Spark Plug type 70 LT-1

[Wolf S.]

hi everyone,

what type of spark plug, tapered or flat with washer, was used on the 1970 LT-1 engine?

tapered plugs came in 5/8" hex while the ones with washers came in 13/16" hex.

my 350cu" has an assembly date of april 9th and the left cylinderhead is dated march 31st.

the engine came with champion rv17yc tapered plugs. probably not the correct plug but engine runs ok (plugs are brownish).

owner's manual states ac delco r43 as original equipement and the r44 for city driving which are plugs with washers.

thanks

[Jim T.]

Re: Spark Plug type 70 LT-1

Wolf I have used J12Y Champions in my original owner 1970 350/300HP since it's first plug change. Also use the J12Y in my 1968 327/350HP.

[Duke W.]

Re: Spark Plug type 70 LT-1

Most of us who have owned vintage Corvettes for a long time and been through tons of fouled plugs will tell you that the best heat range for normal driving is AC "5" or equivalent in another brand for all vintage engines regardless of power rating.

I found that heat range "3" was good for race track hot lapping, but fouled quickly in normal road driving. Heat range "4" was better, but would still foul. Heat range "5" is just right, and they won't overheat unless you drive like a maniac all the time or use your Corvette to haul a 6000 pound trailer up Loveland Pass every day.

R45s are an off the shelf plug that should be available at most parts houses.

All '70 small block heads are machined for gasket seat plugs (taper seat small blocks began in '71), so maybe you have the wrong type installed or maybe the heads aren't OE. It's easy to determine the type of plug - gasket or taper seat - by inspecting one of the spark plug ports with the plug removed.

Duke

[Dennis D.]

Re: Spark Plug type 70 LT-1

Most of us who have owned vintage Corvettes for a long time and been through tons of fouled plugs will tell you that the best heat range for normal driving is AC "5" or equivalent in another brand for all vintage engines regardless of power rating.

I found that heat range "3" was good for race track hot lapping, but fouled quickly in normal road driving. Heat range "4" was better, but would still foul. Heat range "5" is just right, and they won't overheat unless you drive like a maniac all the time or use your Corvette to haul a 6000 pound trailer up Loveland Pass every day.

R45s are an off the shelf plug that should be available at most parts houses.


Duke

Couldn't agree more(not that it's needed). I ran R-44 in my 70 L-46 and fouled out one set after the other. Sure that blocking the crossover and eliminating the heat riser was mostly to blame. But after trying 43 with worse results, finally went to the 45 and haven't had a problem since.

[Wolf S.]

Re: Spark Plug type 70 LT-1

thanks for the response
i'm wondering if you can fit tapered seat plugs into plug ports for gasket seated plugs?
maybe the po put in tapered plugs instead of the correct gasket seated ones?
i'll check the spark plug ports and recheck the cylinderhead casting dates.

[Duke W.]

Re: Spark Plug type 70 LT-1

thanks for the response
i'm wondering if you can fit tapered seat plugs into plug ports for gasket seated plugs?

If you have found a way to pound a square peg into a round hole, perhaps it will work.

Come on! AC plugs cost about two bucks each. Look at the seats and fit the proper plug. Otherwise, we're going to start calling you Bubba.

Duke

[Duke W.]

Re: Spark Plug type 70 LT-1

Couldn't agree more(not that it's needed). I ran R-44 in my 70 L-46 and fouled out one set after the other. Sure that blocking the crossover and eliminating the heat riser was mostly to blame. But after trying 43 with worse results, finally went to the 45 and haven't had a problem since.

Blocking the heat riser has nothing to do with fouling plugs. The problem is that Corvettes, even base engine models, have a lot of power for the load of the car and run at light to moderate load 99.9 percent of the time, which is not enough to keep plug tip temperature above the fouling range with colder than AC heat range "5" in any kind of normal road driving, including 80 MPH cruising on interstate highways.

Most of use who have been driving these cars since new found this out a long time ago, and the fouling problem was a lot worse with leaded fuel. Chevrolet's OE heat range recommendations are way too conservative. There are many threads on the subject.

Duke

[Joe L.]

Re: Spark Plug type 70 LT-1

thanks for the response
i'm wondering if you can fit tapered seat plugs into plug ports for gasket seated plugs?
maybe the po put in tapered plugs instead of the correct gasket seated ones?
i'll check the spark plug ports and recheck the cylinderhead casting dates.

Wolf-----


Based on what I can see of the size of the counterbore for the plugs, I'd say these heads are machined for the tapered seat plugs. However, do remove one and carefully examine the machined spark plug seat.

Are you sure these are the engine's original heads? Have you checked for casting number and casting date? All that's needed to do this is to remove the valve covers.

[Wolf S.]

Re: Spark Plug type 70 LT-1

hi,
well, today i first checked the driver side cylinderhead casting and date.
which comes to March 31, 1970, so i do have the correct head -- at least the driver's side.



the passenger side rocker cover i couldn't get off. i didn't have the right tools with me. there some sort of rubber gasket that melted with the engine heat not the normally cork one.

i also pulled one of the spark plugs (champion rv17yc tapered) to see what kind of plug port my head has. can't really see what i should be looking for in a tapered plug port or a gasket plug port. i did notice that a
small area of the tapered port of the port was shiny bare metal
btw, i was able to screw in a gasket plug (without its gasket) that i have and it seated.

