Piston Slap?

[david.g.chapman]

In the last year or so I have noticed a knocking noise from my twincam when cold, which seems to disappear when the engine warms up.

This noise is at valve speed, and is not that loud - I can just hear it with the bonnet (hood) closed. It sounds like one single knock. I had the head refurbished this year, with new valves and guides, so I suspect piston slap.

When the head was off I waggled the pistons in their bores, and there seems to be a fair amount of play there (all about 8 thou, but hard to measure), although there is not much of a wear ridge on the bores. The good news is that the engine does not burn oil (over 600 miles per pint), or smoke. Compression is good on all cylinders.

Am I right, or could it be something else? If I am right, is there something that could help, such as using a different oil to the 20W50 I normally use? It does not seem to be worth doing anything more drastic at this stage.

Dave Chapman

[gjz30075]

I take it the head is no longer off the engine? Because the only way to measure the cylinders is with a bore gauge; the kind with a telescoping 'T', so to speak. How are your valve clearances?

Greg Z.

[david.g.chapman]

The head is back on.

The largest clearance is 14 thou on an exhaust valve (should be 9 to 11). The others are more or less in spec. I don't think it's that - its a lower frequency knock rather than a tap - but I could be wrong.

Dave Chapman.

[Steve Crook]

check the ignition timing is correct and even if it is try retarding the timing by 2 or 3 degrees to about 7 to8 degrees and listen for the sound during a test drive. If the noise is reduced yes it probably is piston slap.

[types26/36]

David,
Probably not your problem but just check the timing chain adjustment.
Another unlikely cause but worth checking (it happened to me) is that one of the cam follower sleeves (pressed into the head) moved round slightly (or the camshaft developed more end float?) and the cam lobe would strike it, the sleeves have a cutout for clearing the lobe. I just ground the cutaway a bit more and it was fine as it seemed tight.
Brian.

[type26owner]

Best way to do a simple quick test is to short out a plug wire while it's still connected to the plug with a low voltage test light. Alternatively while the engine is idling or revving pull each spark plug wire off the plug in turn one at a time but hold it near the engine so the spark can jump to earth. Use an insulated boot puller specially made for this purpose or risk getting zapped. If the knock goes away you've got piston or wrist pin noise on that cylinder.

Did ya check the squash deck height and valve to piston clearances?
-Keith

[david.g.chapman]

I have had new cams fitted as part of my head re-build a few months ago, and I remember the noise before then. I have also had new guides, valves and seats fitted then. So I can probably eliminate them as the valve clearances are OK-ish.

It may be worth checking about the cams hitting anything as thay rotate, as the cam width may be the same from the old to the new cams. As the engine warms differential expansion may be enough to gain clearance.

I have tried the spark plug test, by pulling each plug in turn. I have to be careful to keep the revs the same before and after, as the noise falls off below 600 rpm, and the twink does not want to idle smoothly on 3 cylinders when cold and on half choke. Result - no change on any cylinder.

The noise is definitly at the same speed as the valves - you can hear the normal valve tapping noise come in as the engine warms and the knocking dies down.

The noise, when present, becomes lost in the general engine noise above 1500 rpm or so.

Does the speed mean it cannot be piston slap, or will a piston slap only on a power stroke? Hmmm.

Dave Chapman.

[rgh0]

I have observed a similar noise a number of times and currently have it on my plus 2. Its a heavier tapping sound than the rest of the valve clicking and dissapears on warm up and at higher revs. In most cases it has turned out to be a case of excessive cam follower clearance in its sleeve due to sleeve or follower wear. Did you replace the followers and check the sleeves in the rebuild ?

Excessive valve clearances can also create a similar noise problem depending on the clearance ramps on the cam. If the clerance exceeds the size of the low acceleration clearance ramps and the cam and follower come together during the high acceleration period of the cam that follows the clearance ramp period then a much heavier tapping noise is generated.

rohan

[david.g.chapman]

Thanks for all your replies so far.

I gave the head to an engineering firm to have the valve work done, and the followers and sleeves were not changed. I have inspected them in the past, and the followers looked OK - just some minor wear marks on the top and bottom third of the follower skirt, and nothing in the middle.
There was also no significant play when I waggled a follower in the sleeve - can't say I checked them all for that, though.

I thought that the noise was coming from deeper in the engine - but these things are notoriously tricky to locate. Time to get a toy stethoscope!

Dave Chapman.

[gjz30075]

I"d look again at the cam followers. I believe the clearance there is less than a thou so you shouldn't feel ANY wiggle movement and maybe some difficulty rotating it. I know you feel the noise most likely is lower (tone and location) but changing out the cam followers might be worth looking at. That way, you can also put a gauge to the bore of each follower to make certain those are in spec, too. Since the clearances will now change, this is a good time to put all valves in spec. Easy for me to say since I'm not doing the work!

[type26owner]

Bottom end noises normally get louder as the rpms increase. Try listening for it going away is the clutch is depressed. The other obvious suspect is a mechanical fuel pump on it's way out. Is this noise present only while the engine is cold and goes away when it's up to temp?

If the camlobe buckets are loose kiss off the guides, valves and any positive stem seals in short order cause they are going to wear out quickly. Visually inspecting them is of little value. They require a Location Clearance Fit Class One level with a very small error envelope. If you rotate the head over and they fall out of their bores then they are most likely too loose. Found that the buckets do most of the wearing so just replacing them could restore the required close clearance fit again possibly. Ouch, they are not cheap items!
-Keith

[david.g.chapman]

Thanks again for this useful information.

I had the head off several months before the last time (its a long story), and at that time I recall trying out all the followers. there was no significant play in any of them at that time - by that I mean unable to feel anything. They were also difficult to remove, and I used some long nosed pliers and a steady hand to get them out.

I cannot be 100% certain, however, and when I take the cover off next (soon) I will measure them properly.

The noise definitely goes away when the temp gauge reads above 50 C - well before a fully warmed up engine.

I have felt the fuel pump (a nasty aftermarket affair) for any knocking but cannot feel anything - but as the jackshaft runs at valve speed this is a possible area for the stethoscope.

Dave Chapman.

[steveww]

Not really on topic but - next time you pull out the followers do not use pliers Any magnet of reasonable strength will do the job with out the risk of marking the followers

[types26/36]

Or do what the books say, use a valve grinding stick with its rubber sucker.
Brian

[type26owner]

The noise definitely goes away when the temp gauge reads above 50 C - well before a fully warmed up engine.

Noises that go away when the engine warms up are not normally serious problems. Ignore it.
-Keith