How to fit an 85cc big bore kit
This is a really basic breakdown of how to fit a big bore kit - i would hope that the instructions are that basic that anyone with little knowledge of anything mechanical can follow it - therefore if you are a trained mechanic it may have more instructions than you actually need - but hey! we all had to start somewhere!, and if there are people on the forum who are willing to try and do the work themselves - i think we should provide them with as much help as possible. So here we go:I am making the assumption that the kit includes: Barrel, piston & rings, carb, rear shocks and smaller rear sprocket - this is a basic up-grade kit - however if you have bought a cylinder-head as well post and say and i will give a breakdown of how to build that too.
Please bear in mind this is for a 85cc kit – i.e. a 51mm barrel and piston – if you are fitting a 88cc kit with a 52mm barrel you may be able to get do the same – however it depends on the alignment of the castings on the crank case – sometimes these need to be bored out to allow the 52mm barrel to fit!
The most essential part of any rebuild is the care you take. The first time you ever do it, it will take quite some time, but after you have done it a few times its easy. Cleanliness is of primary importance. Just because it might look scruffy sitting in the frame, the inner parts are always clean and must be kept this way. Before removing the any bits from the engine make some effort to prepare a working area. Try and get into this habit of cleaning bits as you remove them, taking some time to examine each bit for wear, cracks and other damage. Scrape off old bits of gaskets using a sharp blade, but take care not to remove slivers of aluminium as you do it.
Once you have removed something, try to keep it with other related components. If it has machined surfaces, try and keep these wrapped in kitchen paper towels. Do not store the parts in a damp atmosphere, if it is going to be a while before you rebuild then make sure that the parts are wrapped up and put in lidded cardboard boxes, out of the way.
When everything is in bits you should be able to play around with all the parts without getting your hands anything but slightly oily.
So now you are prepared these are the jobs:
Before starting this job - remove the exhaust!...sorry!...i forgot all about the exhaust when i wrote this (Friar Monk has just pointed this out to me..i thank you sir!). Still not a bad attempt i thought considering it was done one night while i was sitting in front of the computer...without an engine in sight. Oh and at the end put it back on again!. This invloves undoing two 10mm nuts at the front of the exhaust and normally another nut 13/14mm on the top of the rear shock...some other bikes like the Dax have an additional bolt halfway along..14mm. Some of the exhausts have two small collets, which fit inbetween the clamp mand the head...remember this when you come to re-fit it...otherwise the exhaust will blow.
- 1. Remove from mudguard - two 10mm headed screws located under the mudguard
- 2. Turn off petrol and remove petrol pipe from carburettor.
- 3. Remove the spark plug cap – and tape the lead out of the way.
- 4. Unbolt standard air-filter box and or unscrew jubilee clip on rubber hose.
- 5. Unbolt two 10mm headed screws that hold the manifold to the cylinder head.
- 6. Taking care not to rip any gaskets – [unless you have new ones!] – gently pull the carb, and manifold together to one side of the bike.
- 7. Unscrew the top of the carb.
- 8. Remove the slide from the accelerator cable – this is done by depressing the spring – twist the throttle to full – and then hold the slide where it is and release the throttle – you will now see that the cable is loose – you can now pull the nipple out of the bottom of the slide – [be careful as you let go of the slide because the spring is now loose!].
- 9. Tape the accelerator cable out off the way.
- 10. Remove manifold and carb.
- 11. Slacken the tappet hole caps. It is easier to do this with the engine complete than with the cylinder head off if they are stiff.
- 12. Remove both the cylinder head side covers. One of the bolts on the right hand cover runs through the centre of the camshaft and holds the left hand cover on from the inside.
- 13. Rotate the engine until the "O" mark on the cam sprocket aligns with the notch in the cylinder head. Remove the three 9mm headed bolts holding the sprocket on and remove the sprocket. You might have to give the sprocket a tap on one side with a soft hammer to free it from the camshaft, then you will have to unhook the chain.
- 14. On the sprocket side of the head is are two bolts, one holding the cylinder to the barrel and one holding the barrel to the crankcase both 10mm - normally with a cross head as well – remove these.
- 15. Remove the 4 nuts holding on the cylinder head top cover.
- 16. Pull the cylinder head off, taking care that the cylinder barrel does not come off as well. The cylinder head might want some careful tapping with a mallet/ soft hammer to free it. Take care not to damage the cooling fins.
- 17. On the chain side of the cylinder barrel is a bolt. Remove this, as it is the spindle for the cam chain idler gear.
