[harry k]

72 HO 7F6 round port calls for tapered peanut spark plugs 5/8 hex R45TS ,
meanwhile 71 191-197 casting round port calls for gasket seat 13/16 hex R45S

cast iron head threads within these years are same , why the spark plug aplication differences ??

[Johnny406]

Most likely hardened exhaust seats for unleaded fuel.

[b-man]

My 1971 197 heads take the gasket-type plugs.

Had a set of 1969 # 48 heads that used the smaller tapered-seat 'peanut' plugs.

Go figure.

The exhaust seats weren't hardened until the 1975 5C heads came out, so that's not a factor.

[Schurkey]

Originally Posted by b-man

The exhaust seats weren't hardened until the 1975 5C heads came out, so that's not a factor.

I thought that GM hardened (usually by induction heating) the exhaust seats "across the board" starting in '71 or '72. Am I wrong?

[b-man]

I always thought that too, but the induction-hardening didn't start until later on.

You should have seen the set of '71 #66 heads I pulled off my 455. When I first put them on the exhaust seats were perfect.

After 30K miles half of them were even with the chamber floor, they receded by about 1/8". Was running the 068 cam and stamped 1.65s (about .430 actual lift) and Edelbrock Performer springs (#5758) that were pretty weak. I was running 2.56 gears for most of those miles, and 2.93s for some of them so excessive RPMs weren't a factor.

I'm pretty convinced those exhaust seats couldn't have been induction-hardened.

[steve25]

Hardening did not start untill the 1.66 valve became std in all 4bbl motors and the cat converters.
the 5/8" plugs warm up faster, so you get better emissions and less of a chance of fouling.

[chuckies76ta]

Originally Posted by b-man

My 1971 197 heads take the gasket-type plugs.

Had a set of 1969 # 48 heads that used the smaller tapered-seat 'peanut' plugs.

Go figure.

The exhaust seats weren't hardened until the 1975 5C heads came out, so that's not a factor.

So can you run either plug in the #197 heads? (R45TS or R45S) I'm running the R45TS in mine. Should I be running the R45S ones? What's the difference?

Thanks
Charles

[harry k]

Originally Posted by chuckies76ta

So can you run either plug in the #197 heads? (R45TS or R45S) I'm running the R45TS in mine. Should I be running the R45S ones? What's the difference?

Thanks
Charles

that was my question as well , spark plug hole threads are the same , what is the difference from 71 to 72 in order to use flat gasketed seat in 71 to tapered seat in 72 ?

["QUICK-SILVER"]

The threads are the same but the hole around the outside is made different.
Little plugs will tighten up in older heads. BUT big plugs don't work in small plug heads. There's no flat area for the washer to tighten up against. Just a V shaped hole for the small plug.

Little plugs in old heads need to be a little extra tight to seal good. The V shape for the taper isn't the same as the beveled first threads that make the plug easy to start.

Little to big but not big to little

[harry k]

thanks QUICK-SILVER
hmmm good observation and explains it
like Tom V use to say in his sig. " engineers do things for some reasons"

[chuckies76ta]

Thanks for the explanation on the spark plugs. It's just the info I was looking for too.

Didn't mean to cut in on your topic HarryK.. Here are some pics of what Quick Silver was talking about.

Thanks guys.

The left two pics are a 6X head, and next is a 197 head.. Never realized it, but there is quite a difference..

[chuckies76ta]

Found this on another site..

The R45TS means that the plug has a tapered seat with an longer extended tip. Assuming both plugs are 45 series... the threads are the same. 4 in 45 means that the plugs are 14mm and the 5 is a heat rating..(Darkshark)

[screamingchief]

I dont really know why this keeps coming up time and time again,and it seems the info always is a bit murky.

The plug change in '72 came hand in hand with the revision to the exhaust crossover and choke set-up,this is when that little "port" appeared above the exhaust crossover too.

Understand that unleaded gas was coming into full force by '73 as leaded was being phased out quickly,the cat-cons came later in '75,by then unleaded was the law for new cars.

This was the dawn of the "smog era",the folks @ GM corporate knew what was in store for them,and they were scrambling to make the necessary changes.

'73 was the first year for EGR.

Leaner mixtures and no lead fuel was going to be SOP,and they knew it at that point.

The plugs seat is part and parcel to the heat transfer interface for the the plug itself,heat is transfered thru the tip insulator to the plug body,and from the plug body to the heads cooling jacket via the threads and that seat contact area.

