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Old 02-04-2013, 09:53 PM
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400 SBC Build - Spark Plug Reach

I am in process of building a 400 CID SBC for installation in a Jeep Wrangler. Long story about how I got here but not the point of this post. From what I have been told the engine is a 1970 vintage.

I had the heads (original cast iron) machined to install hardened exhaust seats, etc. and installed new stainless steel valves. To check valve sealing I installed spark plugs and noticed that the plug threads extend into the combustion chamber which doesn't seem correct.

The heads appear to be machined for tapered seat plugs (original 1970?) and the plugs I installed are AC CR43TS spec. From what I can determine on the AC plug specs these are 7/16 reach plugs and the shortest reach that they make with the tapered seat.

Any ideas what may be going on here? Unfortunately I am not going to be home where the heads are until the weekend so questions about the casting numbers or measuring the plugs, or confirming that the heads are suppose to use tapered seat plugs will need to wait.

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Old 02-04-2013, 11:53 PM
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Quote:
Originally Posted by WICruiser View Post
I am in process of building a 400 CID SBC for installation in a Jeep Wrangler. Long story about how I got here but not the point of this post. From what I have been told the engine is a 1970 vintage.

I had the heads (original cast iron) machined to install hardened exhaust seats, etc. and installed new stainless steel valves. To check valve sealing I installed spark plugs and noticed that the plug threads extend into the combustion chamber which doesn't seem correct.

The heads appear to be machined for tapered seat plugs (original 1970?) and the plugs I installed are AC CR43TS spec. From what I can determine on the AC plug specs these are 7/16 reach plugs and the shortest reach that they make with the tapered seat.

Any ideas what may be going on here? Unfortunately I am not going to be home where the heads are until the weekend so questions about the casting numbers or measuring the plugs, or confirming that the heads are suppose to use tapered seat plugs will need to wait.
How far do the threads extend into the combustion chamber you think?
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Old 02-05-2013, 06:38 AM
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I can see 2 threads of the plug inside the chamber.
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Old 02-05-2013, 08:57 AM
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Are you sure these heads are machined for taper seat plugs? Casting no? Date code?
Sounds like someone put pre 70 heads on the engine.
I see the problem all the time. Kids bring in old Camel back heads with incorrect taper seat plugs installed in them and this leaves about two threads exposed..
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Old 02-05-2013, 10:43 AM
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I can see 2 threads of the plug inside the chamber.
Two threads wouldn't scare me but I bet if u try and shop around, u may find a plug or method that will bring those two threads back out a little to where you want them...I know having some threads sticking out could potentially heat up and cause run, detonation, but both are obvious if they occur
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Old 02-05-2013, 10:55 AM
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400 SBC Build - Spark Plug Reach

If the heads are 1970 or older they use a R44 gasketed plug. After 1970 GM went to the tapered plug R44T. AC no longer makes a R44 plug so you have to decide on a colder plug or hotter plug. Still it will be a R43 or R45 part number. The plug you purchased CR43TS is a commercial grade plug. C= commercial, R= resistor, 4= 14mm thread, 3= heat range, T= tapered, S= extended tip. If it is a 9.1 compression ratio engine I would use the R45.
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Old 02-05-2013, 05:49 PM
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The heads are casting number 3951598 and the have a double mountain peak ID mark on each end (not a camel hump shape).

After looking at the pictures my wife took for me today I am fairly sure that the spark plug holes are not tapered seat. They do have a chamfer leading into the thread but they also have what appears to be a farily large flat surface that I suspect are actually the gasket seat. I should know better than to trust the auto parts counter guy who took a quick look when I picked the heads up from their shop and proclaimed that they were tapered seats.

They are rumored to be 1970 vintage (same as the engine) and it sounds like that is the last year GM used the gasketed plug before changing to tapered seats so that makes sense.

Based on measurements and calculations my static CR is in the 9.85 range so I will probably start with a colder plug and adjust if necessary.

Thanks everyone for your interest and input.
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Old 02-07-2013, 12:25 AM
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400 SBC Build - Spark Plug Reach

Quote:
Originally Posted by WICruiser View Post
The heads are casting number 3951598 and the have a double mountain peak ID mark on each end (not a camel hump shape).

After looking at the pictures my wife took for me today I am fairly sure that the spark plug holes are not tapered seat. They do have a chamfer leading into the thread but they also have what appears to be a farily large flat surface that I suspect are actually the gasket seat. I should know better than to trust the auto parts counter guy who took a quick look when I picked the heads up from their shop and proclaimed that they were tapered seats.

They are rumored to be 1970 vintage (same as the engine) and it sounds like that is the last year GM used the gasketed plug before changing to tapered seats so that makes sense.

Based on measurements and calculations my static CR is in the 9.85 range so I will probably start with a colder plug and adjust if necessary.

Thanks everyone for your interest and input.
Lower compression engines like 9.0-10.0 would benefit with a hotter plug. Usually colder plugs are used once the compression ratio gets up to 11.0 and higher.
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Old 02-11-2013, 09:03 PM
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Confirmed that the heads are machined for gasket plugs with 3/8" reach. Installed one plug to confirm and the threads on the plug are flush to just slightly exposed in the chamber.

Thanks for the insight - closing this thread out.
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