Causes of top end noise

[yeadon_m]

Hi folks,

I had my UK GT550B engine totally rebuilt as you might recall, and there were complexities arising from the builder fitting KTM 200 rods in place of the originals. As these are 1mm longer, I now have 1mm alloy spacers under the cylinders and all is well there so far.

The remaining issue was with top end noise. Idling cool it was pretty swishy but as it warmed up it started to rattle at idle, though it rang smoothly and quietly under power. This seems to be the wrong way round as I'd heard that pistons often make noise cold before they expand, and when hot, they fit better and noise levels fall.

These cylinders are nikasil-plated and the engine shop had farmed that job out to Langcourt UK. I found from Langcourt who stamped the cylinders that the builder had not sent them pistons so they had 'plated them to 60mm'.

By agreement I am now about to send the top end complete back to the builder for inspection.

Now I've taken the top end apart I cannot tell whether or not the piston fit is loose, but you might tell me I won't be able to determine that without precision gauges (which I don't have, nor would I know how to use them).

Here's the question: other than incorrect piston / cylinder clearances, what other causes of top end mechanical noise are possible? I'd hate to miss something to do with rings, ports, piston pins, piston pin hole etc while its in bits, put it back together and find its no better!

My hunch is it will be that the plater, being unable to match the pistons, could not be sure what the in-use clearances would be (strikes me as an odd way to prepare a cylinder anyway, but what do I know) and mine are incorrect.

Cheers for any advice or info.

Mike

[Craig380]

Hi Mike,

If the port edges are chamfered and there are no signs of the rings catching on sharp corners (you'd see small scratches on the piston rings that correspond with the sharp edge) then I would think top end noise could only be piston slap, or small end bearing rattle.

Sharp edges can be smoothed lightly by hand using a fine jewellery polishing-stone (you can get them in Pound shops, they're fantastic for this kind of work).

I don't know what the plating process is, so I'm not certain how the plater goes about matching bore size to piston. For example, do they bore the cylinder to a given size, then add the plating knowing that the layer of plating is always a given thickness? That is, do they bore to 59.8mm, knowing that the plating is always 0.1mm thick to give a 60mm bore when finished? Or can they alter the thickness of the plating?

[yeadon_m]

Craig,

Thanks - that could be very valuable info. On two of the cylinders, some of the ports on the intake side are sharper-edged than I think they should be, certainly sharper than the other ports in the same cylinder or the same ports on the 3rd cylinder.

Both pistons have scores in as do the rings, only on those pistons and only where the sharp port areas are.

I've said to the builder that while intake side scoring can occur with lube failure, I'm certain that isn't the case here. So if they agree, I have asked at their cost they replace the pistons and then have the cylinders prepared and plated to 1.8thou clearance again at their cost.

Maybe in addition to clearance issues I was hearing ring/port noise. The port edges feel sharp just as you slide a finger over and into the port so I've no way to know whether a ring sliding past would 'notice'.

I know what 'normal' feels like on plated cylinder port edges as I have a set of slightly worn but otherwise stock plated cylinders from a 550A. These are definitely not sharp in any way.

To your question and its a good one - I have no idea. I know with a conventionally bored steel liner, they are bore to be slightly too small, then honed to the right clearance. With plating I don't know if they plate then hone in the same manner, or plate directly to final size. Like you, I'd be interested to learn from anyone who does know!

Cheers,
Mike

[Vintageman]

If you think clearance. look for signs of piston wear. If just the bottom of skirts show wear (polishing for example) too loose. Corners too tight.

You can get thin flat feeler gauges to check clearance. It is not exact but, close. Put the piston in the cylinder and push it to one side (intake or Ex) and slide in the feeler gauge opposite side (all the way since thickest part of piston is up a way). When the gauge holds the piston steady in bore due to friction that's clearance.

I do have access to gauges at work and easy to use (nice costly instruments). I have compared this crude technique above and better than +/- 0.001" accurate particularly where bore is new.
a good sanity check. If I think out of spec then i'll get access to more accurate measuring tools.

[tz375]

They strip off the old plating and re-plate it and then they hone it to finished size. They must have the pistons to get the correct clearances. Upou can do a quick and dirty check with a feeler gauge between the front or rear face of the piston and the bore

Plating tends to build up on sharp corners and the ports have to be chamfered after they are plated by the engine builder or by the plater. But the engine guy is responsible for building and checking all that.

A sharp edge on a port will catch the rings and in very short order would hammer the rings and ring grooves to death.

[Coyote]

+1 to what TZ says. In chrome plating, there is no way to plate to an exact size. As stated the chrome always builds up faster / more on the sharp edges. So the bores must be plated undersize and the ground or honed to fit the individual pistons. No 3 pistons are exactly identical so it is critical that the shop doing the sizing have the pistons in hand. Otherwise they are just taking a shot in the dark. Sounds like exactly what happened in your case. That coupled with insufficient or missing chamfers.

[yeadon_m]

Thanks for that, I'll let you know what they say. Top end now en route to the builder for inspection. I expect at minimum one new piston and at least two replates (outers were very noisy).
Mike

[Craig380]

The port edges should not feel sharp at all, even with plated bores you can gently smooth those with a fine jeweller's stone with light finger pressure until nothing snags or tries to cut your skin.