Bosch WR 8 AP spark plug issue

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rjk40
750 GT
Posts: 183
Joined: Thu Dec 05, 2013 10:54 am
Location: Malabar, Florida

Bosch WR 8 AP spark plug issue

Post by rjk40 »

I've discovered an interesting dilemma, and would be interested in hearing from anyone who might have
experienced the same thing.

My 1975 860GT kick start only Ducati , engine 851539, came factory equipped with champion L88A spark plugs.
The diameter of the threaded portion of the Champion L88A is 0.517"

I purchased Bosch WR 8 AP plugs from Steve.

The diameter of the threaded portion of the Bosch WR 8 AP is 0.523" ,
fully 6 thousandths larger in diameter than the L88A.

The extra size makes the Bosch plugs not want to go in very far.

I have a new set of L88As that I bought years ago.
They, as well as the originals, fit just fine.

Anybody else seen this phenomon? It makes no sense to try to force the Bosch plugs in;
six thousandths (.006") is a significant difference.

Thanks in advance.
Richard in Florida
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81mhr900ss
Parallel Twin
Posts: 144
Joined: Tue May 31, 2016 3:49 pm

Re: Bosch WR 8 AP spark plug issue

Post by 81mhr900ss »

Odd. I just measured my spare set of plugs and the threaded section at it's widest is 0.5417" and I know these plugs fit as I tested them. 81 MHR.
rjk40
750 GT
Posts: 183
Joined: Thu Dec 05, 2013 10:54 am
Location: Malabar, Florida

Re: Bosch WR 8 AP spark plug issue

Post by rjk40 »

Yeah, it is odd.

When I had the heads off for servicing, I cleaned the threads in the plug holes, and put the old plugs in before cleaning the holes
(something I learned on my model airplane engines), and there was no problem with the threads.

I should have tried the Boschs then, but did not anticipate any issue with fitment.

The original Champion L88As are no longer made, so I am at a loss on what to do. Fortunately, I have an unused set that I will
put in. For break-in after final assembly, I will first put the old ones back in; they are still quite clean.

Another unsolved mystery.

Maybe some other individual who goes back to the Champion L88A days, will have seen this phenomenon.

Anyway, thanks for the reply.

Regards,
Richard
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Craig in France
Paso 750
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Joined: Tue Mar 23, 2004 3:58 pm
Location: Montpellier, France

Re: Bosch WR 8 AP spark plug issue

Post by Craig in France »

rjk40 wrote: Thu Sep 10, 2020 11:00 am <snip> The original Champion L88As are no longer made, so I am at a loss on what to do.
Hi Richard,

Personally ... :)

Just fit NGKs - B6HS, to be precise. It's the equivalent of the old Champion L88A and I've been doing it for the last 40-odd years ... :-D

All the best

Craig
NGK.jpg
rjk40
750 GT
Posts: 183
Joined: Thu Dec 05, 2013 10:54 am
Location: Malabar, Florida

Re: Bosch WR 8 AP spark plug issue

Post by rjk40 »

Thanks Craig,

I shall do that.

Regards,
Richard
rjk40
750 GT
Posts: 183
Joined: Thu Dec 05, 2013 10:54 am
Location: Malabar, Florida

Re: Bosch WR 8 AP spark plug issue

Post by rjk40 »

Mystery solved, much to my embarrassment.

As before when fitting the rear wheel, I merely needed to give the spark plug a bit of a nudge to get it to seat properly.

I'm really embarrassed about making the same "torque" - related mistake; it's the second time. First time was when setting the rear wheel, and forgot
that I was pushing a bit against the new cush drive. Finally, my feeble brain figured it out, and all was well.

The NGK plugs went in just fine, when I put a bit more torque to the socket; a bit more than the original Champions, as the NGKs are
about .006" thicker, but not that big a deal, as I have now learned.

I'd like another slice of crow pie, please.

Regards,
Red - Faced Richard in Florida
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Craig in France
Paso 750
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Joined: Tue Mar 23, 2004 3:58 pm
Location: Montpellier, France

Re: Bosch WR 8 AP spark plug issue

Post by Craig in France »

No worries - we all have the right to get things wrong from time to time.

Don't forget to put some Coppaslip on the plug threads, btw. Makes life easier when you need to spin them out :-D
rjk40
750 GT
Posts: 183
Joined: Thu Dec 05, 2013 10:54 am
Location: Malabar, Florida

Re: Bosch WR 8 AP spark plug issue

Post by rjk40 »

Thanks, Craig, will do.

" an anti - seize by any name...."

I have a can of it with a brush in the cap, bought long ago. It's silvery,
and very messy if it goes to the wrong place. But, it works great.
Probably not the best choice as a laxative.

Lesson learned long ago after getting tired of cursing at stuck bolts. A while back, I remember posting a question
about reducing the torque wrench setting when using anti-seize, versus the "dry" setting assumed by the author.
That makes sense, but is not something often seen.

Best Regards,
Richard
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Craig in France
Paso 750
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Joined: Tue Mar 23, 2004 3:58 pm
Location: Montpellier, France

Re: Bosch WR 8 AP spark plug issue

Post by Craig in France »

rjk40 wrote: Sun Sep 13, 2020 7:39 am A while back, I remember posting a question about reducing the torque wrench setting when using anti-seize, versus the "dry" setting assumed by the author.
Yes, strictly speaking you should/could, the critical question always being: "By how much?"

Dunno ... How slippery is your slippery stuff? ;)

And anyways, for something like spark plugs, the best torque wrench is the one at the end of your arm :-D.
rjk40
750 GT
Posts: 183
Joined: Thu Dec 05, 2013 10:54 am
Location: Malabar, Florida

Re: Bosch WR 8 AP spark plug issue

Post by rjk40 »

Hi again, Craig,

I looked at a few sites and found that a general rule of thumb is to reduce the Dry torque by
20 to 30% when using anti - seize. One link had a guy from Loctite say that just reusing a bolt tends to smooth the threads enough that it can make a
huge difference in the torque actually applied; the smoother the threads, the less the necessary torque.

My anti - seize, from NAPA (US major auto parts chain...) is a greasy - oily goo, with what looks like untrafine aluminum powder in it.
The instructions on the container say to torque to manufacturer's specs. Really?

So, once again, there are as many opinions as there are mouths (or other orifices) to speak them.

Personally, I like the idea of reducing the torque applied by about 20%. I have done that in the past on the plugs on my 1971
Chevelle's aluminum - headed crate motor, and have never had an issue with the plugs backing out, and they always
come out nicely.

From a physics point of view, it makes common sense to reduce the applied torque when using anti - seize.

Additionally, my wife's late uncle had a Honda NSX, a sporty little auto, and found out that Honda never reused certain engine bolts because
they were teflon coated, so reusing them was not an option.

Anyway, it was great exchanging chat with you, and I hope that you and yours stay safe and happy.

Regards,
Richard
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