GolfGTIforum.co.uk
General => The garage => Topic started by: golf-sib on 24 June 2013, 00:50
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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.
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Different stroke on oversize stuff?
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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
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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.
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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
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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
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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
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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.
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Is this engine Chinese!!!
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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. :grin: :grin: :grin:
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If it's Chinese its a through away design. However I used to make gaskets for a Suzuki ts 185er using cereal box and vauxhalls green gasket rtv worked fine. Tapping a little hammer against card whilst resting it on head to indent shape.
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hi m8 am a bike mechanic and own cleveland county scooters in teesside NE,
have you got the piston the right way round as the inlet valve (think it is) is bigger and the piston has a bider grove to take it,
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I've spoken to a couple of people and recon you might get away with putting 2 gaskets together to make up the difference. Not guaranteeing that though. But another point if relevant if fitted with a cam chain is it going to be long enough to fit with taller head gasket thickness?
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Piston is correct way round, it seems a common issue with these bore kits. What I've decided to do is make a 2mm gasket at bottom end and use stock 0.6mm head gasket if that doesn't give enough clearance then I will use the cb150 gasket for head at 1.5mm
only worry is timing chain should slip on but the more you space things out the more you knock timing out.
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I also have an entertaining video of how we removed the camshaft
www.youtube.com/watch?v=bCYtyGaRIB8
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Right!
Build the gasket, yada yada, got the engine running but the timing chain was under some stretch, when I cut the engine out it snapped the chain, no damage to valves etc, but now I'm in a dilemma can anything actually be done?
One idea is to add two more links to the chain, links are 8mm long and the stretch is 6mm overall, so with the tensioner in thory it may work, but I'm sure adding additional links would knock timing out, correct?
Other method is to put it all in the bin :grin: :grin:
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extra links in the chain won't affect timeing.
have you thought about chmber volumes ect, just wondering with spaceing th ebore up your effectivly lowering your compression ratio also doing the same with thick head gaskets.
another approach is woud it be worth moveing the piston down the bore by doctoring the con rod or piston itself
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In essence the compression should be fine as the 150 kit uses a shorter con-rod, but since no one has yet documented this upgrade its literally like walking in the dark too continuous surprises.
What is this doctoring you speak off?
Hmmm the link option is a high possibility then, can i break the links the same way as a bicycle chain by pushing the rivet out, or would it weaken the chain to much?
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I think if the chain has snapped once its fit for the bin. there is every chance it will break again.
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Would it be easier to look where the contact is on the piston and machine/grind a bit off so it clears?
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rallye, I've sorted the valve issue, I did want to debate to dremel it out, but I had the pistons ceramic coated, all is well now
I've ordered a new chain, some new links as well, lets see what happens!