Punching out a Cobra Jet to .070?

[cwareham]

Hi Folks-

I put my hands on a 428 cobra jet block (69 casting,) but I have a bit of a problem....It's bored .060, and is going to need another .010 to get all the cylinders "right." currently, a couple of the cylinders are 2 thousands big and a couple are 3 thousandths big.

My question is this - understanding that I'm starting to push it, can I go .070 over on a cobra jet block? This engine will likely never see the far side of 5000 RPMs with the possible exception of dyno day...it's going to be a street motor with some relatively mild track time now and then. I picked the block up for a song, and I have most of the components to build the short block for a pretty good price....in other words, it's worth it for me to seriously consider it rather than dropping $800-$1000 on another 428 if I can get away with it.


All the best,
Chris

[blykins]

Do you have access to a sonic test? I wouldn't continue until I had each wall tested......Even .060" on a stock 428 block seems awfully high.....You may wanna see how thin the walls are before you continue.

Or you could always have the block sleeved.....

[cwareham]

Blykins-

I'm about to get it sonic tested. Just had it magnifluxed, and that checked out fine. I've heard that .060 over on a standard (non-CJ) block is a no-no, but I'm not sure how much "extra" I have with the CJ block....book says "thicker cylinder walls" but doesn't say how much.

I thought about sleeving it, but man I'm thinking that the machine work doing that would get close to what another CJ block would cost...

Thanks for the quick response.

All the best,
Chris

[blykins]

The sonic test will tell the tale.....or you can do the "drill bit" test. That may give you something to go on....and maybe it could tell you whether or not a sonic test was necessary. May save you a little money.

I was told by a reputable machine shop definitely not to go less than .110" on the cylinder wall thickness.....

[cwareham]

Thanks for the guidance on the .110" thickness. What's the drill bit test?

C

[blykins]

It's been awhile since I've performed it....so I can't remember the specifics....but if you have the freeze plugs out, you can in most cases see how thick the cylinder wall castings are by sticking a drill bit in between the cylinders through the freeze plug hole.

Now if I remember correctly, if you can fit a 15/32" drill between the cylinders, then the castings aren't thick enough to bore a lot out of them.

You can do a search on drill bit test and probably find out more. Like I said, it's been about a year since I've messed with an FE.....and I was using it to see if a 390 block I had was thick in the cylinder walls or not....

[Tout]

Sonic test the block B4 you spend any more . my experience on CJ engines is that up to .030 is ok and anything over that is asking for a problem. Some of the castings were not as good as others and the only way to find out is to have it sonic tested or x rayed.
Tout

[FFR428]

Drill bit test....the shank of a 13/64" drill bit will fit snugly between 428 wall cores. If for some reason a 8/64" bit fits those are 427 thickness walls. With the std 428 wall cores it will be to close to tell and sonic check is advised. Knock out the three freeze plugs on each side to access the wall cores. This will really only tell you what wall cores were used in casting the block. Core shift will also determine the sonic results for the .070 overbore. I've run 428 CJ blocks up to .060 with no problems FWIW. G.

[Ron61]

Chris,

I had my 428SCJ bored .065 and have never had a problem with it. It was sonic tested before and after the bore, then blue printed and balanced. I have ran 11:1 compression in mine since I got the Cobra in 1969. Only problem and I cured that was that on our 110+ humid days it wanted to run around 235/240 degrees. Both of my engines normaly run around 210 and that is what the engine builders told me they should run. I mounted one of thos e spray type units that they used to use on the old trucks so I can just push a button when the temp gets to high and it sprays a shot of cold water across the front of the radiator. A couple of quick shots will lowet the tmep by around 10/15 degrees for a while.

Ron

[Excaliber]

Why .070? Bore it another .003 or like Ron said, .065. Then order new pistons. 427's (I know it's a 428) typically bore at .003 segments. Mine, with final hone is actually .019 overbore (was .017). My new pistons have .0035 clearance. Of course these are CUSTOM ORDER pistons and cost a bunch!

By the way, YES, my new pistons are ready to ship from Gessfords!

[K.Wilson]

Chris,
I tried to get this to post before but it would not post so I will try again.
There have been a few threads in the past on this topic, but here are the basics:
Take the shank of a drill bit (not the drill end) and slide it between the cylinders thru a freeze plug hole. Find the largest size bit that will go in and measure the diameter of the bit. Then take a caliper and measuer the distance between the same two cylinders at the top of the block (this assumes there is no "ridge" at the top of the cylinders). Measure it several times and use the smallest number, then subtract the drill diameter and divide by two and you will have the cylinder wall thickness.
Web thickness minus drill diameter divided by 2 equals wall thickness.
This is only true if there is no core shift and it assumes that the cylinders were bored in the center of the casting.
I my opinion this test should only be used to see if the block is worth spending the money for a sonic check. In other words it can prove the block is not usable and save the cost of the sonic check but it should not be used to prove the block is usable.
Keith

[cwareham]

Thanks a ton guys....this is the kind of thing I was looking for.

All the best,
Chris

[Frank Messina]

Chris,
If things turn out ugly with your block you might want to look at possibly having it sleeved. It's a good way to save a difficult to replace block and is not as exotic as it sounds. All big diesel engines have sleeves. When they have to do a rebuild they just replace the liners (sleeves). FWIW
Frank