BJ7 spark plug threads

[RestoreThemAll]

I have no wish to spark a spark plug brand war.

From my lurking around the BCF I noticed that several trusted BCF members use NGK BP6ES spark plugs so I bought some for my stock BJ7. The new plugs fit loose, even sloppy all the way to the washer seal. The head was completely rebuilt by a machinist and has yet to be run.

The Bentley book shows the spec at 14mm threads. I figured out that means the plug width. I used a set of calipers to examine the new plugs and they measure 3.78mm a difference of .22mm from spec. Being a back yard mechanic I don't have a good way to measure the spark plug hole. Any suggestions?

Is the difference of .22mm something to be concerned about? The sheetrock wall in the garage should slow the plugs down some, if they miss my head, when they blow out.layful:

The TPI on the new plugs measure 20 TPI or 1.25mm TPI. That seems to be correct.

Not that it really matters but why are the plugs specs in mm? I don't recall any other metric specs.

 

[Keoke]

That must be modern Math Dale- I do not understand it!!
But if the engine run good I guess you are good to go too--:highly_amused:

OH! My Bosh WR7DP Plugs do not wiggle around in no holes--

 

[Randy Forbes]

As long as I can remember, and certainly for every engine I've had apart__more'n 50 years, much to my dad's chagrin__they've always been Metric threads on the plugs; even when the hex was 13/16" and the reach was 3/8".

I'd guess you'd have to go back to 1860 and ask Étienne Lenoir the reason why...

https://en.wikipedia.org/wiki/Spark_plug

 

[RestoreThemAll]

So Randy, I know that you recommend the NGK BP6ES plug. Is that for all six cyl Healeys?

Is .22mm difference anything to worry about?

What an easy way for a DIY guy to measure that plug hole?

 

[Randy Forbes]

0.22mm = 0.008661417322834645" (divided by 2 for actual plug to bore clearance)

Not something I would lose any sleep over.

Starrett makes "SMALL BORE" I.D. transfer gauges, if you have your mind set on it...

 

[DerekJ]

NGK are the best plugs and are generally used by most racers. I have 6's in my performance engine as 7s were too cold. 5's also work well in standard 6 cylinder engines. Not sure what you exactly mean by 'sloppy' fitting. When its tightened up it shouldn't move. Mine wobble a bit for the first couple of threads.

 

[RestoreThemAll]

Mine wobble all the way to the washer. They do wobble less and less as they get closer to the washer. Don't move when tight.

I don't doubt the plugs now but do wonder a bit about the plug holes.

 

[drambuie]

It is possible, whom ever rebuilt the head used a wrong size tap to chase the threads.. Metric and some standard taps can be very close in thread pitch sometimes and when the wrong one is used it will cross thread and remove material leading to a very sloppy fit. One way you can check the I.D. Of the spark plug holes are to use a small telescopic gage and mic the telescopic gage with a micrometer.. Even though telescopic Gage's are not a precision measuring tool, they can get you very close if used properly. A previous owner of my old 1971 fx Shovelhead Harley Davidon superglide stripped the spark plug holes, So I machined special sleeves with a thin lip at one end, pre drilled and tapped the sleeves In The lathe, then drilled out the old spark plug holes over size. Turned the O.D.of the sleeves in a lathe for a .003 shrink fit, heated the bores of the spark plug holes with a butain torch and dropped in my new spark plug/head sleeves. After it cooled, I simply chased the pre tapped sleeve bores for a perfect fitting spark plug. I did this without taking the motor out... I just put some small rags in the top of the bore to catch all the metal shavings and used grease on the tap to catch most of the chips.. Carefully inspected the bores for any lose shavings and that was it! Worked like a charm.

 

[RestoreThemAll]

The plug moves back and forth (wobbles) about 1/16", a little less just before it hits the washer.

 

[drambuie]

As long as the spark plug tightens down enough to seat the washer for a good seal you should be fine. Just don't over torque the Spark plugs and strip the threads! A firm snug fit is enough! Using a little never seize on the spark plug threads will take some of slop away if it bothers you that much.

 

[Bob_Spidell]

As long as the spark plug tightens down enough to seat the washer for a good seal you should be fine. Just don't over torque the Spark plugs and strip the threads! A firm snug fit is enough! Using a little never seize on the spark plug threads will take some of slop away if it bothers you that much.

I'm not so sure of that. Yes, the plug should seal OK, but if was 'sloppy' going in it won't have as much 'strength'--for lack of a better word--and a plug could conceivably blow out; esp. if the engine has high compression (I blew a plug out once--not on a Healey engine--when I failed to torque it properly). If you know who did the head work, you might see if he'll 'fess up' to tapping the hole oversize, and compel him to install Helicoils or similar. But, like Randy and drambuie said you'll probably be OK.

