Valves hitting piston after rebuild

[golf-sib]

Timing is 100% spot on the bike engine I am working on, all I did was fit +1mm lift cams and a 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 same big bore kit on its own (without uprated cams).

Anyone have any ideas on anything that can be done, such as maybe a spacer/wider gasket as timing isn't the issue. It's the first time I've done a rebuild and I had a mechanic help me do it, but we never checked the engine turning over until he left when I discovered the piston & valve issue, so I have no idea what options are out there.

Any help is appreciated.

[Chris.]

Different stroke on oversize stuff?

[golf-sib]

Spec wise the 125 and 150 engines have the same stroke just wider bore, but I bet the 150 engine has a shorter con rod due to the bigger wrist pin which is 2mm wider which is possibly why the piston is now hitting the valves. Which is just a pure guess.

Problem is I'm amongst the first people to do this mod and hardly any information exists which put me in a pickle. There's very little information out there. I think I need a solution of some sort

[tshirt2k]

Cut outs in the pistons? Can get them machined. Or swap the old pistons back and do some measurements to see if the piston is higher up the bore. Piston may have a higher CR.

[G601950cc]

You need to make a measurement from centre of gudgeon pin to crown of piston on bid bore kit you may find your new piston to be taller wIth potentIally higher  compression hence the touchy feely.
Your option is modify piston, or shorter con rods.
YOU NEED A THICKER HEAD GASKET

[golf-sib]

Piston has valve cut outs which are bigger and deeper than stock piston, think thicker gasket is the answer but no thicker gasket sets exist unless I double up but I don't think that will give enough clearance

[golf-sib]

really need 2 or 3mm gasket, any ideas what material would be suitable between the head and the block?

Stock gasket looks like this:

http://www.ebay.co.uk/itm/125cc-ATV-QUAD-BIKE-HEAD-GASKET-SET-KIT-125-cc-MONKEY-PIT-BIKE-54-3mm-/140866418825?pt=UK_Motorcycle_Parts&hash=item20cc4af889

[G601950cc]

I'm sure I've seen on internet a company that makes gaskets, usually for engines that are bit vintage however your gasket will be sandwich of gasket paper between two thin sheet of tin. Pressed together also high temp ring is pressed together. I'm very surprised you can't get a 2 or 3 hole gasket thats thicker.

[G601950cc]

Is this engine Chinese!!!

[golf-sib]

Hell yeah it chinky, well I've took head bolts up and measured the distance I need to clear it seems about 2.5mm, ordered two honda cb150 gaskets at 1.5mm which I am tempted to sandwich or as an alternative possibly use 2mm copper and make my own gasket as copper can take compression quite well and a 1000c melting point considering its becoming a popular material for head gaskets again on cars.

Ideally this engine needs to be running by friday but it is a pretty w@nk design its more designed to be disposable than actually reworked.