[TRADERMIKE 2012]

Originally Posted by SD455DJ

Trader Mike, the 428HO is a '68 WJ code (manual) for a '68 Catalina 2-dr post coupe my brother is restoring currently. I believe Steve25 is right on with only one dished piston used on the 428 for all 3 years. I don't recall the CC's now. When Dan rebuilt the engine, he used flat-top Icon or Diamond forged pistons to achieve a true 10.75 compression ratio. The dishes would have dropped it down to low 10's. 428's used the same head as the 400's (670's in '67 and 16's/62's in '68/'69). Their combustion chambers all measured in the 72-75 CC range, so needed dished tops to lower. Pontiac went with dished pistons on the 421/428 rather than have specific head castings, like they did for the 455. In the 60's Pontiac adjusted compression ratios by swapping different heads. I attached a copy from the '68 and '79 parts catalogs of the 428 piston P/N's. I see the '68 catalog lists different P/Ns for the '67 & '68 428's while the '79 catalog list only one for all 3 years (most likely superseded P/Ns over the years to one common part) and most likely very minor differences between the '67 and '68 pistons to notice.

(Got Tree'd by tekuhn!)

Dennis

Good Info., thank you. Now, I don't see the words "HO" included in your documents that you sent for one thing says 428 only plus you mentioned that your brother had to use a Flat Top type version Piston to reach 10.75:1 CR. So, I ask "How did the Factory provide Vehicles with 10.75 CR using the Pontiac 1967 "D"- Port 670 Head 428 ci HO (YK) in my case unless they were using the Flat Top version like the 400 ci Engines. I have learned they would have installed the "Y"-branch Exhaust system under the Special Equipment order. There are 1045 YK code 428 ci "HO" Manufactured in 1967 only. Someone sent a list of Special Equipment like dual springs, special dist., higher pressure oil pump, "Y"- Branch Exhaust. In the Brochure it is mentioned that 10.5:1 CR and 10.75:1 CR were available upgrade options to the Big Vehicles in 1967. I can't help to think that the Flatter Top Piston would have been used to achieve 10.75:1 CR to arrive at those results. We are excluding any dealer upgrades like Royal Bob-catting the Engine here. Since the Dish is to lower the CR on the Big Vehicles. Problem is I don't know what Vehicle my 428 ci went into because it is installed in my 1979 Pontiac TA today. Your thoughts please, Mike out. P.S. On your Brothers Pontiac 428 ci "HO" 1968 WJ code (manual) for a '68 Catalina 2-dr post coupe Engine "What is the Factory stated CR with those Dished Pistons, see the Brochure". I would think the CR would have been 10.75:1 and if it could put out that CR "what type of Pistons were used to obtain 10.0:1 or 10.5:1 CR"? Also, the Pistons in the Photos look like the Dished version or low compression version?