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    Haibeck Automotive Technology



Spark Plugs for the LT5

I recommend the Denso IT20TT as the best spark plug that is available for the LT5 engine. It has an iridium fine wire center electrode and has a platinum insert welded to the negative electrode. It has a tapered seat. All LT5 cylinder heads are machined for tapered and flat (gasketed) seats. Carefully gap the spark plug to .045", or keep the factory .040" gap. Be careful not touch the center wire or the platinum insert. Pressure on these parts could crack the welds and damage the spark plug. We use anti-seize on the threads. We apply silicone dielectric grease inside the spark plug wire boot. The IT20TT is also known as Denso part number 4714. The heat range, Japan specification #20, is the same as what was used when the engines were manufactured. This plug can be run for 30k miles.

History of GM spark plug use on the LT5 engine.

From '90 to '92 ACDelco FR2LS was installed at the factory. This is simple copper spark plug. They were gapped to .035". They have flat seats and gaskets. The heat range is ACDelco #2. This spark plug works great. In about 2000 it was discontinued and replaced with ACDelco 41-602. It also works great. Also discontinued.

In '93 and until the end of LT5 production the ACDelco 41-907 double platinum tapered seat plug was used. They were gapped to .050". These plugs have a problem with the platinum insert detaching from the negative electrode. Then the gap steps open to about .060". In about 1998 they were discontinued and replaced with 41-913. It is the same design as the 41-907 and they retain the platinum insert on the negative electrode better. However the insert still falls off of some of them. The heat range is ACDelco #2. They were discontinued in about 2022.

A very good low cost spark plug for the LT5.

The Denso K20PRU is a simple nickel plug similar to the '90 - '92 production FR2LS. Gap them to .035". It has a flat gasketed seat. They work very well. They are also known as part number 3145. This plug can be run for 10k miles.

Why the IT20TT is the best spark plug for the LT5.

 The LT5 coils are what are known as wasted spark coils. They spark two plugs at the same time. One of the plugs is on the exhaust cycle so the spark is "wasted". One of the posts on the coil has a positive electrical polarity. The other is negative. Therefore on half of the spark plugs the spark grows from the negative electrode or strap. The platinum insert provides a pointy surface which is best for developing an electric field. It also wears much longer than a plain nickel strap.