• When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.

Spark plug reach aluminum head big block

68 Sport Satellite

FBBO Gold Member
FBBO Gold Member
Local time
11:57 PM
Joined
Feb 10, 2010
Messages
2,255
Reaction score
1,473
Location
San Jose, CA
i was originally running iron head 906’s on my street driven flat top piston 451 B motor and used the short reach NGK big block XR5 plugs (.375” reach with projected tip). When fighting a pinging issue last summer I had switched to a colder NGK plug with the same reach (-8 since NGK Heat range is higher # equals colder, with .375” reach). I have since learned that driving around town 99% of the time with a lot of non-highway miles with this colder plug and my 10.2:1 compression motor in California 91 octane high ethanol summer blend can load up the plugs so I ordered and received some NGK’s in the -7 heat range (NGK 5671-7). These are non-projected flat tip but I was suprised to see that these have a much longer .75” reach. Reading up on this it seems that the longer reach .75” plug is what is needed for the aluminum headed big block just like an iron headed small block plug and all along I have been using the short reach plug as I had for my big block iron heads.

What problems may I expect if I continued to run the shorter reach plug with the aluminum head and by swapping over to the longer reach non projected plugs for 2500 rpm street driving can I expect to gain anything (cooler running, power difference, less chance of pinging, etc)?

After swapping plugs do I need to do any other engine tuning after the install if my tune was perfect with the short reach plugs?

541895C7-534B-4DD8-A737-3DAE1A1C4FE0.jpeg
 
So you now have aluminum heads?
 
1 problem you may have using a short plug on an alum head , is stripping the threads in the plug hole. Alum, being the softer metal , needs longer threads to hold the plug. Plus the short reach will not allow the charge to fire well, since the spark will be down in a small hole. Use the long reach plug. It won't foul or load up as easily.
 
using a 3/8" reach plug in a head that is designed for 3/4" reach plugs will really screw up any flame propagation. use the correct plug for the head.
 
It is hard to figure out from your post exactly what is going on but if you now have aluminum heads on your engine then you need to read the instructions and use the correct spark plugs. Using the wrong spark plugs will eventually cause a big problem.
 
Sorry for the confusion. Yes I have Edelbrock rpm aluminum heads now and didn’t really realize I needed to install plugs that were head specific. I only knew big block plugs needed. I will install the long reach plugs. I hope no damage has occurred by running the short reach plugs for a few thousand miles.
 
the short plugs won't break anything but performance will be dismal. the 3/4" reach plugs in the picture don't have a projected tip. if you have flat top pistons use a projected tip. I don't get lost in who's name is on the plug. I look for proper heat range and where the tip is at in the chamber. the more centrally the tip can be located, the better the burn. I use autolite 3924's but i'm sure an ngk plug will run just as well.
 
The racing plug is not worth the extra $$,I saw no difference in performance.If you install the projected tip,roll the engine over a few revolutions and make sure the the strap does not contact the piston.If it doesn't hit cold it won't hot at op temp.
 
I use a NGK 6987 for my street cars with aluminum heads. It is an economical OEM spark plug that works with pump gas engines.

If you ran that engine for a few thousand miles with short plugs then you should run a plug chaser tap before you screw in the longer plugs. You might have carbon built up on the threads. You should notice a performance improvement with the correct spark plugs since before you were starting the flame down inside a hole. I wouldn't be surprised to find out that you were misfiring on a regular basis since the spark plug was so shrouded.
 
Thanks to all of you for your valuable feedback. I can’t believe I made such a simple mistake. You guys are the best:)
 
Run a thread chaser to clean threads so you protect what you have
as one piece of crap can cause the plug to grab a thread and tear them out
 
Update: I have to admit I had a major moment of CRS (can’t rememember shi*) when I first posted this. When I originally installed the aluminum heads it looks like I did install the correct 3/4” reach plugs. I pulled these NGK BCPR6ES yesterday (heat range 6, one colder than the XR5’).

2352FA75-7E9A-42EF-9518-2EA21871A15A.png EB3DDAE3-16CB-4FDC-950D-83326EB5B1AB.png
 
I have now installed the heat range 7 non-projected tip NGK 5671’s based on the NGK chart recommendation for my 10:1 compression and will see how they do.
 
I wonder what the difference between the 6987 and the BCPR6ES are? Both 3/4” reach, both projected tip, both resistor, both heat range 6...
I use a NGK 6987 for my street cars with aluminum heads. It is an economical OEM spark plug that works with pump gas engines.

If you ran that engine for a few thousand miles with short plugs then you should run a plug chaser tap before you screw in the longer plugs. You might have carbon built up on the threads. You should notice a performance improvement with the correct spark plugs since before you were starting the flame down inside a hole. I wouldn't be surprised to find out that you were misfiring on a regular basis since the spark plug was so shrouded.
 
I wonder what the difference between the 6987 and the BCPR6ES are? Both 3/4” reach, both projected tip, both resistor, both heat range 6...

Standard Versus V-Cut Electrode
.035 Versus .043 Gap

When I called NGK on my 10.5-1 Compression E Street Aluminum Head

They recommended NGK BKR6E 6962

V-Cut with only .031 Gap

To many choices
 
for street use I gap all my plugs at .035". I know we can get by with larger gaps, but why not just change plugs a little more often?
Spark plug gap is head and compression specific not plug specific, correct?
 
Auto Transport Service
Back
Top