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Author Topic: original big block heads  (Read 2651 times)
70lt1z28
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« on: October 14, 2006, 02:36:55 PM »

some told me I should try and get john hinckley to answer this question.  here goes,  a 69 camaro built 10E  of 1969, engine assembly date is july 24th 1969. Is it possible for this car to have 1 may dated 840 head and 1 june dated 291 head? the 291 head has a flat seat and uses the same plug as the 840. I have heard that this 291 with the flat seat plug was used as a  service head only. if this was the case would it have been dated  june 5th 69??  also if the car came with a set of 291s wouldnt they be the peanut plugs?? any help I can get will be great   thank you, john
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Ed Bertrand
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« Reply #1 on: October 14, 2006, 03:25:24 PM »

Basically John contacted me with this question off-line and I gave him my "opinion", but I would like to hear JohnZ's answer regarding the "official" Gm policy of placing mismatched parts on an engine from the factory.

The 840 heads had a CC of 106.8 and the 291's had a CC of 109.037. In addition, the 840 heads used an exhaust valve size of 1.72 while the 291 used an exhaust valve size of 1.88, thus making the assembly (for want of a better word) not "balanced" if both were used on the same engine. The 840's also used the "N" (flat style) spark plugs while the production 291's used the "Peanut" (tapered style) plugs. The "Service Replacement" 291's were cast in both the "N" and the "Peanut" versions, but again, according to CBTN, the "N's" were for service use only.

My question to you John (Hinclkey) are twofold:

1. Would the Tonowanda plant install one 840 head and one 291 head on a production engine?

2. Am I correct in that any "left-over" 840 heads that were still around Tonowanda after the 291 heads were introduced in May, 1969, would have been transferred to the Service Department?

Ed
« Last Edit: October 14, 2006, 03:31:19 PM by bertfam » Logged
JohnZ
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« Reply #2 on: October 15, 2006, 08:57:11 AM »

Ed -

1. No, they wouldn't mismatch heads on the same engine - not intentionally, anyway.

2. Yes - either Service would take them, or they'd be scrapped (re-melted at the foundry next door).
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'69 Z/28
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Ed Bertrand
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« Reply #3 on: October 15, 2006, 12:52:55 PM »

Thanks John. That's what I thought, but I just needed to make sure. I didn't want to give John (above) bad info.

Ed
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KurtS
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« Reply #4 on: October 18, 2006, 10:36:59 PM »

I thought this was discussed on one of the other forums too....

Have you confirmed the different valve sizes and spark plug seats?

John can confirm this: If the 291 had the same fit and function as the 840, the engine plant would use either.
cc volume probably isn't different enough to be an issue. Different valve size and spark plug design would not be the same funtion, but I'd like that confirmed on this engine first since I've heard different valve sizes and plug designs on the 291 head. 
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Kurt S
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paceme
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« Reply #5 on: October 19, 2006, 07:52:33 AM »

Kurt I believe this subject was discussed before. I had a 6a built L78 and one head was a 840 and the other was a 291. Based on the date of the 291, I believe it was a warranty replacement. Also my understanding was that 291 with older style seats were specifically used as warranty replacements. BTW functionally the heads were not a mismatch, as that car ran great with no issues.
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Steve Shauger
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nuch_ss396
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« Reply #6 on: October 21, 2006, 08:44:24 AM »

Hey guys!

This response is a little off target, but applicable.

Over the years, I've heard of a few cars having one '840 head & one '291 head.  Other than the spark plug seat, these heads are supposed to be the same ( functionally ). 
As was previously mentioned, the '291 heads were available ( in service ) with the older style spark plug seats.  As a matter of fact, the older seat style
was used on all aluminum BB heads throughout production.

There is an inherent mismatch from combustion chamber to combustion chamber on the same head.  This is due to machining, casting, etc..  So, using one
'840 head & one '291 head shouldn't pose any real problems.  As to the question of one of each being used on the same production engine, I would tend to doubt it.

I'm doing some research right now concerning manufacturing tolerances in the engine ( BBC in my case ) machining process.  I have learned some startling information
about how sloppy the tolerances really were on our beloved engines.  It has been reported that piston to deck height clearances ranged all over the place and out
by as much as 0.020 - WOW!  When you add combustion chamber volume variations, actual piston design fluctuations, rod centerline-to-centerline variations; then throw
in intake runner volume variations coupled with exhaust port variances, you start to see the real picture.  I was told that production engines were designed with these
fluctuations in mind.  It was just a matter of manufacturing reality in those days.  One very knowledgeable builder told me that an L/72 was pulled apart and upon
inspection it was found that the "as advertised" compression ratio of 11:1 was actually less than 10:1 - HOLY SCHNIKE-EES !  Another high-end race engine builder told
me that they took new engines apart, decked the blocks to get them more accurate, port matched the heads, milled the combustion chambers to like-volumes and
mixed rods to find 8 of the same centerline-to-centerline length.  Until I started to build an L/78 engine, I never considered these issues.

I guess this starts to explain why some cars ran better than others ( having the same engine HP ratings ).  Just thought I would share this information.

Steve
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69 SS 396, Hugger Orange, D/80, D/90
Chambered Exhaust, N/66, THM400, 3:73 posi

Steve A.
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