Hi all. Got my engine back from Steve Sell recently. Sent to him as I wanted gearbox sorting. New rod kit, all bearings, small end etc. New rings, piston (std) and bore OK.

 

 Have obviously had barrel off to put engine in frame, new gaskets. Have pulled down again to check rings, all good.

 

 Jetting and fuel mixture make no difference to noise. Bike pulls well, all of the exhaust is new, runs nice at walking pace and pulls fine.

 

 So is this just a normal Ossa thing?

 

 Video below

 

 

 

 

 

 

I've never heard anything that bad before. I'd suggest a check on the new bearings though it sounds like something loose and moving around.

It does, but there isn't. Small end is great, piston good in bore and inspected, pin nice and tight. Big end no float.

 

No excess smoke, heat, exhaust in tight.  It is a reed valve one, nothing in there that shouldn't be.

 

 Strange

Did you put the alloy spacer between the head and cylinder on assembly,

And if so what thickness.

Just a thought

As in the head gasket so to speak?  Yes a new one supplied by Steve. About 3mm I suppose. It did make this noise before it went to Steve BTW, Big end was floppy which is what I presumed it was, Noise is top end ish it seems.

Edited November 8, 201510 yr by nickyburnell

Did you put the alloy spacer between the head and cylinder on assembly,

And if so what thickness.

Just a thought

 

PS. I did think of this, so ran two with super low compression as a 10 second test, my thought was that the ring wasn't being compressed. Same noise.

 

The video only shows on tickover. Is the noise speed related i.e. when you rev it does it change ? Does sound pretty bad was wondering if could be primary drive or even the exhaust if you have changed and checked everything in the engine. Ossa motors are noisy compared to modern stuff in fact all 70's stuff is as are TY's right up to and including Pinkys but this isnt normal for a MAR.

It is sort of rev related. Entire exhaust new.  Back to lockup!

 

Sounds like piston slap.

 

Mine sounded like this before I had it rebored, the skirt of the piston slapped the bore causing an identical rattle.

 

You'll need to get the bore and piston measured properly all the way down.

 

 

Last time I heard a noise like that was on a Beamish Suzuki where the piston was hitting the head on each revolution.

Looks like you have a mikuni,but not sure.My 1970 Plonker had a pretty loud rattle at real low rpm - ended up being a very worn slide in the IRZ carb.

This isn't a scientific way of measuring, but if it's worn you'll feel the clearance and if I'm right you'll be able to reproduce the slap/knock.

 

Remove the barrel and piston.

 

Turn it upside down and fit the loose piston with rings to the barrel so the pistons skirt is flush with the bottom of the liner.

Put your thumbs inside the piston skirt and try rocking the piston back and forth in the bore (exhaust port to intake port)

 

Due to the need for a certain clearance, you should feel some movement and perhaps the slightest "tic" as it moves back and forth.

 

What is shouldn't do it slap noisily back and forth with a knock.

 

Next fit a ring to the bore on it's own, square it up by pushing the piston in behind it and measure the ring gap.

Do this top, middle and bottom of the bore.

The ring gap should be 1.9mm.

2.1mm is max.

 

Better still would be to get the piston and bore measured accurately (a local engineer should be able to do this for a drink)

Piston to bore clearance should be around 0.07 or 0.08mm, anything more and it needs a rebore.

 

From what I remember original pistons only come in

.0  (standard)

.2  

.4

.6mm sizes

 

So for a 250 the largest original piston is 72.6mm. (and the bore at max would be 72.68mm)

 

You can go to a 73mm pattern piston, but you need to take a waffer of material out of the crankcase mouth for it to clear.

Easy to do with a bit of patience and a Demel.

 

You could also get it relined if it's too far gone and I think stock liners are still available (an Ossa dealer in the USA).

I had one done once on a TY and it was quite possibly the worst experience of my life, the muppet had it nearly a year, didn't dress the ports so it mullered the new piston and rings and left a gap under the lip so it sank the first time it heated up!

Edited November 9, 201510 yr by goudrons

Hi Nicky,that is to my mind piston noise. When chasing a mechanical noise I put a long screwdriver to my ear & touch on different points on a motor to home in on a noise. A long piece of rod or even a thin piece of wood should be worth a try. I have never done an Ossa,but plenty of Bultaco's & the Mahle Pistons are always stamped with a + or - with a number. This is a guide for final hone to be above or below the overbore size. This is critical,it must be right. If you still have the original piston check for the numbers on the top,then see if the new piston is marked the same. Just a thought,if the new piston is a Wiseco,they need a much greater clearance as they are forged & will seize from cold if a too close clearance is used. Let us know how you get on. Hope this helps,John.

Many thanks to all. I'm now sure it's piston slap at bottom of bore.

No head contact.

 

In Motion have pistons 72.00, 72.25, 72.50, 72.75, 73.00.

 

 I'm on 72 std so hopefully 72.25 will cure it.

 Strip down again tomorrow to check for bottom slap, then off to the re-borers

They do tend to wear from the bottom up, methinks the throw of the crank tends to wear the bore front and back at the bottom.

 

Get the bore measured before buying a piston.

It could be .25mm won't be enough and you'll have wasted your money.

It's rare but it happened to me once and once is enough!

 

I believe Steve keeps a few OE pistons, though I am using a 73mm from In Motion as my reed converted barrel was on it's last knockings.

 

Though it does appear good quality I had to remove some crankcase mouth as mentioned above and a little from the piston skirt on the intake side so it cleared the intake port properly.