1967 427 deck height, should be 9.800 inches, is this consistant? always

[William B.]

I have a low mile 67 427/400, I am freshing it up with rings and bearings. At the machine shop the machinist measured my deck height with a special fixture installed.
It measures the deck height from the center of the crankshaft to the top of the deck, gm calls for it to be 9.800 inches plus or minus .001, mine measures 9.799. With that measurement the machinist assured me my block had never been decked.
My question is with the thousands of 427 blocks gm made were they constant in the 9.800 measurement or did the vary by much more the, + or- .001?

[Joe L.]

Re: 1967 427 deck height, should be 9.800 inches, is this consistant? always

I have a low mile 67 427/400, I am freshing it up with rings and bearings. At the machine shop the machinist measured my deck height with a special fixture installed.
It measures the deck height from the center of the crankshaft to the top of the deck, gm calls for it to be 9.800 inches plus or minus .001, mine measures 9.799. With that measurement the machinist assured me my block had never been decked.
My question is with the thousands of 427 blocks gm made were they constant in the 9.800 measurement or did the vary by much more the, + or- .001?

William------

I do not know what the allowable tolerance was from the original specification of 9.800"; it could well be the .001" you were told. In any event, I would not expect it to be much more than .005". So, I have no doubt that at 9.799" your block has never been decked.

[William B.]

Re: 1967 427 deck height, should be 9.800 inches, is this consistant? always

Joe,
Thank you for your response, the reason I asked the question was, another engine shop machinist stated blocks could vary as much as 40 thousand plus or minus. I thought about his statement, with that much you would have piston to valve clearance problems?
Could you even deck a block that much?

[Duke W.]

Re: 1967 427 deck height, should be 9.800 inches, is this consistant? always

9.800" is the nominal blueprint dimension. You are lucky that yours is within thou. I don't have much data on big blocks, but small blocks (nominal 9.025") are usually high - as much as .015", and I've seen side to side deltas of as much as .010". My theory is that decks are high due to broach tool wear, and I suspect big blocks follow a similar pattern. So your block was likely processed with new tools.

Deck height is a very important compression ratio parameter, and a high deck will not achieve the nominal advertised compression ratio. On a small block a .015" high deck will drop the CR nearly half a point.

I always recommend that deck clearance be checked prior to teardown, and if this is neglected or as a cross check, better machine shops have tools to measure actual deck height on a bare block.

A few years ago I co-authored an article on compression ratio management with John McRae when he was rebuilding a '67 327/300 to "Special 300 HP" specs. It was published in the Corvette Restorer and is on the Web. Search the Corvette Forum for threads started by SWCDuke and download the pdf.

Duke

[Dick W.]

Re: 1967 427 deck height, should be 9.800 inches, is this consistant? always

Never seen one .040 but have seen several in the range of .025 measured from side to side and end to end.

[William B.]

Re: 1967 427 deck height, should be 9.800 inches, is this consistant? always

Joe, Duke, and Dick, thank you for the responses.
I can be assured I have an original undecked block, based on your comments.

Last question the machinist's claim that he has seen many Chevrolet big blocks with a factory .040 plus or minus variation on deck height, can anyone back that up or is he confused?

[Duke W.]

Re: 1967 427 deck height, should be 9.800 inches, is this consistant? always

Like I said earlier, from my limited set of what I consider to be reliable small block deck height/clearance measurements, the highest I've seen is .015", none low, maybe .010" side to side variation, and only one had any significant taper (end to end slope).

In your case your big block appears to be right on the nominal deck height dimension.

Duke

[Bruce B.]

Re: 1967 427 deck height, should be 9.800 inches, is this consistant? always

I find it hard to believe that with the technology in 1967, that in a high volume manufacturing operation they had the capability to hold the deck height to within a plus/minus 0.001 inch. Jigs and fixtures, tool wear, temperature and overall environmental conditions would make holding a "tenth" pretty difficult IMHO.

[Duke W.]

Re: 1967 427 deck height, should be 9.800 inches, is this consistant? always

A "tenth" means .0001", one tenth of one thousandth. I don't know what the blueprint deck height tolerance is, but as stated by me and Dick, there is considerable variation in deck height, with many being high that I attribute to tool wear.

Duke

[Rick N.]

Re: 1967 427 deck height, should be 9.800 inches, is this consistant? always

I've seen up to .005" variance in stock BB deck heights over the years.
Still keeps stock quench at .035"- .040" with a factory head gasket.

[Bruce B.]

Re: 1967 427 deck height, should be 9.800 inches, is this consistant? always

A "tenth" means .0001", one tenth of one thousandth. I don't know what the blueprint deck height tolerance is, but as stated by me and Dick, there is considerable variation in deck height, with many being high that I attribute to tool wear.

Duke

WOOPs my mistake.

[Dick W.]

Re: 1967 427 deck height, should be 9.800 inches, is this consistant? always

Joe, Duke, and Dick, thank you for the responses.
I can be assured I have an original undecked block, based on your comments.

Last question the machinist's claim that he has seen many Chevrolet big blocks with a factory .040 plus or minus variation on deck height, can anyone back that up or is he confused?

I would not dispute his findings based on my limited knowledge. I have built more engines that had to have the numbers on the pad than I have performance engines that I was worried about the deck height