lift....

I can have the pistons fly cut but i know they are really heavy anyway...And don't want to loose any compression but the selection for inexpensive 427 pistons @ std. bore seems limited.
thanx
mike

I am going to say its a 402 that somebody bored to 4.250. Yes it can be done.
Its this simple SHBH, look at the bottom of the cylinder bores. If they are straight across with no notches for rod clearance, its a 402

If you chose to go to a 6.385 rod, I could give you even more choices. All with a 119 chamber Are you running closed chamber heads?

LINK: 427 #1
LINK: 427 #2
LINK: 427 #3
LINK: 427 #4

Originally Posted by obnoxious001

Does "the book" actually state that it can be a 454 block?

Seems like it depends on which book you read...several include 454 and many don't.

Yes...that is what confused me....not to many oem blocks that i ever messed would take almost a full 1/4" over bore....but it is now...

Originally Posted by gn7

I am going to say its a 402 that somebody bored to 4.250. Yes it can be done. And i can prove it! lol
Its this simple SHBH, look at the bottom of the cylinder bores. If they are straight across with no notches for rod clearance, its a 402

Nope no notches....and looks like a poor place to be wanting to add any....

Originally Posted by gn7

If you chose to go to a 6.385 rod, I could give you even more choices. All with a 119 chamber Are you running closed chamber heads?

LINK: 427 #1
LINK: 427 #2
LINK: 427 #3
LINK: 427 #4

108/9 cc closed chamber

I know i'm pissin in the wind with 'old school' heads but the hp is
pretty close to what i need. The heads have some port work that i was told 'they knew what they were doing'...2.240 intake valves.
#1 is a full point less comp
#2-4...open chamber

thanx
mike

No notches = a 402 block. Yes they could take a 4.250 bore, but thats the limit. You don't need to notch any thing for a 396/427 crank. Only if you deside to go to a 454/4.00 stroke. Notching the bottom of the bores is not a problem. Its solid metal and only a cylinder extension for piston stability at BDC. The bottom of early tall deck truck blocks are the same way and I have dropped 4.5 stroke cranks into them.

Tough to get where your headed with the stuff you have. I would just fly cut the TRW pistons. But first I would check the valve to piston clearance. Remember, its not a lift issue with clearance, its a duration issue. Its never how far the valve opens that causes the problem, it WHEN it opens. Exhaust is very seldom the issue with BBCs, its the intake. Where is your piston when the intake valve is wide open?

Originally Posted by gn7

No notches = a 402 block.

Early 427's also came without notches... but chanches are pretty good this fella has a later 402 block.

Originally Posted by gn7

Yes they could take a 4.250 bore, but thats the limit. You don't need to notch any thing for a 396/427 crank. Only if you deside to go to a 454/4.00 stroke. Notching the bottom of the bores is not a problem. Its solid metal and only a cylinder extension for piston stability at BDC. The bottom of early tall deck truck blocks are the same way and I have dropped 4.5 stroke cranks into them.

Agreed... 4.250 on a 402 block is it... I've done a few of these and 4.250 is all I would ever go on one... did take one to 4.280 and ended up splitting a cylinder... apparently found an extra thin spot.

Originally Posted by gn7

Tough to get where your headed with the stuff you have. I would just fly cut the TRW pistons. But first I would check the valve to piston clearance. Remember, its not a lift issue with clearance, its a duration issue. Its never how far the valve opens that causes the problem, it WHEN it opens. Exhaust is very seldom the issue with BBCs, its the intake. Where is your piston when the intake valve is wide open?

x2

I'd go with GN7's suggestion and just cut the notches if you need them... you won't see hardly any compression drop.... or rather... the minor amount you'd see with cutting the pockets a little bit deeper the prop/jet/rear tires/ would never know. Any other option that will work with all you have will end up costing a bit of loot in pistons. To be honest, depending on the cam profile, you may not have to notch the pistons as much as you think... just adding .050 worth of lift doesn't necessarily mean you need .050 more pocket. Hell... I've put in "smaller" cams in stuff before and lost piston to valve clearance compared to the "bigger" cam... You just don't know what you need until you check it out.

Originally Posted by six-oh-nine

Early 427's also came without notches... but chanches are pretty good this fella has a later 402 block.

.

Yes the early 427 didn't have notches. Like I said, neither did the 427 truck blocks. The reason I say its a 402 block is that they never used that casting number for a 427. But they seemed to have "recycled" the number for 454s. If it had notches, it would be a 454, no notches, its a 402. No notches AND and different number, could be a 396, 427,or 402. But that number, and no notches, its a 402.

Originally Posted by gn7

No notches = a 402 block. Yes they could take a 4.250 bore, but thats the limit. You don't need to notch any thing for a 396/427 crank. Only if you deside to go to a 454/4.00 stroke. Notching the bottom of the bores is not a problem. Its solid metal and only a cylinder extension for piston stability at BDC. The bottom of early tall deck truck blocks are the same way and I have dropped 4.5 stroke cranks into them.

