Spark Plug Length on 350

Ducati single cylinder motorcycle questions and discussions, all models. Ducati single cylinder motorcycle-related content only! Email subscription available.
Moderator: Morpheus

Moderator: ajleone

eib
Posts: 5
Joined: Sat Jul 28, 2018 2:13 pm

Spark Plug Length on 350

Postby eib » Sat Jan 12, 2019 11:26 am

Hi Guys,

OK, I am a bit of a newbie here.....and I admit that it is around 40 years since I last took a Ducati single apart, so what little knowledge I had then has long been lost! You will find a short introduction to me and my scrambler in the 'People pics' section.

I have a bundle of questions, and I am sure most will be pretty basic or even daft, so bear with me, I will start with two that are bugging me and slowing progress:

Sparkplug length:

I have a 1971 350 Scrambler and am a little perplexed by how high in the threads the sparkplug (Standard = B7HS) seems to lie, so deep that it appears to be totally covered by the threads. This was noted in a post by bikester250 back in Jan 2019, but no-one seemed to be forthcoming with any thoughts on this, so I wondered if I could ask again. Is this normal, Is it a problem and what is best way forward.
IMG_20190105_165637_DROa-sm.jpg
plug loose in head without insert, insert will make it 2.5mm higher still

I have the additional problem that this head has a rather industrial thread-insert that has a 2.5mm collar on top, which therefore lifts the plug further still up the threads.
IMG_20181226_141514_DROa-sm.jpg
Brass insert, outer thread 18mm x 1.5pitch

Although the engine runs OK, this doesn't look right to me and I am trying to work whether to ignore, use a different plug or try something different. As bikester250 pointed out, using a projected plug (B7PHS) will go some way to correct this, but with the insert, it will still be pretty low. Going to a 3/4in reach plug (B7ES) is a big jump and I worry will be too long, unless I used a washer above the insert, to raise the plug a couple of mm more. This could be a loose washer, or if I was confident of how it would work, I could solder a copper washer onto the top of the brass insert, making it permanent, but of course would need to do this before returning it (hopefully permanently) into the head. Any thoughts on any of these questions would be appreciated. What is the best way forward?
IMG_20190112_105522_DROa-sm.jpg


Removing the Bevel tube from head:

IMG_20181230_155305_DROa-sm.jpg

I realise this question will be pretty basic and I am sure a little gentle research would answer the question, but as I have your ear... I have the head off and want to remove the bevel tube to replace the failed gasket between it and the head. The Haynes manual seems to suggest it will just 'come off' leaving the bevel shaft it place, but if so, it needs a knack or method I don't have. Never been good at puzzles, but it looks to me like the bevel shaft/bearing is going to have to come out to allow me to remove the tube. So first, is there a way of removing the tube without the shaft? If not and the shaft has to be removed first, what is the best way to get it out. Manual suggests tapping it out from above, which I can understand, but never like pushing/hammering when the bearing is taking the pressure. What is the safest way to remove it?

Thanks in advance

eib
***************************************
Ed I Bremner
350 Scrambler
MG 850T3 cali
Montesa Cota 247 Trail
Plymouth, UK
***************************************

ranton_rambler
Posts: 207
Joined: Sun Jun 04, 2017 11:33 am
Location: Stafford UK

Re: Spark Plug Length on 350

Postby ranton_rambler » Sat Jan 12, 2019 12:10 pm

Just going from memory when I did my 175 top end last year, I don't think the flange on the tube will come past the various bits sticking out of the engine, so you need to remove the shaft and tube together. The bearing at the top sits in a collar which should be a tight fit in the head, but you'll need to tap it out so the whole thing will swing away from the engine. I haven't run mine enough to know if it leaks yet, but I have read that this joint can be tricky to get right, as the gasket needs to be thick enough to compress, but not so thick that it doesn't clamp the collar properly.

Ian

LaceyDucati
Posts: 280
Joined: Sun Sep 19, 2010 10:30 pm
Location: Wales UK
Contact:

Re: Spark Plug Length on 350

Postby LaceyDucati » Sat Jan 12, 2019 8:32 pm

Hi,
Firstly, I've seen Loads of those thread repairs (I know the culprit!) and often the length is incorrect. Mostly they were intended to be used with long reach plugs. Unfortunately they always come loose and they are impossible to keep secure. I've tried Loctite, pins screws all to no avail. The only solution I've found to work is to make a custom bronze insert the correct length for a 10mm long reach plug. Then tighten it in with the head when hot (hex on insert flange). The insert will have a thicker wall which holds the tension better. The 10mm plug needing less torque to tighten or loosen helps the insert stays in the head.

It's not possible to remove the tunnel without the top bevel shaft and bearing. As this removes easily there is no issue with this.

Regards Nigel

Samurai
Posts: 112
Joined: Wed Nov 28, 2012 9:53 pm
Location: Somerset, U.K.

Re: Spark Plug Length on 350

Postby Samurai » Sat Jan 12, 2019 9:09 pm

Regarding the loose spark plug insert, I too had this issue on my 175 TS recently, as Nigel says, it seemed impossible to get it to stay in place for any length of time with Loctite and he was kind enough to suggest the same solution. The insert was the same as yours, in that it shielded the plug by about 2mm and was obviously not helping combustion efficiency.

Upon taking the head to a reputable local engine reconditioner to do the work, they reported that the reason the standard insert was not staying in place was that the person who had previously done the repair, appeared to have 'wobbled' as they had created the external threads and that some of them were giving quite a loose fit.

They measured up to see about doing an oversized custom insert, bit did not recommend this in the end as it would have come too close to the valve seat, giving the chance of a crack developing.

