31/2 Front Forks - Damper Rod Bolts

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Archie Adv
Posts: 65
Joined: 27 Jan 2016 17:54
Location: Clqypole UK
Location: Nr Newark Lincs

31/2 Front Forks - Damper Rod Bolts

Post by Archie Adv »

Hi All,

Been wee while since I last posted and you will be thinking only time I post is when I have a question..!!

I have to admit I got side tracked building up to a trip to Spain in June and Holiday in Cornwall but also lost my MoJo a little on this. That said I'm back at it and getting on with it again.

As the subject says I need some help, info on the front Marzocchi Forks.

1. The bottom bolts that are in the forks seem to be longer than necessary, topping out before fully seated and thus allowing the fork oil to leak out! I did wonder when I stripped them why they were fitted with two soft washers however, even with two washers they are still not sealing the on the mating surface and threads. I believe these to be the original allan bolts. So could someone advise what the thread and overall length is on these bolts please?

All advice greatly appreciated.
Thanks
Archie
Last edited by Archie Adv on 13 Jul 2016 14:45, edited 1 time in total.
George 350
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Re: 31/2 Front Forks - Damper Rod Bolts

Post by George 350 »

Hi,

From my notes it is an M12 screw, 30mm long. However, that seems rather large a diameter to me, so my notes could be wrong. I do know they needed an 8mm hex key to undo them.
What diameter are your 'too long' bolts? Also are your bolts threaded full length? As I said, the originals were cap head screws, i.e. full length thread. If yours are partially threaded, you could be running out of thread?
What I do know is that the original bolts have a reduced height head on them, probably to ensure clearance to the wheel spindle at the non pinch bolt end.

Hope this helps,
George.
George
350 sport 1978, 350 Strada 1978
650 Norton 1967, 650 Kawasaki 1977 and 650 Enfield 2019
Archie Adv
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Location: Nr Newark Lincs

Re: 31/2 Front Forks - Damper Rod Bolts

Post by Archie Adv »

George,
Thanks for your prompt reply, I think you're on the money there with your spec on the hex head bolts, and they are threaded all the way down the bolt with a shallow head. This shallow head screw allows for free fitting of the axle shaft. I'm away from the bike at the moment but will check the dims again after another strip down tomorrow.

Unfortunately the parts list only groups all components for the shock as the hydraulic brake set with no other details.

@ EV Guru. Paul, can you shed any light on what size Dowty washers you use on the 350 Marzocchi Forks please?

Thanks again gents.
Regards
Archie
EVguru
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Re: 31/2 Front Forks - Damper Rod Bolts

Post by EVguru »

A 30mm length sounds about right.

One problem is preventing the damper rods from turning whilst you're tightening the bolt. Any burrs or dirt in the threads can cause problems and it's not uncommon to find a huge burr in there from the drilling of the compression damping holes. Marzocchi would have had a damper rod holding tool.

Of course if someone has removed the bottoming out cones, then the bolts will be too long.

If I can, I like to use a Doughty washer rather than the soft Aluminium washers. The later can have tiny hard to see cracks and cause leaks.
Paul Compton
http://www.morini-mania.co.uk
http://www.youtube.com/user/EVguru
Archie Adv
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Location: Nr Newark Lincs

Re: 31/2 Front Forks - Damper Rod Bolts

Post by Archie Adv »

Hi Paul,

Thanks for your follow up. I've been back out and measured the bolts and they are as George stated. I have taken photos but having difficulty in finding how to post them up. Hey ho!

As you state I have checked for burr's and also note that there is a very slight ridge just below the damper hole, I suspect to prevent over tightening the bolt and closing up the hole. Makes sense to me.
So you say if the units have been stripped down which they have and everything removed to clean, including the bottom out cones but have been reinserted as per your video (excellent) btw and everything now committed to memory. So as I see it now maybe remove a couple of threads from the bolts or add another washer or make something like a 5mm alloy spacer. I think another washer just adds potential for more leaking unless of course I were to add some thread fix of some kind hoping to create a better seal. Thoughts?

