Hi all,
Just started to pull my '42 engine apart. Seems that the exhaust valve guide thread in the case is stripped so bad, you can pull the guide up and down with NIL resistance.
Is it even possible that the engine had been running like this and in which case the case as been bored out by the Guide?
In any event, it needs to be resurrected, as I want the cases for their '42 year, and, they are in much better condition otherwise, than my '41 engine I have.
Q: What is the best way to fix the crank case ? Will a helicoil work? Or is it best to weld up and re-tap? or, drill out and make a interference fit bush and pin it?
Cheers for any help :)
email (option): marty.whyte@gmail.com
I wouldn't weld it personally...Results can be erratic due to casting material variations, corrosion and oil impregnation and you only get one go at it...
A helicoil would make good repair but you might have trouble finding one in the correct thread...An oversize bush fitted to the crank case and then tapped to the correct size would be my choice if no helicoil is available...
You could screw and Loctite (with retaining fit Loctite) the bush into the case before tapping for the follower guide..
You'll also need access to, or the services of, a machine shop to set the crank case up to ensure the guide thread is perfectly vertical by clocking the top face of the crank case...
The thread is 20TPI Cycle Thread....Ian
email (option): ian@wright52.plus.com
It might not be the best repair but the crankcase hole which takes the exhaust guide is very long. I doubt if it ever comes loose again if you simply 'loctite' it in place......
email (option): m.wijbenga@hotmail.com
Michiel,
You need a proper permanent repair on this one as above. What you need to think about is where have all these bits gone too from the stripped thread.
Tim W
email (option): t.j.walker@btinternet.com
Hi Tim, Glueing can be very permanent :relaxed: . I wonder what could have stripped the threads?, never seen that before. Maybe someone has tried to replace it in a wrong way and damaged the threads?? These guides are useally very, very stuck in the case.
And yes it probably calls for a good repair with threading but still think, if the fit is still reasonable, Loctite could be an option as there is lots of surface which is needed for good glueing.
email (option): m.wijbenga@hotmail.com
Michiel,
A good quality loctite at the correct grade will fill and hold a gap of around 0.012' so that is only 0.006" a side. I would still go for a permanent repair there is enough material around the damaged hole to fit a threaded plug. Maybe you could just tap the hole out oversize and make a new oversize guide. But don't know if this is an option as the original guides may be hardened.
As you say its hard to belive that this has stripped its thread but who knows what has happened to these case over 70 plus years.
Tim W
email (option): t.j.walker@btinternet.com
Personally I wouldn't consider Loctite for this job...I think a proper repair would be the correct way rather than just going for the easiest option...
It is also worth remembering the tappet guide has to be removed and refitted if the exhaust cam follower has to come out or if the cases have to be bead blasted at any time...
A machined repair properly executed will be permanent and will ensure the tappet guide is vertical in the case and that consequently, the tappet head is square to the valve stem etc....Something that cannot be guaranteed if the guide is Loctited into a stripped thread....
An oversize guide could be an option but they are hardened, so some care would be needed in selecting the right material, applying the correct hardening and tempering and ensuring the correct clearance...
Taking that into account I would consider machining the case to be the best possible option...I have altered crank cases in this area previously and the machining itself is not difficult...The most important thing is to set up the cases accurately prior to commencing work...Ian
email (option): ian@wright52.plus.com
""" A machined repair properly executed will be permanent and will ensure the tappet guide is vertical in the case and that consequently, the tappet head is square to the valve stem etc....Something that cannot be guaranteed if the guide is Loctited into a stripped thread...."""
For the good order, what I mean is loctite not the threads but the plain surface below the threads which is more than 35 mm long. If the long corresponding hole in the case is not affected by the loose guide its correct position cannot be beaten I would say..........And further more, it looks like quite a challenge to me to position a helicoil or threadbush exactly in line with this long hole so that the guide will turn in correctly...... who will guarentee that in front??
Removing for replacement is indeed more difficult when glued but when a good tappet and guide are fitted, with todays average use they will probably last more than a lifetime.
email (option): m.wijbenga@hotmail.com
Thanks All. Great info.
Ian, is it 3/4" 20TPI NSC as in this website's 'Threads' information? I've never heard of NSC (not that, that means much) and my supplier of Helicoils has not heard of a thread repair kit for this size.
Is NSC compatible with another thread type?
Pretty desperate for a Helicoil to work, as I don't think there is enough meat in the Case to bore out and make a substantial enough bush to work.
If I can't fix this '42 Crank case (for my '42 bike), I'm stuck, as my '41 cases have had a rough life, cracks, welds, flogged out Mag hole and recess (worn massively oval) among other damage. I have a ZM engine, but dont' want to use that!
Marty
email (option): marty.whyte@gmail.com
NSC is simply the abbreviation for 'Non-standard Cycle'....motorcycle manufacturers were very fond of them. The problem will be that it is a 60° thread...and it's probably so unusual that there is no commercial thread repair kit available.
Bugger.
email (option): marty.whyte@gmail.com
Better go for the easy option !!:laughing: :laughing: :wink:
Just checked, I have a 3/4" x 20 tap, so can make you a threadbush if your stuck, let me know
email (option): m.wijbenga@hotmail.com
Hello Marty.Fun & games,just wait until you start on the gearbox.Before you go any further ,have you checked that the cases are a matched pair.They are stamped on the front about half way down under the part number 66-1600.I have a very good set you can have for the postage cost if you want,1943 I think,WM20 78323.Even without the fly wheels they they are not light.Any chance of a photograph of the stripped part it is something I have not seen or heard of before.Regards,Sam.
email (option): smarwick@iprimus.com.au
Hi Sam,
I'll post a photo this afternoon. Most Odd thing I've seen... but, I have the worst luck with anything I turn my hand to despite trying to do everything right. I must have stood on a leprechaun in a previous life!
Yes, My '42 cases are a 'matching pair'. My '41 cases are also, however, they require too much repair to contemplate using.
PM'd you about the cases you have.
Michiel, Thank you for the offer of making a bush. Might take you up on that! Cheers.
email (option): marty.whyte@gmail.com
You can clearly see how stripped and worn the thread is. I'd say, the engine was running for some time with the guide loose!
Is there enough meat to make a threaded bush?
email (option): marty.whyte@gmail.com
Hi Marty
Not that I can be of any help, but was interested if and how you managed to fix your worn thread.
Regards
Pat
email (option): sacombsashtrees@hotmail.com
I went through this with a pair of ZB Goldie cases. On the inlet side though.
I suppose the guide comes loose in the case and then the tappet movement rocking back and forwards just wears it out.
The answer was to source a new timing side case and have it matched to the primary side case with the engine number by a good engineer.
email (option): mfarnay@bigpond.net.au
Hi Pat,
So far, to answer your question, I have sourced another set of good cases. I spoke with my engineer, and once he told me how much it'd be to fix, I thought it best to find another set of cases.
email (option): marty.whyte@gmail.com