In the end they made a custom insert the correct length and same diameter for the standard 14mm plug, but with a more pronounced 'top hat' so as to bite into the head better. They also used a special metal 'glue' to fill up the uneven gaps in the external threads. They shaped the end of the thread insert to follow the combustion chamber profile.

This repair cost me the princely sum of £48 and has so far seemed to have completely solved the problem. I hope that this may give you an alternative way to carry out a repair if you wish to retain the standard plug size, providing you can find a reputable engineering shop near you to do the work.

LaceyDucati
Posts: 280
Joined: Sun Sep 19, 2010 10:30 pm
Location: Wales UK
Contact:

Re: Spark Plug Length on 350

Postby LaceyDucati » Sat Jan 12, 2019 10:47 pm

I hope your new insert does stay put, but my experience (albeit with race bikes) is that after they get really cooked they come loose when you try and remove the plug hot. Not sure any "special glue" (a loctite of some sort I expect) will stand up to that sort of repeated high temperature. I've tried high temperature structural adhesive and that cooked. Also there is the possible concern of poor heat transfer though epoxy. Anyway time will tell.

Nigel

ducwiz
Posts: 291
Joined: Tue May 22, 2012 12:52 pm
Location: near Frankfurt, Germany

Re: Spark Plug Length on 350

Postby ducwiz » Sun Jan 13, 2019 10:43 am

Hi,

whenever I have seen this solution for a plug thread repair, it had more or less failed. Always caused trouble like loosening, not gas-tight, etc.

In the past i succeded using the following inserts, which moreover do not reduce the plug's reach by a too large amount, or even at all:

http://www.timesert.com/ (in the EU marketed by Würth Group)
https://www.boellhoff.com/de-en/products-and-services/special-fasteners/thread-inserts-helicoil.php
or less expensive variants of Helicoil:
https://www.baercoil.com/en/products_thread_repair_kits.html
https://www.afsrecoil.net/

I had repaired one of my 350D heads with a Bear-Coil insert and used it for ~30000km without any problems. Today, a complete repair kit for 14mm plugs costs ~50€, while it was ~45DM in ca. 1990.

I recommend to disassemble the head entirely, clean it thoroughly and get the spark plug hole build-up welded, then re-threaded in a machine shop or an engine repair specialist.

cheers Hans

eib
Posts: 5
Joined: Sat Jul 28, 2018 2:13 pm

Re: Spark Plug Length on 350

Postby eib » Sun Jan 13, 2019 10:51 am

Thanks all for your input.

I have two possible engineers to ask about making up a new insert, so I will see what the verdict or estimate of cost is.

I am not sure that I will be getting the engine up to the heat that a full race engine would get to.....so glue might help.....but I get your drift. I can see that a 10mm sparkplug would also help for the reasons you give.

I am still curious why when a normal plug (B7HS) is offered up into its hole, without the insert as it would of originally been....it looks so deep in the threads. Is this normal? I would presume that the bottom of threaded section of plug should be flush with the edge of combustion chamber(and electrode standing proud) so why so deep? I presume if I get a new insert made, then I am aiming for this plug depth. What is the optimal protrusion of electrode into combustion chamber? Obviously not touching piston!

I will have another go at the bevel shaft today. A light tapping did not move it, but I will try a little more. As I said, just get nervous about hitting bearings. Where should I hit - centre of bevel gear? Would a little heat help here? Get worried in case I am working against some other weirdness, like the bearing has been loctited in or something.

Thanks again,

eib
***************************************
Ed I Bremner
350 Scrambler
MG 850T3 cali
Montesa Cota 247 Trail
Plymouth, UK
***************************************

Samurai
Posts: 112
Joined: Wed Nov 28, 2012 9:53 pm
Location: Somerset, U.K.

Re: Spark Plug Length on 350

Postby Samurai » Sun Jan 13, 2019 10:54 am

Hi Nigel,
I did query if the 'glue' was a type of Loctite and also regarding the electrical and heat conductivity, but was assured that it was not and actually contained metal particles and should be more than sufficient combined with their slightly wider 'top hat' flange to bite more firmly into the head.

I guess time will tell and if it ever does fail, then I can always go down the route you suggest and use a 10mm plug, but for now all seems well.

Incidentally, your suggestion to save time chasing down elusive ignition gremlins by changing the entire system has actually worked! I fitted an Electrex World electronic ignition kit to replace everything, together with sorting the spark plug insert as described above and it fired up and ran well within a few kicks :D .

I'm still fine-tuning the fuelling, but I actually ride it down to West Bay in Dorset and back to Taunton without any issues on New Year's Day. I can't thank you enough for pointing me in the right direction as to be honest I was starting to lose heart.

At least I can concentrate on getting ready for the Motogiro d'Italia next year and putting on some miles. Eventually I may retrofit certain components, to try and regain a bit more originality, but for now I'm a happy bunny :D

LaceyDucati
Posts: 280
Joined: Sun Sep 19, 2010 10:30 pm
Location: Wales UK
Contact:

Re: Spark Plug Length on 350

Postby LaceyDucati » Sun Jan 13, 2019 7:14 pm

Hi,
Glad all working out with the new ignition, keep us posted and wish you a successful Giro :-)

Both Helicoils and timeserts work well if correctly installed, assuming its not been tapped to M18 for a bronze insert! Although I have welded up plug threads in the past when necessary, I would caution doing this lightly as doing so will distort the valve seats and possibly compromise retention. I would suggest trying any other solution first.

Regards Nigel

double diamond
Posts: 454
Joined: Thu Apr 28, 2011 1:20 am

Re: Spark Plug Length on 350

Postby double diamond » Sat Jan 19, 2019 3:38 am

Here’s how your spark plug should look.
Matt
Attachments
image.jpg


Return to “Ducati Singles Main Discussions (& How to Join)”

Who is online

Users browsing this forum: No registered users and 13 guests