[Duke W.]

Re: Spark Plug type 70 LT-1

It appears that head was originally machined for tapered seat plugs, but the large diameter spark plug port could also be machined for gasket seat plugs.

So the correct plug would be AC R45T or equivalent in another brand, and the Champion RV17YC is okay. It's roughly equivalent in heat range to AC 5 to 6, but has an extended tip so it's equivalent to the R45TS or R46TS.

This is an anamolous situation because '70 LT-1s with 186 heads "should" be machined for gasket seat plugs.

Both my Corvette P&A catalog and old AC Spark Plug catalog call out a conventional gasket seat spark plugs as does your owner's manual. The first use of taper seat plugs on small blocks is "supposed" to be 1971.

Maybe Terry M or some of the other LT-1 guys can sort this out.

Duke

[Terry M.]

Re: Spark Plug type 70 LT-1

It appears that head was originally machined for tapered seat plugs, but the large diameter spark plug port could also be machined for gasket seat plugs.

So the correct plug would be AC R45T or equivalent in another brand, and the Champion RV17YC is okay. It's roughly equivalent in heat range to AC 5 to 6, but has an extended tip so it's equivalent to the R45TS or R46TS.

This is an anamolous situation because '70 LT-1s with 186 heads "should" be machined for gasket seat plugs.

Both my Corvette P&A catalog and old AC Spark Plug catalog call out a conventional gasket seat spark plugs as does your owner's manual. The first use of taper seat plugs on small blocks is "supposed" to be 1971.

Maybe Terry M or some of the other LT-1 guys can sort this out.

Duke

Duke,

I believe you are correct: 1970 LT1 used gasket seat spark plugs 13/16-inch hex, and 1971 and 1972 LT1 use tapered seat 7/8-inch hex plugs. There was a different head casting number from 1970 to 1971, but that may not have had anything to do with the spark plug machining operation. We know it for sure had to do with combustion chamber size.

[Joe L.]

Re: Spark Plug type 70 LT-1

hi,
well, today i first checked the driver side cylinderhead casting and date.
which comes to March 31, 1970, so i do have the correct head -- at least the driver's side.



the passenger side rocker cover i couldn't get off. i didn't have the right tools with me. there some sort of rubber gasket that melted with the engine heat not the normally cork one.

i also pulled one of the spark plugs (champion rv17yc tapered) to see what kind of plug port my head has. can't really see what i should be looking for in a tapered plug port or a gasket plug port. i did notice that a
small area of the tapered port of the port was shiny bare metal
btw, i was able to screw in a gasket plug (without its gasket) that i have and it seated.

Wolf------


These heads are designed and machined for GASKETED spark plugs. Someone apparently previously installed tapered seat plugs but these are not correct for these heads. They will "work" due to the normal thread lead-in chamfer, but they're not correct, functionally or otherwise.

The left 2 photos below show a KNOWN tapered seat machining in a GM #3991492 cylinder head. The spark plug counterbore is about 0.675".

The photo on the right shows a KNOWN gasketed seat spark plug machining on a GM #3927186 cylinder head. Note that there is a thread lead-in chamfer just like yours but this does not represent a seat for a tapered seat spark plug. The spark plug counterbore on this head is about 0.885" which should be about the same as yours.

So, when you reinstall spark plugs, use the gasketed type plugs. Also, make sure you fully clean the seats at the base of the counterbore of dirt and grease. As a result of the installation of the tapered seat plugs this area has been exposed to such and obviously needs thorough cleaning.

[Duke W.]

Re: Spark Plug type 70 LT-1

When I first looked at Wolf's pictures, the head casting was clearly designed for gasket seat plugs, but the gasket seat appeared to have not been machined!!!

On further inspection, what I think I am seeing is debris buildup on the machined gasket seat surface due to the use of the small diameter taper seat plug.

I believe once this area is cleaned of debris it will be clear that there is a machined gasket seat.

Recommend you loosely install the "peanut plugs" and use solvent and a toothbrush, toothpicks,... whatever will work to clean out the debris and keep it from falling onto the threads and getting into the cylinders.

It all seems perfectly clear, now - Bubba was there. I should have known!

The best plug to use is the AC R45 or equivalent in another brand.

Duke

[Wolf S.]

Re: Spark Plug type 70 LT-1

thanks a lot for all the input.

so some po put in tapered plugs! as you say bubba was there! in fact he was all over the vette!

i, never, in 7 years of use -- having covered only 1600miles -- checked the spark plugs until this year. the vette always ran well... still runs well... as the proverb says.....

i'll definitely clean the plug port surroundings before putting in ac r45 gasketed spark plugs. i'll post some pictures of the cleaned ports.




ps would extended tip plugs be better (with 's' suffix)?

[Duke W.]

Re: Spark Plug type 70 LT-1

There's no real definite answer. Extended tip plugs can be more resistant to fouling, but if you use AC heat range 5 or equivalent in a conventional non-extended tip, they should not foul unless all you do is idle along in parades.

Way back when guys would use them because they reduce combustion chamber volume by about a cc for higher compression, but that's not generally a good idea today because available octane numbers are a point or two less than the sixties.

It's my understanding that the highest octane available in France is 98 RON, which is equivalent to US 93-94 PON. Back in the sixties, typical US premiums were 98-100 RON.

Duke