That situation affects the heat range a given plug will exhibit.

So by changing the seat,they also altered the heat transfer into the head,by increasing the heat transfer traits,thus they could keep the plug tip temps in check without having to drastically alter the heat range or tip design of the plugs they were previously using.

Just because two plugs share similar heat ranges,it does'nt mean they are all that comparable,earlier plugs were often standard tip plugs vs the later plugs that used extended tips,that too affected the heat transfer traits of each given plug.

If a fella goes trying to put a taper seat plug where a gasket plug lived,he's basically taking a big gamble that wont drastically affect the situation.

I would'nt do that,,,I'd just find the right plug for the seat design the head uses...

Also understand that situation can affect the plugs "reach" into the head,and that too can have certain implications too.

Try running both plugs into each type of head,watch where it puts each plug tip in relationship to the end of the threads in the head.

As for the hardened seat situation,I dont know if I totally agree with the '75 up statements.

I know my '72 7F6 H-O heads sure seemed to have hardened seats,as did the three sets of "smog" '73-'74 #46 350 heads I had a while back,all those appeared to be hardened when they were cleaned,you can see it pretty clearly when the heads are freshly cleaned,the seats have a blueish tint to them.

They knew by '70 what was coming down the road,they dropped the compression ratios in '71,then the other changes in '72,then came EGR in '73,then cat-cons in '75.

I know the text in the owners literature for the '72 GTO said the engine was designed to run no-lead fuels too.

IMO the hardened exhaust seats came earlier than '75.

Till somone can convince me otherwise,I myself feel '72 was the start of the hardened exhaust seats,that to me helps explain the change in plugs and the corresponding change in the exhaust crossovers too,all that was to help get some heat outta the chambers to protect the exhaust valves & seats as the unleaded fuel/emissions era hardware was coming into play.

Just my .

[chuckies76ta]

Originally Posted by screamingchief

I dont really know why this keeps coming up time and time again,and it seems the info always is a bit murky.
Just my .

Thanks Brett for your knowledge. The reason it probably keeps coming up it that we as a group are always learning. There is always something that gets over looked.

I assumed I had the right plugs in my #197 heads. I didn't even give it a second thought. when I read the post, the light bulbs went off.

For myself, I feel I have come along way from the days when I bought my car and had to change the leaf springs. Couldn't find any shop in the City to do them for me. I know, they were just leaf springs. Didn't think I had the knowledge to do the job.

Well, today is a different story for me, but still learning as I go. At this point all I can say is a Big Thank you to guys like you, and to the membership on this board.

No offense was taking from your statement Brett, Just an opportunity to say Thank you for all your knowledge and help over the years.

Charles

[harry k]

Originally Posted by screamingchief

I dont really know why this keeps coming up time and time again,and it seems the info always is a bit murky.

i used the search function in the whole forums sections and did not come up with anything prior posting this .

as far the specifics of my thread goes, my only concern is ,its very simple,
to the unaware persons , you can easily mix up and use vise versa plugs in a head (just like chuckies76ta did)
i was seeking why as Pontiac did this.

[screamingchief]

Originally Posted by harry k

i was seeking why as Pontiac did this.

Because they really did'nt have much of any say in the thread pitch used on spark plugs.

14mm is the industry standard spark plug thread pitch,95% of spark plug applications use that thread,been that way since spark plugs have been in use,certainly the most common spark plug thread used since OHV engines tooks over.

It's pretty hard to find a plug that does'nt use 14mm threads...

Plug "reach" is usually all that changes as far as the threads are concerned.

That is changed merely to position the plugs tip properly in the chamber,as you dont want the tip sunk way back in the threads,nor does one want the tip and/or threads hanging way out into the chamber either.

Seat design,well that's just one odd area the plug gods played around with the basic plug design,and do note they only did so on iron head applications,most all aluminum head engines will still employ the gasket style plugs as the tapered seat plugs have a common tendency to seize in the aluminum heads (due to dissimilar metals with a small current going thru it,as that usually = electrolysis),and nobody likes it when that happens.

I know this has been discussed here in the past quite a bit,and I really dont know why a search did'nt bring any of that info up,I'll hafta maybe see what I can get to come up,I do know it helps to limit what forums your searching for info in,and to choose the keyword for the search carefully,but other than that,I dunno why little to nothing came up.

There also is a ton of good info on this stuff on the plug makers websites too.

Here are a couple graphics to help explain the concepts I mentioned in my posts.
(courtesy of the NGK website tech info)

HTH.