Hello and my 1st post after my 250/1 sezied.

[DAVIDGUEST123]

Right then, I’ve finally got the barrel back from the engineering chap( top bloke but takes his time) before chucking it all back together is there anything I should pay particular attention to? Also what should the piston ring gap be? Any help would as usual be gratefully received.
Thanks Dave

[Puffs]

Ta for keeping us posted!

On page 21 of the "MZ_TS250-1977-Official_Service_Manual" (Blurredman's website, see his signature) they give an end gap of 0.2mm, service limit 1.6mm. But I suppose you have a rebore + a new piston/rings/gudgeon pin/circlips, so you can't do better than that. You may find 0.3mm or so.

Other than that:
- Lube piston & rings well on installing; makes it easier. Be careful not to break a ring, and position them properly. I often put a bit of grease on the mouth of the cylinder sleeve, to ease the rings in. In fact, I also round the entry of the sleeve with some sandpaper (or a file), to make it easier for the rings to enter. Of course you compress the rings with one hand, while the other holds the barrel.
- Also excess lube on big end & small end. In this case, a new needle cage in the small end would be a good idea.
- With a new piston, it is also be worthwhile to see how much gap you have between the top of the piston & the head. See page 29 of said manual.
- Depending on how much piston-barrel clearance your engineering chap gave it, and what piston you bought, you have to brake it in gently. Many people (including me) mix richer during the break-in period.

Good luck!

[DAVIDGUEST123]

Thanks for that Puffs. I keep you informed as to how I get on. One last thing, is there any reason I can’t pop the piston into the barrel before sliding it down onto the con rod?
Thank again Dave

[Puffs]

You mean: in order to make entry of the rings easier & safer, I suppose.

I never tried that. It will be quite a heavy block of stuff you'd have to manhandle, while inserting piston pin & the last circlip. Of course inserting the piston requires quite accurate alignment on the bearing, while it's a tight fit in the piston. (I believe they say something about heating the piston in some manuals.) And the barrel's cooling fins might be in the way, preventing easy access.

I don't know - up to you. I believe the customary procedure is to put a rag over the crank (to catch the circlip when it falls, so that you don't have to take the engine apart), and then install bearing & piston, take the rag out & put the gasket & barrel. If you help the rings with your fingers, it's not all that difficult. But if you feel the other method seems more attractive, give it a try & let us know.

[DAVIDGUEST123]

Well... it’s back together and running.

Starts well, pulls fine, no horrible bottom end noises, but it sounds like either the small end is worn ( yes some people just don’t take advice. Low funds made me deaf) or piston slap. I’m going to run it for little while as is, but the noise is quite pronounced.

I’m a bit loathed to strip it again as I did examine all the bearings for play and in my humble opinion they seemed fine.

So I a little lost as per what to do?

Thanks Dave

[Puffs]

Piston slap is not very likely, as you just had a rebore. With a feeler gauge you could measure the clearance between bore & piston (at the skirt), and you are likely to find something around 0.04 - 0.06mm. But indeed, if you find 0.10 or more, it will be piston slap & your rebore tech made an error.

However, it is more likely to be the small end. Putting a new bearing cage & a new pin may help, but it is no guarantee. Look at the hard-faced internal surface of the small end. It may have some signs of wear. If you have worn through the hard facing, you'll need a new conrod set. Then also replace the main bearings.

Sorry to be the bearer of bad news, but I recommend you take the barrel off again & install a new needle bearing cage (+ piston pin, if not done before), and see if has play with that. Taking the barrel off is not so much work, and then you know more. The manual gives clearance numbers, I believe.

If the small end is going, it will not stop, it'll only get worse & you risk more damage. So best see what's the cause of the rattling.

[DAVIDGUEST123]

Bad news it is. All back together with new long pin needle roller bearings and although the engine does sound very slightly better it still has a rattle. So it's engine out time.
There didn't seem to be any wear to the bearing surface of the con rod at the top end although I could detect a slight left/right movement at the lower end. I think this maybe the issue ..so out it comes.

I think my next move may be to have the engine stripped by someone with a liitle more knowledge than myself...Any recommendations?