 

[RestoreThemAll]

Bob,
The guy who did the work on the head is no longer in business. Retired and moved on.

I had a friend, who's been a boat mechanic for many years, look at it. This morning he put a plug in and torqued it. He did say its loose but felt OK when it torqued down. I may try leave it as is or put in helicoils. I'm undecided. The helicoils would buy some peace of mind. I do like a tight tolerance.

d

 

[RestoreThemAll]

Thanks for wiki link Randy. https://en.wikipedia.org/wiki/Spark_plug Interesting read.
d

 

[RestoreThemAll]

Thanks Drambuie. I like your ideas. A proper measurement of the spark plug holes would answer the question. I'd certainly do that before modifying the holes.

I'll let you guys know in the end.

Thanks guys for helping me think about this!

 

[tahoe healey]

Try another brand of plug to see how it fits. 1/16 of an inch is a lot of movement in my opinion. That amount could reduce the compression. Maybe you could run the engine and while warm , spray soapy water around the plugs to see if you get bubbles (with the engine running).
TH

 

[Patrick67BJ8]

Bob,
The guy who did the work on the head is no longer in business. Retired and moved on.

I had a friend, who's been a boat mechanic for many years, look at it. This morning he put a plug in and torqued it. He did say its loose but felt OK when it torqued down. I may try leave it as is or put in helicoils. I'm undecided. The helicoils would buy some peace of mind. I do like a tight tolerance.

d

My first car was a Corvair and I stripped a sparkplug hole in the aluminum head. A helicoil fixed it right up and made it the best sparkplug hole on the engine.

 

[Randy Forbes]

While Heli-Coils (and Time-Serts, preferred by a lot of pros: https://www.belmetric.com/14x125-de...62_1029.html?zenid=9vuucvffasju6c40d319c6cpj4 ) are the best thing for a damaged/stripped sparkplug hole, I would only use them as a last resort. Though rare, it's not unheard of to have fasteners/plugs gall inside the inserts, making for a world of hurt to rectify things.

Random Time-Sert image (verify correct depth for your application)

Are you familiar with the term "fix it until it's broke"? While I'm not the one standing there wiggling the plug, that term might apply. If it was mine, I'd run it and see what happens; might try what was suggested about trying a different brand, but I'd be surprised if there was any improvement. All my cars have a smear of Kopr-Shield on their threads (reflex action w/me) and it does give the impression of a tighter fit, whether it really does or doesn't is another matter.

 

[drambuie]

I have no wish to spark a spark plug brand war.

From my lurking around the BCF I noticed that several trusted BCF members use NGK BP6ES spark plugs so I bought some for my stock BJ7. The new plugs fit loose, even sloppy all the way to the washer seal. The head was completely rebuilt by a machinist and has yet to be run.

The Bentley book shows the spec at 14mm threads. I figured out that means the plug width. I used a set of calipers to examine the new plugs and they measure 3.78mm a difference of .22mm from spec. Being a back yard mechanic I don't have a good way to measure the spark plug hole. Any suggestions?

Is the difference of .22mm something to be concerned about? The sheetrock wall in the garage should slow the plugs down some, if they miss my head, when they blow out.layful:

The TPI on the new plugs measure 20 TPI or 1.25mm TPI. That seems to be correct.

Not that it really matters but why are the plugs specs in mm? I don't recall any other metric specs.

I just went out in the garage and measured the I.D. of my spark plug hole on my 1967 BJ8 (factory built motor never rebuilt) For your information the I.D of the spark plug bore is 0.500 (1/2 inch) maasuring the crown of the threads. The champion spark plug major O.D. Measures 0.545 which means the useable thread depth is 0.0225 per side. The thread pitch is 14MMX1.25 Now that you know the factory dimensions you can compare your plug bores to mine.

The first photo shows a sleeve blank I made similar to the one I used to repair the my Harley Davidson spark plug hole, except I machined it without a hex and a thinner step. Be sure to use the same material as the head so the new sleeve expands and contracts at the same rate. The threads are pre bored and tapped in the new sleeve before shrinking it in the head, then chased with the 14mmx1.25 tap after the new sleeves are sweated in and let to cool. A .002 to .003 shrink fit maximum. Length of sleeve should be a little shorter then the existing spark plug hole bore In the head.

I sincerely hope you never get to this point, but if you do...the head can saved and repaired for a permanent fix. I have seen many heli coils fall out! Naturally your head will have to be removed for this repair unlike my Harley Davidson which gives a full access for repair with the head on.