How far can you bore out a Pontiac 350?

I am thinking of building a Pontiac V-8 and would like to avoid any major blunders. I understand that 389s and 400s command a premium, while 350s are relatively cheap. I suspect boring it out by .275, producing a 406, would be a rather large blunder. Boreing by .068 would produce a 366 with the same bore as a 316 or 347. This would require pistons that might be hard to find. All this is beginning to sound rather expensive and possibly more trouble than it is worth.

 

Why not just bore it 0.030 and get 0.030 oversized 350 pistons??? Then you would have enough meat left in the cylinders for a future overbore if it is required.

To find out exactly how much the maximum bore it could take would require that the engine gets sonic checked for thickness. Every engine is different, and with core shift and other casting defects, some engines can't handle a big bore job. Of course you can always cut out the cylinders and sleeve it to just about any bore you want, but that gets expensive, very quickly. As a rule of thumb, you will want to leave a minimum of 0.170 wall on the thrust side, and not less than .110 wall on the non thrust areas, unless you plan on filling the block, but that changes the equation completely.

Good Luck.

 

Ok...looking in my trusty Motors manual I see we need to stick with the 350s 3.75 inch stroke to get pistons with the correct comprression height for your application. You're right, the 400 pistons are just too big at .245" more than the 350s puny 3.875 bore...maybe 389 Pistons? My calculator says they're .187 bigger at 4.0625...still a pretty healthy overbore but your machinist may find the 350's bore will take it, I dunno without sonic testing. If so, a set of stock-bore 389 flattops shouldn't cost much..
A set of flat tops from another make might work but my book doesn't show piston-pin location or compression height. My machinist has such a book as should any machinist. Find a piston with the same piston pin diameter [.9802] and same compression height and pick your bore size...
I'm not familiar with 350 Poncho blocks...sonic test your block for possible .187 overbore for stock 389 pistons.
I bored my 347 block out .125" to use Pontiac 370 pistons [both engines use the same stroke] and still had plenty of meat left according to my machinist.

 

Since when are 400s expensive? Get one and rebuild it at .030 over. Be happy.

 

Oh don’t get me wrong, if I can find a hi-compression ratio 400 or 389 I will take it. I’m somewhat concerned about not being able to find one, especially in six or seven years when I have finished collage and have (hopefully) some disposable income.

 

Pacific Performance Racing makes a crank with a four inch stroke that drops into your Pontiac 350 block and transforms it into a 383. Available using Pontiac or big block Chevy rods (Pontiac costs more) it shows lots of promise and it's affordable. Plus 350 blocks cost even less than 400 blocks.

 

Pontiac 389's are super cheap, 400's are cheap. Pre-64' 389's you can't give away(starter bolts to bellhousing not to block). Just how many would you like?