lapping flywheels

[EML]

Any of the gurus out there have info on lapping flywheels on. We have one that continually flys off and I have read somewhere that they should be lapped on well as well as loctite on the keyway.
It makes sense but I have not had to do it in the past on anything I've had.

[Mike52]

I always lap but never locktite.
Even Briggs and Stratton recommend lapping and they only use a zinc key.

[EML]

Thanks Mike, seems mechanics these days just want to loctite it all together and rattle the nut up. This one is based on the TX motor with the reverse (threaded crank to take a bolt, not to take a nut) bad design and they only had it for a couple of years. They expect that the bolt holds it all together, not the taper.   

[Gippslander]

Get the finest grade of valve grinding paste -- a new tube or tin -- so you can see the consistancy required -- some are oil based -- some are water based -- I prefer oil based -- no particular reason.

Remove the key.

Check for any keyway damage -- on both the shaft and the flywheel -- VERY carefully dress/remove any damage with the finest file -- do not remove base metal -- only remove damage/raised metal --- buy and use the correct shaped file -- flat for the shaft -- half round for the flywheel -- small and very fine and best quality -- I prefer Bahco brand.

Make sure your kew fits snugly -- the key does not stop the flywheel turning -- the taper does -- the key only locates the flywheel to keep the timing correct -- by getting the flywheel on the shaft in the correct location -- you may have to get an oversized key and file it down -- allow say one hour to do that -- slow and cautious job -- pin vice and magnifying glass etc'.

Clean the tapers with WD40 or similar.

Place a dab of grinding paste on the shaft taper and put the flywheel on and push it gently on whilst rotating back and forth -- you can push harder but firm push only seems best.

After say one minute remove flywheel and clean the tapers and inspect -- you are looking for a complete coverage of smooth dull grey -- you will most likely see incomplete coverage -- there will be some "unground" parts -- so put on a fresh dab of grinding paste and repeat until you get the tapers "matching" and that is "proved" by each taper having complete "smooth grey coverage".

When all matched up -- clean with the same WD40 type spray -- do not use thinners or straight petrol or contact cleaner -- they will strip all "oils" from the shaft and the shaft needs to be lubricated in the most minimal way -- because without some lubrication to form a barrier it is possible for the tapers to bind up too tightly -- so there is a trick -- if you have a "moving taper" you can assemble with neat petrol -- it is very thin and although it will allow for "slippage" when assembling it will form almost no barrier/film and will allow the closest possible fit of the taper -- but beware -- it will be difficult to get off.

Make sure you have clean and good threads on the shaft and the nut -- test the nut by hand fitting it -- must be firm and smooth and not loose -- and then lubricate the threads and the contact part of the nut with light machine oil and use a tension wrench to tighten to exactly the factory specs -- and if you have not got a tension wrench that reads for left hand threads -- get one -- this is a part that absolutely must be to the correct tightness because the "push" onto the taper is an engineering calculation that causes the flywheel taper to stretch over the shaft a very little bit -- and that is how a taper actually works.

And as others have said -- balance is important -- and it is possible to do simple home balancing -- but it is not anywhere near as good as a professional job but on single cylinder engines it can be done" in the "ball park" at home especially if you have access to a lathe and two tapers and the ability to make a test weight and of course very acurate scales and a press to get the crank apart and back together.

[cappra]

Montesa's use a very small flywheel key, so any motor I build, I alway's lap the flywheel to the shaft.  I use the valve grinding compound
which consists of a medium grit and a fine compound tin.  Just lather up the shaft and start turning the flywheel.  Make sure the medium compound
is completely removed and finish up with the fine compound.  The lapped area should be a dull grey colour.  I usually heat up the flywheel a bit
on the stove and then mount on the shaft and torque to spec.  I also alway's use a new lockwasher.

[Gippslander]

Cappra -- you learn something every day -- I never knew about warming up the flywheel -- that would be a neat assembly method.

And just looked at Husky manual for MK series -- at page 1A-6 it says -- "assemble using locktite 241" and "tighten three times, once on assembly, once after 2 minites running and once after 2 hours running".  (And I thought loctite was a no no)

Wasp -- the comments about balancing are true -- I would have thought that a rebalance was a normal part of a restroke job.

