Refurbishing threads

The 3 1/2 forum
julianharty
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Location: High Wycombe
Location: High Wycombe Area (Bucks)

Refurbishing threads

Post by julianharty »

Like many bikes, it turns out that my recently acquired 3 1/2 sport has various worn out or stripped threads. These include one for the tank mount which is an uncommon size of M7 diameter. Here's what I've found so far, I wonder what recommendations you have in terms of repairing and restoring these? I do have a helicoil style kit for common sizes:

- Rear cylinder head, of the 3 screw holes for the tappet cover, 2 need new threads
- Tank mounts, one of the two, perhaps both need new threads (and now at least one new bolt as the LH one escaped on my last test ride).
pikl
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Re: Refurbishing threads

Post by pikl »

A quality helicoil in aluminium will last a long while (for the valve cover screws).
For the frame mount, you may have trouble finding a correct helicoil insert (although I imagine lots of stuff can be found online, f.e. ebay).
julianharty
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Re: Refurbishing threads

Post by julianharty »

Thank you for your suggestions. Amazon.co.uk (I live in the UK) has M7 helicoil kits for £25 ish, eBay does have some but they're harder to track down and it's harder to confirm when the items would arrive from eBay. The M7 bolts are also available from online vendors, not from local stores AFAIK.
MickeyMoto
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Re: Refurbishing threads

Post by MickeyMoto »

Hi Julian,

Another Morini? When does the treatment start? Check out Time sert. I have used these successfully in the past, including mending a stripped cam bearing housing thread in a cylinder head.

Wurth do a 7mm x 1mm pitch.

Mike.
pikl
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Location: Slovenia

Re: Refurbishing threads

Post by pikl »

Local mechanic shops may also have sets (at least the standard ones), and they normally do not charge too much for inserting them (less than buying the whole kit). But if you often do mechanical stuff, it's not bad to have a few at home either way.
julianharty
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Re: Refurbishing threads

Post by julianharty »

Mike,
yes, last year I sold the Corsaro 1200 back to NLM, it's not a bike I want to service longer term - too many special tools and quite expensive. I then bought a K2 Kanguro which needed lots of repairs as long unused parts were brought screaming back into the land of the living - some died instead e.g. the ignition coil in the stator, one transducer, the swinging arm pivot lost it's head, etc. etc. I sold that to someone on the forum who wants to take it for a long distance ride in Spain and decided it was time to have a plain 3 1/2 sport since I've not had one for several decades. What I'm also seeking (probably instead, but who knows) is a Dart to ride for a while. I don't think it'd be a bike I'd keep for years, but I've never ridden one and would love to try.

I guess my long term ideal stable would be a K2 (again I've never owned one) and perhaps my 501 engined 3 1/2 - I've yet to resolve how to fit the front starter motor into the K frame. I've lots of ideas and know it's possible with some engineering. At the moment I'm reading various classic magazines and asking around to try and find someone competent who'd do the machining and welding I envisage.

How are things with you?
julianharty
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Re: Refurbishing threads

Post by julianharty »

I'd neglected to answer your question - the treatment starts probably mid-late January for the cylinder head repair, I first want to order spare tappet cover and head gaskets and NLM are closed until around 3rd Jan (when I'll be in California for work). For the tank mount, it'll start tomorrow with a helicoil style insert Amazon promise will arrive. The bolts and washers probably wont arrive until the end of the week. I did think of fitting an M6 insert for now, but it could then be harder to remove when I want to fit the M7 insert so I've decided to do the work once, properly rather than twice.

If anyone local to me (High Wycombe area) has spare gaskets that'd enable me to repair the head this week :)
'It must be a .....'
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Joined: 12 Sep 2010 12:25

Re: Refurbishing threads

Post by 'It must be a .....' »

Hi, you could make the front tank mounts M8 and it is very easily done; I did this a few years back when NLM couldn't supply front tank mount rubbers bushes etc all they had were the M7 bolts and then I discovered one mount was badly cross threaded.

