I have a 68 Cutlass S with a 4.1L inline 6 cylinder .Most of the spark plugs have a 13/16 hex and I need a 5/8 to fit the socket around the plug to get into the block. Does anyone know what plugs fit this engine?
Thanks
Glenn

 

Champion plug # is 405 ngk # is 6578 these plugs should be 13/16 I ment to say sorry

 

AC R46N is what is called for or equivalent or the same part number you pulled out.

 

Are the engine and the cylinder head definitely both original? Is the seat/seal area in the cylinder head flat and would use a gasket or is it tapered?

 

I checked on these and they say they're still 13/16 hex. Guess I'm just going to have to take the plugs I have to a store. Thanks for the info!
.

 

Are you sure that engine hasn't been replaced with a later Chevy six?

 

What plug brand and # is in it. They may or may not be correct. Is the head tapered or flat seat. Seen the wrong plug used too often.

 

I believe it is a Chevy 250.

 

The plugs I took out are AC Delco R45TSX but everywhere I look it says these are not for this engine.

 

Here's a few pics of the plug hole an engine side shot.

 

I'm working off of a phone at the moment but TSX is tapered seat. That head looks flat and would use a ring washer seal plug not a tapered plug. Somebody with better image access please jump in.

 

I don't understand why you need a 5/8" to fit the socket around the plug. I have changed plugs on second generation Chevy L-6's dozens of times with nothing more than a straight (non-ratcheting) 13/16" plug wrench on a 13/16" plug. 5/8" shouldn't even enter into the procedure.

 

I'm glad it worked for you but a 13/16 socket does not fit in the hole. Only a 5/8 deep socket will surround the hex in the hole. I'm not a mechanic but I know when something doesn't fit. I just came on here for some advice.

 

Looks flat from my armchair.

 

AC R45XLS or NGK 6578 should do it.

 

Around 1970 or 1971 Chevrolet switched from gasketed spark plugs to tapered seat spark plugs. That's why I asked if this is the original engine. If a 1968 car requires tapered seat plugs either the cylinder head or the complete engine has been replaced.

Give us a picture of the spark plugs currently in the engine, and a good close-up picture of the spark plug hole.

 

Are the tapered seat plugs "peanut" plugs (5/8") or 13/16"?

 

Are you making progress with this? The problem is it had the wrong spark plugs in it, seen it before. It had tapered seat plugs by the part number provided. It uses non-tapered plugs so the plug # you took out will not work. Try ACR45XLS.

 

Lets see some pictures of the car and a few standing back showing the engine bay with the econo 6. We don't see these often.
Is it a 3 on the tree trans?

 

dr, Isn't it "tree on da' tree"?

 

Here's some close ups of the plug hole, the plug and the engine bay.

 

Its a 2 speed automatic.

 

It must be a shock to most people when they open the hood, 6 bangers lives matter!

 

Most people want to know when I'm going to replace the engine with an 8 ;-o .This one is one of 410 built with an inline 6, convertible. I like that its different.

 

Its a 2 speed automatic.

 

Not certain what the deal with the plugs is, but that sure is a nice car!

 

Cool car. Bet not too many 250 Six 2 speed verts existed in 68 let alone today. The stainless steel wheel well trim would really set this car off.

To me that plug hole looks like its designed for a 13/16" hex gasketed plug not a 5/8" hex tapered plug.
Though it looks like a slight starter taper/chamfer exists as it goes down into the hole. Chamfering is a common machining practice. If it wasn't for that chamfer the tapered R45s would not have worked at all. Doubt that starter chamfer is the correct angle for a tapered plug.

You have to be concerned about the plug depth. Chevy commonly used 3/8" reach gasketed plugs in 68. 3/4" reach was for aluminum heads.
Tapered plugs in SBC started around the 71 model year.

Doing a little searching on various 250 web sites would agree with all of this. Id surf around and post the question or three if you need further validation, as I'm not a 250 Six expert.
AC R44XLS,NGK BPR4ES, etc are all gasketed plugs. A quick gander at Summit, plugging in the 250 Six resulted in gasketed plugs.

https://www.summitracing.com/search/...ine-6-cylinder

 

By your picture, the recess in the head does not look deep enough to interfere with access to the hex on a gasketed plug.

Yes, it would prevent a 13/16" socket from bottoming, but that may not be required to engage the hex on the plug.

Buy or borrow a single 13/16" gasketed plug to see if this is correct.

BTW, beautiful car!

 

Yes, I believe it should have a gasketed plug. Its hard to see in the picture but it is flat so the gasket makes sense. The AC R45XLS looks like it might do the trick. Its always run a little rough and doesn't smoke at all so maybe the tapered plug that was in there made it run a little off. With new plugs and a timing reset, I'm hoping it will run smoother, though it doesn't run poorly at the moment. I may replace the plug wires too depending on how it runs. Couldn't hurt.

 

Confirm that the thread depth of the new plug is ok/correct. The R45 XLS is 3/4" reach, I just don't remember the 3/4" reach being correct but it has been decades since I've worked on one of them.

Three areas to check on that engine for idle problems:
Carbs get loose between the fuel bowl and the throttle plate,
Intake manifolds leak vacuum at the outer runners, and
distributor wear causing dwell/timing fluctuations. Move the dist shaft side to side to check and look for fluctuating vs rock steady base timing with a timing light.

Your engine bay looks great!

 

Thank you for the compliment! I will try and find out the depth. If it was too long, would that cause any issues in the firing or maybe just burn less efficiently?

 

It could potentially hit a piston or valve. Look at how deep the threads are in the head before installing. The potential problem could occur if the head is after '73? or so, I'm not certain on the year the design changed. This engine is rare in Olds world and common in C___Y world. They were very durable engines.

If you search on Chevrolet six cylinder plug reach you'll find a ton of info about this at Steve's Nova Site.

 

If your head is built for a 1/2" reach plug, and you install a 3/4" reach plug, there would be no firing issues, but there could be 2 problems...

• The threads that are inside the combustion chamber may heat up and cause preignition
• The longer reach may hit the piston or valve

With the reverse situation...

• The spark would be shrouded by the longer threads in the head, which could cause misfiring

The 3/4 inch reach plugs are usually used in aluminum heads. I think you will find that your threads are about 1/2".

 

What is the cylinder head casting #? From that we should be able to figure out the year range and spark plug reach for sure if you'd like.

 

I will try and find that for you.

 

Here are two pics of the rear passenger side block and the passenger side head.

 

Block is a 64-76 230 or 250 CID Chevy block. The head casting # is different than supplied, it should be raised and all numeric characters similar in format to the block.

 

3892858???

 

Here are 2 pics on the right driver side headers . The small tag picture on the front passenger side of the valve cover gasket and is stamped K 725 but faintly behind it ( you cant see in the picture) are the code E then O or a zero,915TA. helpful? Thanks again for all the help. I do appreciate your time!

 

3923418 is the intake manifold and it is the correct casting number.

We need the cylinder head casting number to figure this out. It will be raised not indented/stamped and I'm guessing it is 3892858.