As before mentioned I have the mechanical knowledge of a labrador. Please speak to me as such.

My rebuild is essentially complete, so I am now solving problems I have created.

I have two problems, knocking piston and clutch that does not work.

Clutch: previously worked fine. The only things I have changed is clutch cable (pulls clutch pulley the entire distance of travel) and I removed clutch cover, removed oil pump drive shaft inside, then re-installed. I did not touch the clutch of gears. The clutch however does not engage.

Piston: I have installed new piston, and it was (I believe) ‘interacting’ with the head. This was remedied by installing 2 head gaskets, although it sounds like it is still just touching. It moves entirely smoothly without the head installed so there are no obstructions in the barrel etc. Worth noting that before I had the cylinder bored, I did a sort of trial top end rebuild (first time) with old piston, and it did the same thing. Confusing, because previously the piston did not knock at all. What could cause that?

Many thanks,
Olly

I guess for starters, what bike are you working on.

As far as piston hitting, the only way that will happen is if head was shaved too much, cylinder deck surface milled or wrong piston.

Are you using a stock cylinder base gasket?

Olly, since you say nothing was disturbed on the clutch 'side' (RHS) of the engine when you had the clutch cover off, your problem is probably on the LHS and manifested itself when you changed the clutch cable. I suspect the cable change has disturbed the way the spring loaded clutch actuator arm interacts with the clutch push-rod that passes through the hollow clutch shaft. The good news is that it's fully adjustable for your new cable. Remove the LHS front drive sprocket cover and give the area a good clean from any chain lube, muck etc. You should see a hexagonal bolt head (about 14mm) and within this head is a threaded pawl which can be adjusted slightly (to the left or right) using a crosshead screwdriver. You only need to turn it by a few degrees either way to have the full limit of adjustment so go easy and experiment yourself. When you make an adjustment you are actually raising or lowering the tapered clutch actuator mechanism that pushes on the clutch rod, which in turn pushes a ball bearing and then the clutch mushroom etc. Good luck with it all.
As far as the piston goes the obvious culprit as a previous person said could be the wrong piston. To require a second head gasket which is after all quite 'chunky' and still you think there's touching suggests something radically wrong. Do you know if the barrel top mating face was 'skimmed' when you had the rebore done? Also I don't think it would explain the measure of intolerance you're experiencing but you did hopefully replace the barrel lower gasket, accepting this is fairly thin card material? Finally, and it's a long shot but is the replacement piston a NOS design that perhaps uses an old fashioned piston ring configuration where the top ring actually sits right on the top lip of the piston? Don't quote me but I think they called it the Dykes configuration and it used the engines combustion chamber pressure to seal the ring against the bore. Anyway you DON'T want to be using this type anymore, since modern ring material, design and engineering tolerances have superseded this configuration. Hope this helps and good luck with it all. I'll follow the thread to see how it goes. Andy.

Thanks for replies!

I'm working on a 1976 DT175 here. I should mention the clutch was operational previously, and the piston was not knocking.

Clutch: Ok, I suppose it is possible that I have upset the spring loaded arm, that was actually my thought and I had a go with that bolt yesterday to no avail, but I was fairly aggressive with it and took it the whole way off initially. I'll have another go, with a more gentle approach. Thanks Andy.

Piston: I am using the correct piston, and the head was bored for the piston. Cylinder side was skimmed, and cylinder head I skimmed with a piece of glass and sandpaper. Critically though, this problem was evident with the old piston too, before I had the cylinder bored or skimmed. So essentially, I took the head apart, then put it back together with the same parts, and it was knocking. Rather confusing. I am using the correct base gasket. It is not a NOS design. I've run the engine more yesterday, and I believe it is no longer knocking at all with the second gasket installed. I'm going to take the head back off again today and see if there is any visible scoring on the piston roof. If there is not, I will remove the second gasket, and put a tiny piece of blu tac on the piston roof. I'll then manually turn the engine over, and see how much it is squished.

What do you think? This all feels positively sherlock holmes-ish.

Thanks for help so far.

If you can video the engine running, post on YouTube.

Olly, maybe use 'engineers blue' on the top of the piston rather than blu-tac, for more accurate assessment. Best regards Andy.

Best way to check is with a length of small diameter soft solder laid across the piston.

Can be done with the head on but much easier to remove the head, lay the solder across, rotate the engine, remove the head, measure the thickness of the solder.

Here's one tutorial.

www.bansheehq.com/forums/index.php?/topi...re-squish-correctly/

As requested here are some photos, busy week so have only just had the chance to pull it back apart. Certainly looks like piston is giving head a bit of love. Not to mention rear right hand head bolt was loose, I assume knocked loose by piston hitting head in that area.

Also, everything is an unhealthy looking brown colour, and oil in the cylinder is black.

What the hell do I do now? Stack base gasket?

A new head will be a fucking nightmare. I’m well over budget on this project and just want to ride the thing... 99% there.

Your head looks like it has been skimmed . It also looks to have been done unevenly .
There appears to be a small lip by your finger , but possibly nothing on the opposite side .
The marks on your piston , are only on the intake side . If the skimming was level , then if there
was contact , it should be all the way around . A good machinist should be able to recut the squish
into the head . To do it properly , you'd have to measure existing clearances , as Mark has already
suggested , and he'd probably need your piston , to get the correct angles off the piston dome .