As an after thought although the bike was laid up for more than 25 years there was practically no fluid came out when I stripped them initially, obviously on someones garage floor.

Agree, on the Dowty washers I just need to try and source some now. Also the lack of a rod locking mechanism doesn't help although I have found with the fork cap in place creating some spring tension is just enough to hold it tight enough to get a purchase on them so far!

Thanks for all suggestions.
Archie
George 350
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Location: Northampton

Re: 31/2 Front Forks - Damper Rod Bolts

Post by George 350 »

Archie,
Whatever you do do to resolve the issue, please don't use anything on the threads themselves. They are a bugger to undo as they are, let alone having to fight sealant or thread lock in the future! That said, a smear of sealant under the washer could be worth it.

Are the forks still in bits, or at least in a position to remove the bottom slider? If so, with the bottoming out cone on the damper rod, screw the bolts in and find out where they bottom out. You should then see if it is burrs on the holes, distortion in the tubing or something like that that is preventing them from screwing almost all the way in. Again I'm testing my memory here, but I'm pretty sure that the slider where the screw seats is around 5mm thick, so the gap between bolt head and damper cone should be smaller than that.
Re shortening the bolts. When you are installing them, how many turns are you getting before they get tight? You should have at least 8 turns .Any less won't be enough engagement for safe use.

Finally, is this issue on both forks or just one side?

Regards,
George.

PS: For pictures, you will need to load the pics onto photobucket or similar and post the link into the message.
George
350 sport 1978, 350 Strada 1978
650 Norton 1967, 650 Kawasaki 1977 and 650 Enfield 2019
3potjohn
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Re: 31/2 Front Forks - Damper Rod Bolts

Post by 3potjohn »

Archie Adv
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Location: Nr Newark Lincs

Re: 31/2 Front Forks - Damper Rod Bolts

Post by Archie Adv »

George,
Thanks again for the info. I have the main offender RH on the bench again and have been checking the dims etc.
I agree re the use of thread sealant and as you say a little RTV sealant under the washer might do the trick. However, I've ordered some
12 mm Dowty washers and will have them next week. So will just have to be patient. Your note on safety re the number of threads engaging is noted and pertinent,
That said whoever stripped them previously didn't consider that when they added 3 washers to the right fork tube when they reassembled them.

I've checked them out thoroughly and there are no burrs or dirt in the tube threads or bolts. The bolts screw into to top out flange in the damper rod just under the
first hole and leave 11.8mm gap to the underside of the bolt head. Therefore ~9.2mm threaded into the the rod. If as you say the bottom of the fork leg under the cone is ~5mm and the designed washer is a 2mm crush washer! that means I have ~ 5mm excess thread and was how I determined maybe a 5mm alloy spacer might be more useful than a stack of 3 washers. A 5mm of excess thread seems a very wide tolerance and will still exist with either a spacer or washer. In fact a single washer will mean there would always be a very loose tolerance and the bolts would never have enough torque on them to keep them tight. Hence my thoughts on removing a couple of mill from the bolts. These things are set to challenge us/me to find sensible solutions and I'm sure I'll fix it in the end, whichever method I choose.
Thanks for the input George very helpful indeed. Not sure if these will work..!!
Image

Image

John, Thanks to you to the forks rebuild.

Regards
Archie
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Re: 31/2 Front Forks - Damper Rod Bolts

Post by EVguru »

Archie Adv wrote:The bolts screw into to top out flange in the damper rod just under the first hole and leave 11.8mm gap to the underside of the bolt head. Therefore ~9.2mm threaded into the the rod. If as you say the bottom of the fork leg under the cone is ~5mm and the designed washer is a 2mm crush washer! that means I have ~ 5mm excess thread
Why are you measuring the length of thread from the bottom of the damper rod?

Install the bottoming out cone on the damper rod and try again!
Paul Compton
http://www.morini-mania.co.uk
http://www.youtube.com/user/EVguru
mgelder
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Re: 31/2 Front Forks - Damper Rod Bolts

Post by mgelder »

Are you missing the 'top hat' spacer that fits between the end of the damper rod and the bottom of the fork leg?

