427 blocks 4 or 2 bolt mains

[Joe Peplinski]

My '69 L68 is a 2-Bolt! :-(

Wayne,

Thanks for taking the time to answer my question in such detail. After all of the discussion in this thread got my hopes up that my block might be a 4-bolt after all, I looked at my car again and (unfortunately) found that your description of the 2-bolt block describes it perfectly (ignore my previous post - I managed to get "forward" & "to the rear" mixed up in the descriptions). This finding along with PAT68's posting seems to confirm what I had always been told: that 390/400 blocks were 2-bolts, while 435/L88 blocks were 4-bolts. Thanks again.

Joe Peplinski - '69 L68 Conv.

[Tom B.]

Re: 427 4 bolt mains - More questions

Wayne, and all,

Until now, and since I wasn't previously aware of the external boss identification of the 427 blocks, I have found much of it very informative.

I am also wondering if it presumes too much. I question whether it truely identifies the 2 bolt mains from the 4 bolt mains or if it more correctly identifies the lesser horsepower engine options (the 390hp L36 and the 400hp L68) from the higher horsepower 427 engine options. There's more to the higher hp 427 engine components and assembly than just the 4 bolt mains. I believe the lifters, heads, and cam, differ just to name a few.

I can accept the certain amount of truth and evidence as Joe L. has brought up about 427/400 hp L68's, particularly from 68-69, having the four bolt mains. If or since that would be the case, then I'm wondering if the drilled and tapped bosses, particularly the one with the large square hole plug directly above the oil filter was only present if the engine had ALL the components of a higher hp 427 engine option and not JUST dependent on the presence of 4 bolt mains.

Were all 427 engine blocks manufactured to accept 4 bolt mains or do the additions have to be drilled to add 4 bolt mains to an original 2 bolt mains block? TBarr #24014

[Wayne M.]

Re: 427 4 bolt mains - More questions

Hi Tom --- Excuse the delay; I was taking the cast-iron heads off my 396; I think I gave myself a hernia. "Long live 842's".

All big block chevy engines, at least in the era '65 to '74, were manufactured with the main bearing support sufficiently wide to accept either 2 or 4 bolt main caps. In fact, all cast-iron big blocks came in either 2 or 4-bolt configuration, all with the same casting #, with the sole exception of the '65 396's, where # 3855961 was the 2-bolt, and the 962 was 4-bolt. This alone indicates that only machining differentiated the pre-assembled blocks. Now I suspect that the external oil fitting boss machining was part of the "run" that produced 4-bolt main blocks; ie. they didn't do one step without the other. I admit this is an assumption, but many Chev books say this is the way to identify the mains configuration. Probably, the rest of the high performance parts (crank, cam, pistons, lifters, heads, etc.) were specified for assembly, but they could have just as well built an L36 or L68 engine with the 4-bolts and larger drilled oil bosses.

I'm from Missouri on MOST L68's having 4-bolts along with large bosses w/plug, and the responses I've had confirm my suspicion that they came just as GM said they were supposed to be -- with 2-bolt mains.

How does the '66 through '69 tech and judging guides treat the configuration above the oil filter for 390, 400 and 435 cars ? All I have is the '65 manual, and, of course, only the 4-bolt is discussed.

[Tom B.]

Re: 427 4 bolt mains - some answers (long)

Wayne,

Thanks for the information, I was searching for something like you suspected, that machining the boss for the external oil filter would / could have been part of the run that produced the 4 bolt main blocks.

As far as the 66 through 69 Tech Guides addressing the big block boss configuration, all the info I am familiar with is under the Mechanical Sections for Cylinder cases. I don't know of it addressed elsewhere in the TM's.

66 TM: Under the heading L72 425 hp: "Check the area above the oil filter for provision for an external oil cooler which is indicated by a single square plug."

67 TM: "Check area above oil filter for provision for an external oil cooler on L88 and L71/L89 435 hp engines which is indicated by two holes, one oil line fitting and one plug for external oil cooler provision. The L36 390 hp and L68 400 hp engines have only one hole for the oil line fitting."

68-69 TM: More interesting. There's no description I could find that was similar to the 66 and 67 Tech Manuals, but there are two pieces of info. The first is a chart at the beginning of the Mechanical section which shows the 1969 427 3963512 block to be:

"as 2-bolt late-October 1968" and "as 4-bolt late-December 1968"

The second piece of 68-69 TM info is a brief paragraph used to deal more with date codes and engine stamp pads than anything else, but I found as an interesting application here. It has to do with the engine "hot" test done at Flint and Tonowanda where each engine was ran with natural gas before each it was shipped. If the engine assembly failed the test it was disassembled in the "heavy repair" area. Salvagable components were returned to the line to be used in another engine and depending on the malfunction: "the cylinder case may be reconfigured by hand or returned to the line to be rebuilt into another configuration."

PS, I hope you're just joking about that hernia, they aren't any fun. TBarr #24014

[Jerry Clark]

Re: My '69 L68 is a 2-Bolt! :-(

Hi Joe:

Remember, as previously stated, if strength is your concern there are no structural differences between your two bolt and a four bolt and your two bolt can be easily "converted" to four bolt and if the proper parts are used be even stronger than a stock four bolt.

jerry

[Bob Lanham #32271]

427 blocks 4 or 2 bolt mains

Are 427 blocks 4 bolt or 2 bolt mains? I looked at some materials I had but I couldn't find anything on it. If there are 2 bolt and 4 bolts how do you tell? My particular interest is my 69 427/390. Had some time to start really looking it over and I got curious.

[Dave]

Re: 427 blocks 4 or 2 bolt mains

I've got the 427 out of my 66 in a bag. The (external) difference in the blocks is the drillrd/tapped fitting for external oil filter/cooler on the left side of the 4bolt blocks. People have been known to add 4bolt mains to block that came 2bolt from the factory. They are also stamped differently. The pad on the right front of a 66 shows an IL suffix on the date for a 2bolt and an IP suffix for a 4bolt. Look out for restamped engines. The right valve cover came with big yellow stickers on the back that had IL or IP, etcetera, etcetera.

[Tom B.]

Re: 427 blocks 4 or 2 bolt mains

Bob,


The 390hp and 400hp versions of the 427 came with 2 bolt mains. The higher horsepower 427's (425hp on up) came with the 4 bolt mains. TBarr #24014

[Jerry Clark]

Re: 427 blocks 4 or 2 bolt mains

Hi Bob:


As I understand it any two bolt Big Block can be machined to accept four bolt main caps, all that is required is a set of four bolt caps, the block must be drilled and tapped to accept the extra bolts and the new caps must be installed then line bored, Bingo ! four bolt mains.


jerry

[Roger Coen 30751]

Re: 427 blocks 4 or 2 bolt mains

Does that mean the "new block" is as strong as a factory 4 bolt?

[Jerry Clark]

Re: 427 blocks 4 or 2 bolt mains

Hi Roger:


Yes, as the web ,wall and bulkhead configurations are the same, there are no structural differences.


jerry Clark