Good luck
julianharty
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Re: Refurbishing threads

Post by julianharty »

As a quick update: The M7 helicoil kit arrived early yesterday and I drilled, tapped, and fitted the helicoil in the afternoon. This morning the bolts and washers arrived and fit both original and helicoiled threads (as expected). The fuel tank is off the bike so I can address rust and other mess underneath. Currently the rust has been treated, the loose paint, dirt, gunge, etc removed, the tank masked and 3 coats of Zinc 182 primer applied. I'm now waiting for this paint to harden before painting the tank's underside with the next paint.

I've also learned lots about different tools for holding drill bits and taps in confined spaces as I was curious to see if it'd be practical to helicoil the rear head while it's on the bike. The front cylinder is much more accessible, of course. I've ordered yet more tools, some arrive tomorrow (via Amazon), one will arrive in a week or so but is likely to be the most strategic as it fits on a 3/8 inch square drive (as used in many socket sets). I'll probably still end up removing the head to do the work as that makes the swarf more manageable and easier to remove entirely.

More news as I continue the explorations.
3potjohn
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Re: Refurbishing threads

Post by 3potjohn »

The other favourite is the rocker stud threads so be ready!
julianharty
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Re: Refurbishing threads

Post by julianharty »

Thankfully I've enough helicoils to do all the rocker studs as well as the tank mounts. Although if I have to do the rockers then something's amiss (or a previous owner/mechanic was ham-fisted).

The tools have arrived, apart from the 3/8" drive tap. I'm holding off, partly as I've been busy refurbishing the petrol tank and partly as I'd like to buy the gaskets for the head first. There's a risk the exhaust port thread will be damaged and for now I'm still trying to ride the bike enough to get a feel for what (else) needs addressing before I take it too far apart to use for a while.

I hadn't mentioned that one of the tappets was way out of adjustment - the rear exhaust tappet had a gap of at least 2mm (my entire set of feeler gauges fitted in the gap at the same time!) The bike occasionally made a horrible noise which alerted me to investigate possible causes, hence I removed the rocker covers (and discovered the damaged threads for the rear one). The tappets are now approximately 0.1mm while I see if they bed back in. I'll have plenty of opportunity to adjust them again as I fix things :) And maybe I'll need a replacement pushrod? oh well...
AntietamClassicCycle
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Re: Refurbishing threads

Post by AntietamClassicCycle »

One of the pushrods on my K2 was bent (not sure how it got that way - the bike lead a very hard life previous to my ownership) so I bought a replacement from NLM. The gap kept opening up on that valve, like yours. I finally pulled the pushrod, stuck it in my press and found that the ends weren't pressed onto the rod completely. Now it was a bit too short, so I found a used early pushrod, installed that and now it's been fine for the last 1k miles.
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Ming
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Re: Refurbishing threads

Post by Ming »

AntietamClassicCycle wrote:...I finally pulled the push-rod, stuck it in my press and found that the ends weren't pressed onto the rod completely. Now it was a bit too short, so I found a used early push-rod, installed that and now it's been fine for the last 1k miles.
As far as I am aware, there is supposed to be some 'give' available in the push-rod ends to provide some protection in case if the cam-belt breaking. Should that happen. the theory is that the push-rod can ' compress' so as to minimise damage to valve stems if they come into contact with the piston head.
I had it happen once on a Kanguro and ended up using the compressed rods until I obtained some replacements. It worked just the same, but without the 'safety' aspect.
julianharty
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Re: Refurbishing threads

Post by julianharty »

Thank you both for your insights, when I have the tappet blocks off I'll measure the pushrods and compare their lengths. I vaguely recall something about the pushrods being different lengths on the front and rear cylinders (which the Kanguro parts catalogue confirms). Anyone know what the official lengths are supposed to be please? I can ask NLM when I next speak with them (they're closed currently).
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Ming
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Re: Refurbishing threads

Post by Ming »

julianharty wrote:... Anyone know what the official lengths are supposed to be please?...
I don't have the exact lengths to hand, but according to the original Harglo w/shop manual, the front pushrods are 2mm shorter than the rear.
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