Image
Circled, above.

First time I stripped my forks, one of mine was seized onto the damper rod, the other was stuck inside the fork leg. Without these - and you don't show them in your photo, which is why I mention it - the bolts would seem too long...
Morini stuff on RealClassic.co.uk: http://www.realclassic.co.uk/profiles.html#morini
Archie Adv
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Re: 31/2 Front Forks - Damper Rod Bolts

Post by Archie Adv »

Hello Again,

Thanks Paul, thats why you are an engineer and I'm not..!! However, your point was noted and checked again with the bottoming out cone/top hat as martin referred to it and remeasured. Just over 2mm was my measurement. Thanks for bringing to my attention.

@ Martin, Thanks, all the parts are in situ and I feel quite lucky in that although the bike was sat in a garage for over 25 years it really wasn't in bad shape. All the parts came off without too much difficulty..!! Either that or my memory is not what it used to be.... I did encounter a wee problem with the main stand bolt but overcame that and rebuilt with new parts.

Okay, here is what I have done in the interim until I receive the Dowty washers that Paul likes! I added a spot of RTV sealant (oil resistant) and a 12mm O ring under the washer and the bolt head and it tightened up much better than previously. (This is merely a test to see if it works). It will remain on the bench until the Dowty washers turn up when I will replace the washers.

Now that I know how to add pictures I may be able add a few more as we go for interest.

Thanks to everyone who has responded and heres to a few more in due course.

Archie

PS, This is how it looked when I got it home in January.
Image

Image

Image

Image
baybell
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Location: Swansea, UK

Re: 31/2 Front Forks - Damper Rod Bolts

Post by baybell »

Hi everyone, my first post so please go easy on me...and apologies that this is actually not a new thread but me hijacking a response to a previous thread but it gives all the background needed for my question:

I have been steadily rebuilding my 1978 350 Sport and found this forum and Paul & Soren's youtube channels invaluable.

Picking up on this thread I am now trying to reassemble my forks but I am struggling to tighten the fork lower damper rod bolt as the whole damper rod is turning more or less after hand-tight. All the damper rod parts are there, they're original and seem in good order (Marzocchi) plus all the parts have been thoroughly cleaned to EVguru's previous point on dirt in the bolt threads etc. I have the main spring in and cap on to give tension but it still turns. Grateful of some ideas on how to overcome this? Again, EVguru mentions that 'Marzocchi would have had a damper rod holding tool' - what does this look like and how does it work?

On the assumption we figure out how to tighten and somehow hold the damper rod whilst I nip-up the lower bolt then 'how tight'? I am using a Dowty washer as recommended.

Many thanks

Darren
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Ming
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Re: 31/2 Front Forks - Damper Rod Bolts

Post by Ming »

I have used a long rod down the tube which 'jams' in the top of the damper rod tube. I think a very long Carpenters' Screwdriver (flat head) could make a suitable tool. I think a broomstick has been mentioned before now, as well. Should not need to be rived-up to be leak free.
Stubaker58
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Re: 31/2 Front Forks - Damper Rod Bolts

Post by Stubaker58 »

When I came to strip my forks I had the opposite issue, they turned when I tried to undo them! I solved this with a longer bolt in the drain screw hole tightened down onto the damper rod. This worked well and the rods looked none the worse for it.
baybell
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Joined: 04 Sep 2019 15:13
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Re: 31/2 Front Forks - Damper Rod Bolts

Post by baybell »

Hi Guys,

Thanks for your tips. I eventually found a long piece of steel rod and ground the end to a taper so it would 'jam' in the hole of the top of the damper rod whilst I tightened up the bottom bolt. No idea of torque, but not that high as eventually the damper rod would turn even with the taper-rod inserted and held, but its well nipped-up and no oil is leaking past those dowty washers - so far.

I also tried a longer screw in the slider drain plug but I must admit I bottled it a bit as I didn't want to damage anything, but in a pickle I could see that it would be another option.

Cheers
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