Tough to get where your headed with the stuff you have. I would just fly cut the TRW pistons. But first I would check the valve to piston clearance. Remember, its not a lift issue with clearance, its a duration issue. Its never how far the valve opens that causes the problem, it WHEN it opens. Exhaust is very seldom the issue with BBCs, its the intake. Where is your piston when the intake valve is wide open?

Fly cut....got it.......pretty much what i was thinking....date on block says 1970......so it's a bored to the max 402....lol

Where am i headed? Up river with my foot it it for as long as i can! Boat ran 6400 with 40's and 6500 with 22's and noses over ...not a lot..it might have spun 7 but it wasn't pulling near as hard. Ran it 6k+ for close to 10 miles....

Thinking another .050 lift and 30* duration should be good for another 500 rpm and 25-30 hp.....i'm just guessing....hoping it will pull 7k like it pulls 6500 now....and dreaming for 7k with 40's lol....
maybe 90mph?

second opinion are always welcome!....

Piston is about 104* ATDC at mid lift....It's when the valve starts opening that causes the problem i'm@ 27*BTDC now..the other cam is 35* Cam is supposed to work from 45-8k....I have enough spring to cover it...not counting rev kit.....

I had .060 valve clearance with a .020 head gasket...and ran a .040...Thinking maybe fly cut enough to run the thin gaskets?

Thanx for the help guys!

You are finally going Chevy???

Originally Posted by SoldHondaBoughtHondo

Fly cut....got it.......pretty much what i was thinking....date on block says 1970......so it's a bored to the max 402....lol

Where am i headed? Up river with my foot it it for as long as i can! Boat ran 6400 with 40's and 6500 with 22's and noses over ...not a lot..it might have spun 7 but it wasn't pulling near as hard. Ran it 6k+ for close to 10 miles....

Thinking another .050 lift and 30* duration should be good for another 500 rpm and 25-30 hp.....i'm just guessing....hoping it will pull 7k like it pulls 6500 now....and dreaming for 7k with 40's lol....
maybe 90mph?

second opinion are always welcome!....

Piston is about 104* ATDC at mid lift....It's when the valve starts opening that causes the problem i'm@ 27*BTDC now..the other cam is 35* Cam is supposed to work from 45-8k....I have enough spring to cover it...not counting rev kit.....

I had .060 valve clearance with a .020 head gasket...and ran a .040...Thinking maybe fly cut enough to run the thin gaskets?

Thanx for the help guys!

Whoa... how big is this cam you plan on running... just wonderin'? 35* @.050 IO? And .670 gross lift? Flat or roller?

Originally Posted by six-oh-nine

Whoa... how big is this cam you plan on running... just wonderin'? 35* @.050 IO? And .670 gross lift? Flat or roller?

this
Dur @ .050” Lift: 260°/270° RR: 1.7/1.7Gross Lift:.651”/.677” LSA:107

or this

Dur @ .050 lift 286° @ .050 LBC 108° lift .670 LSA: 108

solid roller/rev kit/girdle..220/500 springs......

Not all that big......is it......

Probably not a good cam for a street car lol but it doesn't much matter what it does below 4500.........

One of those is a recommend from dave crower.......

The one i have now is crower.....
Says it's a good 'marine' cam....thinking jeet boat type marine...

Those 2 are drag race cams... drive shafts and gears like i have

and marine being some kind of contraption that pumps water.lol

Originally Posted by SoldHondaBoughtHondo

this
Dur @ .050” Lift: 260°/270° RR: 1.7/1.7Gross Lift:.651”/.677” LSA:107

or this

Dur @ .050 lift 286° @ .050 LBC 108° lift .670 LSA: 108

Well.... 286 @ .050 and .670 lift... gonna be real easy on parts I suppose... so that is nice.

But yes, it's not the lift that is your problem... it's the duration... that is quite a bit for stock TRW valve pockets.

Personally... I think the first one would have been a better choice... but that's just me.

Originally Posted by six-oh-nine

Well.... 286 @ .050 and .670 lift... gonna be real easy on parts I suppose... so that is nice.

But yes, it's not the lift that is your problem... it's the duration... that is quite a bit for stock TRW valve pockets.

Personally... I think the first one would have been a better choice... but that's just me.

Well... i ended up with a .686/.654 284/286 @ .050....and of course it doesn't quite fit...if it did i would be all confused lol

Looks like i have about .020 on the intake at spec lash .028...
needing about .080?

Exhaust is a crash....it will clear with .055 lash and i need .100?
I suppose there is a formula to compensate for valve angle/fly cut depth, would anybody happen to know the math?


Me just guessing thinks about .060 + .010 in case i advance the cam and maybe another .020 so i can mill the head a little compression...they need to be surfaced anyway.....so .100 cut...

And if i need .100 on the ex. ....about .030 gets me to zero....080 gets the number + .010 to retard cam and .020 to mill.... .110?

The intake valve relief is a dead ass match for the valve angle....the exhaust crashes into the corner of the dome...I know exactly where...i have 6 pistons with kiss marks from the last cam...probably from running lash @ .020 with a .030 spec..oops....It looks like i could do the exhaust with a file

thanx
mike