Bought 65 skylark gran sport with no motor. I can get a 401 easy but would like a 425. what are you your thoughts on boring a 401. How far can I bore it and what are some cheap up grades?
Hi, Won't be able to bore out a 401 to a 425, think it's a 0.125 overbore. Just a guess would be 0.60, but would confirm with one of the experts here. If you want more cubic inches you could always stoke it, not cheap for a custom crank, another option is some combo using Chevy rods and offset stronk on a std crank, believe someone on the board did it if you search, there is more details Regards, Jim
You could go .060 on the 401, but you may have heat issues. The 425 might have a bit more torque, but really, the 425 was not much different as the head flow restrictions had already showed in the 401. The 364 actually had the best flow for cubic inch ratio. If you don't mind telling us what you are after, might be better ways to skin a Wildcat.
I was told the best motors are64,65,66 401's. I won't have #'s matching so for close to same price, I thought I could just put in a 425, but they'er harder to find. So I don't wont to spend a lot of extra money and want best bang for my buck
The very 1st. thing I would do & recommend is to have the cylinder walls sonic tested. Because of terrible core shift in the later years I ALWAYS recommend this. Mostly it's safe to go .030" or whatever. Stroking is easy enough. IF you go with a common available ring size, like 4.211, available rings are 1/16th. 1/16th 3/16ths. whereas you don't have the options for the better rings at 5/64ths. & stroke it to 3.766 you will end up with just over 420 cubes. The "Nail" seems to like this small stoke. Tom T.
As posted and as I very well know from doing porting work on nail heads , the stock heads are a bad choke point even with only 401 cid so to me it's not worth the effort to bring up the cid for what amounts to a small gain in torque that the motor will make! Change your rear gears if you want more torque or run a slightly shorter tire on the back. Maybe put your time and effort into something more power productive like making a close fitting Crank scraper and a windage tray. These have been shown to good for 15 to 20 hp at 5 to 5500k.
the gs skylark comes with a deep rear sump oil pan not much to gain on that one . test on on a 440 mopar was only 5 hp gain after 5000 rpms. ported heads on a 446 cubic inch nail head can support 430 hp and 550 lbs of torque.
I meant to say the best power (not flow) produced by the nailhead given the design and limitations of the heads. (that given the highest stock output of each nailhead as it was delivered). That beyond the 364, even though there was still power to be made with more cubes, the "bell curve" was giving diminishing returns. With a 4bbl, it was over 300hp and 400 ft lb torque, and the 401 and 425 did not give as much "power:cubes". (sort of like power to weight of jet engine thrust) But, when one is pursuing every pony you can harness, going all out and beating all the horses out of an engine with a fat wallet, that's different.
talking factory hp and tq numbers. ok if you have 364, you would need to add 40 horsepower and 65 lbs of torque to equal a 340 hp 465 tq of a 425. i would rather try to add 35 hp and 35 tq to a 425 and be at 375 hp and 500 tq.
Hi guys, I can tell you from experience I have over bored 65-66 Buick 401 engines as big as 4.350 and I have had no overeating issues. in fact, I have sonic tested these blocks at 4.310 and the thinnest part of the Bore was .125 which is no thinner then some of the 455 Buick engines at 4.350 In fact, the 455 in my 1966 GS has a couple cylinders with spots that are as thin as .105 with a final Bore of 4.320 I once built a Buick 401 engine into a 462 cubic inches. I had the crank offset ground to increasing the stroke to 3.89 and the final Bore was 4.350. The combination made 485 HP and 515 lbs/ft of torque. As far as I know the engine is still running today. Now as far as cylinder heads are concerned that is a whole other topic.
Lets keep this in mind gang! Once any motor has hit the rpm where peak torque is made you have then totally taped out the overall flow system for what it's worth, that being heads,Cam , Intake , Exh system and or Carb. Hp will keep climbing because each cylinder is firing more times per minute ( remember hp is a mathematical calculation of torque and rpm) but the hp starts to nose over once the internal friction of motor exceed's the amount of hp being produced.
