[A.J.]

What are the bore tolerances on a -66 389?
It is .040 over (4,1025"?) and I measured variations of + 0,0013 to 0,0027" above that. Hope i got the measurements right as I'm having a hard time converting the figures from imperial to metric and back again.....

[steve25]

Your number of 4.1075" is correct for a .040" 389 which looks like it goes to 10.433 CM in metric.
If the .040" bore in the motor now was run with the wall clearance fit for a 389 cast piston you may be able to get away with running a fordged .040" over piston with just a hone job to gain the new needed fordged piston clearance if the bores are still straight and round, if not a .030" over cast or forged 400 piston would be the way to go for use of a off the shelf piston I would say.
Your choise of heads to use on top will matter a lot in terms having piston to valve clearance issues or not, as a 389 head with a 400 type piston in the block will limit the amount of valve lift you can run to about .420" to .450" if changes are not made to the valve notches in the Pistons!

[A.J.]

I'm planning to use the stock 093 heads as it's matching engine and car, possibly with dished pistons to lower the compression ratio.
How much over can you go on the bore on a 389 XE, 9778789, date code K25?

[gtofreek]

I've done several 389's to .060", and even a couple to .090". .060" should be a safe bet. There are not much in the way of piston choices out there. Sealed Power has a piston[288P, L2269F for the forged TRW], but the 288P is a cast piston, and both are flat top, which would be over 10:1 compression. May need to go to one of these in standard, though I'm not a big fan of 8 valve relief pistons. At least this one has a tall compression height to put it closer to the top of the bore.

https://www.uempistons.com/index.php...0c9e13b54f3fdb

Simply putting in a forged piston will not need to be honed a couple thousands over for clearance as the clearance is built into the piston, so it takes the same bore size as a cast piston.

[78w72]

the 389's are supposed to be very thick blocks. i spoke to butler awhile back about max bore for my 1972 400 block he said 60 is perfectly fine, then proceeded to tell me a story of how arnie beswick used to bore the 389's to .100+ back in the day with never one problem. & this was before sonic checking was available.

that said, you should probably have the block sonic checked for anything over .060 just to be safe, but based on what butler said there is no reason that .060 will cause any problems on a 389. i have no clue on what pistons are available for that size, but its just a matter of matching bore size.

[geeteeohguy]

Ross pistons will make you what you need, to whatever dish size you want. Did a friend's '65 389 that way, not cheap (about $600 for the pistons) but he can run 89 octane with his stock heads, and the car makes a ton of power and drives great.

[A.J.]

Butler seems to have some selections of pistons for 389. I know my engine builder here in Sweden works with them.

[lust4speed]

I've read that Pontiac got carried away in 65 and 66 with their thin wall casting, and I've seen a few that seemed to prove the point that were getting thin at only .030" over. Then just when I was ready to condemn all the late 389 blocks, we had a '66 come through nice and thick. Included are sheets from one of the thin '66 ones and the thick '66. I'd want to sonic any late model 389 before sinking money into it.

[A.J.]

Yes, I feel that a sonic is a must. Browsing thru older post in this subject there seems to be much discussion regarding core-shifts in the mid 66 production of the 389. I guess a November cast such as mine is on the edge based on that. (not sure where early, mid and late production goes date-wise)