Valves hitting piston after rebuild

**arancara** · 06-23-2013, 11:42 PM

Timing is 100% spot on, all I did was fit +1mm lift cams and a 156c big bore kit, I tried turning it over by hand and I can hear the valves hitting the piston, someone on another forum said he came across the same problem with the big bore kit. As he was the person that pre-warned me about the new wrist pin being bigger, which was bored out to accomdate it.

Anyone have any ideas on anything that can be done, such as maybe a spacer/wider gasket as timing isn't the issue.

Any help is appreciated.

**4950cycle** · 06-24-2013, 02:38 AM

If it does complete revolutions there can't be to much interferance coming from the valves hitting the piston . I'd take some ink of some sort, put it on the valves, slip the head back on loosely turn the motor over and see exactly where the valves are touching the piston by seeing where the valves deposited ink on the piston.. I would think there barely touching again because its not completely jamming the motor up. Just touch the piston with a grinder where the valves are hitting the piston. Remember to take a couple thousands more metal out for when the valve stem gets slitely longer from being hot when under operation.

**Morfar** · 06-24-2013, 07:13 AM

Mount extra gasket under the cylinder - don't know the type/thicknes of the gasket there, but maybe one or two extra will do it.....
Cheaper and better/stronger than changing the top gasket.
Good luck!!

**Zorge** · 06-24-2013, 09:35 AM

Аre you sure you can hear valves knocking on the piston? It often can happen that collide piston and head.

Note the bright ring.

sudaranje.jpg

One more gasket below cylinder could be cheapest cure.

Or milling machine...

**spencer2004** · 06-24-2013, 10:50 AM

Are you sure that you got the valve clearances correct? Add a little bit more clearance to each valve and see if the problem persists.

**arancara** · 06-24-2013, 03:00 PM

Yes I'm sure, if you have inspection holes open at the valves, and a gauge down the spark hole you can see when turning over that at tdc the piston hits the valve, you can hear it too. Once collided it won't turn over as the valves now in the way.

Valve clearance has been checked, and re-adjusted still the same issue. I have found someone else who has the same bore kit and he hit the same issue. What I think it is down to, the 150 head as lifan put it has the same stroke as the 125, but the 150cc has a bigger wrist pin so I imagine it has a shorter con rod, meaning putting the bigger head on the taller rod will cause this issue. What I've opted to do is I will release the tensioner and see the clearance of how much things are off by and make a custom head gasket out of copper (possibly 2mm) to create some space. I cant even source a 150cc gasket from the uk quickly at all and I don't think doubling the gasket will give enough clearance if I'm honest.

**Zorge** · 06-24-2013, 04:24 PM

If you ask me, I would tell you not to bother with head gasket. You know, high pressure, heat, etc.

Try with two or three cylinder gaskets - down there conditions are much less harsh.

BTW, did you tried will thing work with stock camshaft?

**Zorge** · 06-24-2013, 04:33 PM

And to tell you this to... please do not think I'm malevolent, but if you ask me, increasing displacement from 125 to 156 cc is ...

**arancara** · 06-24-2013, 04:43 PM

it is pretty wank, measured the clearance its 2.5mm out, which is exactly how much more or less the wrist pin is bigger by. I've ordered two more gaskets at 1.5mm and the copper to see what I can clear.

Putting stock camshafts back in won't make a difference as the same issue is occurring with stock camshafts from the other person I have spoke to and he has a similar clearance issue of 2-3mm