[pancho]

 As was already stated, the key has nothing to do with driving the flywheel, only timing, thats why I throw the steel key over my left shoulder and make up an ali one. [as a steel key shears it rolls partly over the keyway edge, distorts the taper and proceeds to rip $*** out of the flywheel taper surface.]
 What I have done about that damage is turn up a mandrell on a lathe the same as the taper on the shaft and lap the flywheel to the mandrell first.
 This removes the high spots and ridges and makes the whole process quicker and leaves less ridges in the crank taper after lapping.
 I like the idea of heating the flywheel before refitting it cappra. Next time I will too.

[270YAM]

-Gippslander-
Wasp -- the comments about balancing are true -- I would have thought that a rebalance was a normal part of a restroke job.

Yes it would definitely make sense that it is a normal part of re-stroke job to re balance.
However, it is not always the case.

Below is a photo of a stock flywheel (74mm stroke) next to a modified 86mm. Minimal metal has been removed to 'balance' it.
 

The next photo shows the same home made 86mm flywheel next to another home made 86mm crank.
This one has had a lot more metal remove however they both had the same 256 rods, and probably the same pistons. Which one is correct? Dunno I have never used either of them.


And this is a real one:

[270YAM]

EML if your flywheel (alternator rotor) is damaged internally I can bring you another one on the weekend, (assuming it is the same as the ones I have, with male shaft and retaining nut, no bolt) I have 3 spare ones that I don't need. (free of charge) you would just owe me a beer for the freight costs

[EML]

Thanks for the info. I went over to the farm to talk to John the retired engineer and grabbed his two pastes, one water based and quite aggressive and the other oil based for finishing off.
I kinda worked out what was involved and can't beleive that the engine has been at a motorcycle mechanis several times but they have never relapped the taper. It is probably the whole reason we have had so much trouble with spark over the last season or more!!
I have spent much of the day slowly lapping the tapers to match as best I can. I say that because it is one hell of a mess after chopping several woodruff keys off over the years and there is a complete circle of meat missing around the middle so therefore I have essentially two tapers with a gap in the middle-one week from the spectacular I'm not  about to pull the friggin' crank out-there's no sheep stations up for grabs
I now have a reasonable finish on the 'outer' taper and will have to clean it up more to get the 'inner' taper to take up-work with me guys-I'm not pulling it apart now.
I have also spent a fair bit of time matching up a new key but my eyesight is almost shot so I might have to get out a magnifying glass as was suggested.
Anyway, wish us luck.
ps not a 270 crank just std as I wouldn't waste time and cash doing that to such a heavy piece off shit when there are better motors out there to work with. 

[mainline]

this has been very interesting reading guys, should this process be followed for all engines during reassembly? ie small bore two strokes, big 4 strokes etc.

[popeye]

Good work mate, hope it all goes well, Mat would you be able to bring your 16 tooth front down just in case we need to borrow it  thanks mate......... By the way trail ride was brilliant today bugger the racing l recon..........

[Mike52]

this has been very interesting reading guys, should this process be followed for all engines during reassembly? ie small bore two strokes, big 4 strokes etc.

Any flywheel on a taper Mainline.
Interesting fact about Briggs n Strattons . If you put a steel key in instead of the zinc one they have a tendency to shear the shaft off. Heavy flywheel.
Now I know that a Briggs is not a bike engine but the engineering principals remain the same.
Lap your flywheel properly and never have a problem again.

[mainline]

this has been very interesting reading guys, should this process be followed for all engines during reassembly? ie small bore two strokes, big 4 strokes etc.

Any flywheel on a taper Mainline.
Interesting fact about Briggs n Strattons . If you put a steel key in instead of the zinc one they have a tendency to shear the shaft off. Heavy flywheel.
Now I know that a Briggs is not a bike engine but the engineering principals remain the same.
Lap your flywheel properly and never have a problem again.

thanks mike, i'm at that stage in a rebuild at the moment, so the timing is good, if you'll excuse the pun

[270YAM]

ps not a 270 crank just std as I wouldn't waste time and cash doing that to such a heavy piece off shit when there are better motors out there to work with. 

Name 1 engine that is better, more reliable and more powerfull and compact than the XS650 that can be used in a Pre '85 or Pre '75 sidecar. . . 
And then tell us why u don't run one in your bike.