1970 GP with 455
 

Hi, I am considering the purchase of a 1970 with the original 455 motor. Is this motor considered an H.O. motor and is it desirable over the 400s? Thanks.

The 1970 455 (first year, 64 heads AFAIR), tended to have a weak bottom end. Many warranty returns. I'd suggest having it checked out very carefully particularly if it does not have an oil pressure gauge.

I like my RA 400.

ps just looked in Pete's book and have notes that the the D-port 455 (NOT a HO) was rated at either 360 or 370 marketing HP depending on the car line. This was the result of GM relaxing the the CID rule for 1970 and even though Pontiac did not have a "big block" they were forced to make one by stroking a 428 (421/428 was a decent motor) well beyond design limits. This D-port 455 was put mostly in Bonnevilles though some were also optional in Grand Prixs and later GTOs. Frankly I remember seeing rows of them in Pontiac garages getting the engine replaced.

For 1971 the bottom end was strengthened across the board and big cars got a D-port version while for 71 and 72 only Trans Ams got the round port HO. Any 1970 455 with the original engine I would have a lot of trepidation about.

Excellent information Padgett and thank you very much!

Wouldn’t sweat that motor. Most of those 70 455’s with 64 heads are 4 bolt main.

Yes, Pontiac had so many early failures (mostly Bonnevilles) that some strengthening was slipstreamed in. The 71 was a good engine but by then you could not give a 455 away.

Dredging a bit but think the '70 Grand Prix SJ package included a 455 and automagic. My 70 GP had a 400/4 speed and an open rear end. We called it Asphyxiation.

Never heard of anyone having MORE issues with 1970 455s VS later years...(???).......
 

We've had twenty-two 1970 GTOs and GPs with original 455s in them in the past forty years and still have SIX of them.

Also bought and sold around a dozen 455s out of big cars from 1970 to 1975. Again, never a catastrophic failure. Weak rod bearings once in a while, but that's it.

Never had any extra trouble with any of them - actually never, EVER broke one of them. Of course, they were maintained with regular oil changes and we've always kept the RPMs below 5k on the stock 3.25" motors. Most of them actually ran best if we stayed below 4,500......

Had a couple that were basically worn out and needed to be rebuilt (used oil or smoked or noisy lifters or light rod bearing noises) but NEVER had any catastrophic failures.

As mentioned, ALL the GTO and GP 455s were 4-bolt mains. Most of the 1970-72 big car 455s were drilled but only had 2-bolt caps.

And, yes, the 1970 Grand Prix SJ package included the 455 4-bolt engine. Mostly automatics but a handful of 3-speeds and 4-speeds.

Your mileage may vary.

I too have never heard nor read in any literature that there were all those warranty issues with the 1970 455's. Weak bottom end? How so?

My thinking is that is may have been more of an oil pump issue. I looked at the 1969 AMA spec on the oil pump pressures. The standard 428 used the 30-40 PSI pump while the HO 428 used the 55-60 PSI pump. The 1970 AMA spec shows the 30-40 PSI pump under the 455 column with a note that states optional 455 4 bbl uses the 55-60 PSI pump.

It may have been a case of the lower oil pressure pumps getting installed on earlier assembled engines and oil starvation took its toll or those who were used to the higher winding 400's expected the same from the 455's and found out differently with bearing issues.

Would be interesting to know the whole story.

Yep, I figure the failures were due to idiots trying to shift the 455 at 6500. I ran a '70 455 short block HARD for decades! Ran quickest shifting at 5200 with RAIV cam.

Agree, was just a starting GMI student in 1970 and remember seeing A Lot of '70 455 Bonnevilles in the shop and was told "weak bottom end". Do not know if was two bolt blocks, low pressure oil pumps, or overreving just were enough that by 71 many people considered the 455 the same way as a Vega. Do know there was a lot of warrenty work going on and *think* the 4 bolt main blocks were slipstreamed in. Was the same time frame both Chev and Pontiac were seeing a lot of failures of the fiber timing gears. One of the things I looked for was a steel timing gear in my '72. Didn't know about the 7K3 heads though.

BTW ded after 4500 makes a lot of sense. D-port 64 heads and 068 cam.

ps know there were some 70 GPs with 455/4 speed (329 four speeds AFAIR) but were not SJs - that included the THM400.

1970 Bonneville 455 YH code engine with #15 small valve heads recieved 2-bolt main caps.
These main caps naturally were considered weaker the the 4-bolt main caps all other 1970 455 engines got.
4-bolt 1970 455´s were designated "HO", despite the #64 D-port heads.

Weird, most Bonnevilles I would think weren’t bought by customers running them hard like a GTO or TA.
Luxury cars usually attract more mild drivers.

BTW when living in Texas and had the 70 GP a friend had a 70 GP 455 4-speed and remember him having to go through the bottom end.

What I heard many years ago (decades actually) at a POCI meet from a retired Pontiac foundry man was that there was a high failure rate on the then new 455 crank, mostly resolved during the model year. That doesn't mean they were all bad, just a percentage of them would fail very quickly due to bad tolerances in the machining. I recall he also said that by 1970 much of the original tooling dating back to the 1955 engine design was seriously due for replacement. They were doing more and more frequent calibrating on the machinery to keep things in tolerance but the failure rate was climbing. Apparently the 1970 455 black eye to their reputation triggered some overdue capital expenditure at the engine plant.

I can't think of his name but he gave a presentation at a POCI meet back in the late 80's or early 90's.

Just enough that in 70-71 the 455 was considered as desirable as a Vega engine. 67 Camaro 350 (first year) had similar problems but fixed with 4 bolt mains for 68. 68 327 is the best one to have.

I thought I read somewhere that Pontiac started using molybednum rings in 1970, and some engines had oil consumption issues due to the rings not seating properly.

I still have a 1970 GP 455 manual Trans coded block floating around here. As far as 4 bolt small block Chevys they started in 1969 and there is no such thing as a factory built 4 bolt 327. You can easily build one with a 4” bore block and a large journal crank from a 68/69 327.