Roush 427IR spark plug hole stripped

[169mph]

I pulled the spark plugs to inspect and replace last month. I had hell removing four of the plugs. All the plugs were over tightened in my opinion. I had to use a thread chaser on three of the plug holes. The first two went smoothly. I was very careful and I used a borescope to make sure no metal shavings fell into the cylinders / pistons. On third hole, which is the #1 cylinder, I cannot get the thread chaser or a plug to thread in. The threads look pretty bad. Should I pull the head or is there another option? Also, I saw carbon built up on all the pistons but Im not sure if it’s normal or not. The piston below was the worse.

[KarlzEE Bebout]

You can use a steptap without pulling the head. A good glob of wheel bearing grease on the starting end of the tap and also in the flutes of it should catch probably 99% of the aluminum flakes and what ever is left will quickly burn in the combustion. My 2¢

[SBSerpent]

Wow. What could have caused this?

[eschaider]

Are the heads aluminum? What reach spark plug do you use?

[incoming]

i'm thinking the plug is too long and the buildup caused your problems. I always use anti-seize and a torque wrench anytime I'm around anything aluminum. Make sure to research the correct plug thread length and torque yourself.

I once asked a boat wrench what my head bolt torque value and the information was incorrect. I cracked the head on the first nut. I will never make that mistake again.

[169mph]

Yes, they’re aluminum heads and running Autolite 605 plugs. These are the plugs Roush said I need for my specific setup. Thanks, Mike

[eschaider]

Your plugs are 0.705" reach according to Autolite. This is good for an aluminum head. You will need a spark plug repair kit to restore good useable threads. The best thread repair system I have found over the years is the Lock-N-Stitch system. Here is a link to their website => Lock-N-Stitch

This is the Ford recommended vendor for all the four-thread plug hole failures they have had on their three and four-valve Mod motor heads — until they switched to a 9 thread head casting. An interesting side note about the Lock-N-Stitch folks is they were the firm that was selected to repair the cracking cast-iron dome on the US Capitol building because they could do it without welding.

I have used them on a pricey set of ported 4V Modular heads and couldn't be happier. I didn't fully realize how robust the repair was until I read their torque specs for the repaired thread. Their instructions call for a 35 ft/lb torque spec for the repaired plug.

Notwithstanding their repair instructions, I do not use anywhere near that torque level on mine. I spin the plug down until it bottoms in the head and give it a light ⅛ of an additional turn to snug them up. That is all that's required. The plugs never loosen in service and come out easily when I change them. If you want an extra measure of removal safety, you can put a very light coat of anti-seize on the plug threads.

This repair process will save your heads, end the potential for a repeat performance and put a smile on your face.

Pretty amazing guys and stunningly good product.


p.s. The install kit you will want is part # FFT514125LK, which includes five inserts. You will also need another five inserts part # FFT514125L-5 to do all eight plug holes — highly recommended. The kit has the drill, reamer, and install tools yada, yada that you will need to do the job. The equipment and pieces are not inexpensive, but there is none better. You will spend $500 plus shipping and tax, but your plug threads will be better than OEM, and your plugs will show the correct heat signature when you read them. Call their number at the top of the webpage. They are not only very helpful but you can only buy the repair kits directly from them.

[patrickt]

FWIW, I have to use a torque wrench on anything that calls for less than 40 ft/lbs. My aluminum heads clearly state 12 ft/lbs with a dab of anti-seize. To me, that feels "finger tight." If I didn't have an array of torque wrenches, click and pointer, I'd strip every aluminum thread I have on my cars.

[eschaider]

Originally Posted by patrickt

FWIW, I have to use a torque wrench on anything that calls for less than 40 ft/lbs. My aluminum heads clearly state 12 ft/lbs with a dab of anti-seize. To me, that feels "finger tight." If I didn't have an array of torque wrenches, click and pointer, I'd strip every aluminum thread I have on my cars.

I am familiar with the experience Patrick. It's not as if I haven't wrecked my share. It's just over time I've turned down the volume on my torquing enthusiasm — and to my surprise and elation it seems to have worked out well.

These inserts are stunningly strong, hard anodized aluminum and what I liked most was their ability to properly image combustion heat on the plugs

[cbreez]

what would you have done if your scope indicated metal inside cylinder??? Oh my...

[eschaider]

Originally Posted by cbreez

what would you have done if your scope indicated metal inside cylinder??? Oh my...

Start the engine after the thread repair and blow it out the exhaust.

[patrickt]

Originally Posted by eschaider

Start the engine after the thread repair and blow it out the exhaust.

Maybe vacuum out the cylinder with surgical tubing duct taped to your vacuum cleaner hose. That'll at least get some of the big chunks of crap before you blow them out the exhaust. I think even RoadKill would give a quick vac... maybe.

[Kojack70]

I've done exactly this on a motorcycle.

Originally Posted by patrickt

Maybe vacuum out the cylinder with surgical tubing duct taped to your vacuum cleaner hose. That'll at least get some of the big chunks of crap before you blow them out the exhaust. I think even RoadKill would give a quick vac... maybe.