Many thanks Dave

[Puffs]

Sorry to hear that Dave.

And I regret to have not advised you earlier to get a 2nd opinion on the sound, maybe from the rebore bloke, or a motorcycle mechanic in your area.

What you're describing on the big end is axial clearance. That is quite normal, and for the ETZ models the service limit is 1mm. And that's quite a bit. It's radial clearance, so up-down (with the crankshaft in TDC) which is normally the problem, it has a service limit (ETZ) of 0.05mm. Radial clearance is far more difficult to feel properly, unless the big end is really gone.

If you pull the conrod upwards with one hand, while bumping it downwards on the small end, with the muzzle of your other hand, do you hear a loud metallic clacking? That would indicate big end failure. It's a crude test, but the idea is to first pull all radial clearance to one side, and then suddenly push it to the other side. Do not try to test the sideways motion, that is rarely the problem.

On the small end, I do not really see a problem in the bearing cages you show. The one on the left is the original, and I see some signs of touching the inside of the small end, on the edges of the cage, but that probably shouldn't be an issue. With the piston pin in it, can you feel any radial play? I have had a small end failure on my 251 (some 18 years ago, no pictures), and it was quite obvious. Actually a lot more play than I expected from the sound, it did not sound all that bad.

Another problem could be the main bearings, of which there is one at each side of the crankshaft. You can rotate the crankshaft by rhythmically pushing/pulling the conrod up & down, staying in sync so that the crankshaft rotates. Do the bearings make any sort of crunchy or otherwise worrying sound? When the engine was running, main bearing failure is often indicated by a low end grumbling that can often be felt in the footrests. And in catastrophic failure they can seize, in a very loud manner, as I had in another bike.

How much play does the piston have in the barrel? Is it possible you're just hearing piston slap?

If you need another crankshaft, or just a conrod set, you can consider to take the engine out yourself, but have it taken apart & rebuilt by a mechanic. A conrod set is obviously a lot cheaper than a complete crankshaft, but pressing will also cost money. Regardless, if it comes apart you'll need new main bearings & seals.

The only mechanics I can advise you for the MZ is H&S Motors in Aberdeen, but I suspect they have retired by now. If you tell us where you are, maybe someone else can advise.

[Puffs]

Just to show an example of small end failure, as my 251 had some 15-20 years ago. That damaged piston pin is shown here on the left:

Pins with wear.jpg (31.4 KiB) Viewed 157 times

The eye in the conrod had similar wear, but I can't show a pic of that.

The pin on the right is a big end pin of another bike.

[DAVIDGUEST123]

Thanks Puffs for the infomation. The story continues, I decided that as I'm going to keep the bike for some considerable time I'd get it fully rebuilt by a professional. So I've taken it down to Martin Burwins who went though it with a fine tooth combe. The Big end had statred to remove its surface very much like the photos of your small end pin. The clutch had started to spin aon the shaft and there was quite a bit of wear to the various known gearbox slector parts. So i'm now happy that the engone is fine.
The only issue I have is that there is still quite a bit of the noise from the top end which Martin did tell me that the rebore is a little on the large side with a piston clearance of 3 thou. The bike run fine but is a little difficult to start. The question now is do I rebore again or will it get better with use?
Many thanks

[Puffs]

3 thou = 0.0762mm, say 0.08mm.

The book states a maximum clearance of 0.09 - 0.10mm, so you are pretty close to that. You may hear some piston slap. It's up to you, but as long as it's below the stated service limit I'd just ride it. No, it will not get better with use, the clearance will only increase. But there is a limit to the number of rebores you can do.

Clutch spinning on the crankshaft is a common problem, I have not (yet) had it, but I know others have. He put an entirely new crankshaft then, I suppose.

Starting problems are not normally caused by high clearance, unless it has low compression due to worn rings. But I guess it was just bored too big, and that the rings are fine. If the compression is reasonable, for starting issues look at ignition (plug, timing, breaker gap, ..) and at the carb (pilot jet, starter jet if you have that, any other dirt, float level).

Weather & time permitting: ride the bike & enjoy it!