- 18. Remove the cam chain idler gear.
- 19. Remove the cylinder barrel, it may need a few knocks with a mallet to free it from the crankcases. Take care not to damage the piston.
- 20. Keep an eye on the tubular dowels that locate the barrel in relation to the head and crankcase. If they are loose, then note where they should go.
- 21. Place some rag around the connecting rod where it enters in to the engine – this is to stop any dirt or piston circlips falling in – if they do – you need to strip the engine! – so please use a rag, and wrap it round nicely.
- 22. Use a pair of long nosed pliers to grip one end one of the piston clips and gently lever out the clip.
- 23. You should now be able to push the gudgeon pin through the piston from the other side far enough to lift the piston away from the connecting rod. If the piston pin is tight in the piston then you need to lean the piston against a padded block of wood with a hole in it. Now you can tap the piston pin through, although you might want to remove both piston pin rings to do this.
- 24. Carefully scrape off any remains of gasket material. A sharp, straight blade is good for flat gasket surfaces, although you must take care not to remove the metal as well.
- 25. Clean out the carbon from inside the combustion chamber - a worn out flat screwdriver, with well-rounded corners, is pretty good.
- 26. Stop work - clean your hands and have a cigarette or a cup of coffee – and congratulate yourself - you are now half way through!
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- 1. With the new gasket set you will notice there are a number of square section O rings. These fit in holes in the cylinder gaskets for the holes where oil finds its way to the head and back. Make sure you know where they go.
- 2. Fit loose dowels into the NEW barrel.
- 3. Fit the piston rings in the piston – have a look at this picture it should help http://www.honda-dax.com/gallery/albun94/aav
- 4. There are normally three rings – however, on some kits there are five rings – this is because three of the rings make up the oil ring.
- 5. The top ring i.e nearest to the top of the piston - should be the chrome ring – followed by the black ring – followed by a thin ring then a zig-zag ring than another thin ring.
- 6. Fit one of the circlips to the new piston.
- 7. Put the piston back on the connecting rod, noting that the arrow/'IN' word on the top of the piston should be on the inlet side of the engine.I.e pointing up to where the manifold will bolt on later.
- 8. Push the gudgeon pin through and fit the other circlip in the piston.
- 9. Remove the cloth from around the piston.
- 10. Arrange the piston ring gaps so that they are 270 deg apart, and non at right angles to the piston pin.
- 11. Apply a thin layer of engine oil to the piston rings and the cylinder bore.
- 12. Fit the thin paper crankcase to barrel gasket over the engine studs.
- 13. Put the cylinder barrel onto the engine studs and pull the cam chain through - guide the piston into the bottom of the bore. By using your fingernails to compress the rings into their grooves you should be able to firmly press the cylinder barrel down onto the piston. Do not use excessive force as you may not have compressed the rings enough and they may be catching on the cylinder barrel – compressing the rings should not be that hard and there is a tapered lip on the base of the barrel to help them locate properly. Before pushing the cylinder barrel all the way down ensure that the cam chain is not jammed – and that the square section O ring is in place.
- 14. If the cam chain has dropped back down the barrel – don’t worry! The cam chain can either be hooked up using a piece of bent wire, or you can turn the engine upside-down and shake it through. The first is much easier as you will also need to feed it through the cylinder head. Do not use wire that is too stiff.
- 15. Drop the chain guide roller into the cam chain chest in the cylinder barrel casting and fit the spindle bolt.
- 16. Fit the cam chain bolt.
- 17. Get some thin soft plastic coated wire and tie it through the cam chain.
- 18. Fit the head gasket and its 2 'O' rings.
- 19. Using the wire pull the cam chain through the head.
- 20. Push the head down onto the hollow dowels.
- 21. Put a screwdriver or small bar through the cam chain and push if home into the hole in the centre of the cam –[ this will stop the cam chain dropping back into the casing]
- 22. Remove the thin soft wire.
- 23. Fit the rocker gasket and cylinder head top cover – [ I personally put 6mm copper washers under all of the cylinder head nuts this stop any oil leaks – you can buy these from and Halfords type shop]and nuts and tighten. Don't tighten the nuts one at a time, once the slack is taken up, tighten them a little, one after another using a diagonal pattern until you finally reach the final torque.
- 24. Hopefully, if neither the engine nor the cam has moved, the timing should be correct as you fit the cam sprocket to the cam. Check that the flywheel 'T' mark is still aligned when the cam sprocket 'O' mark is aligned with its notch. If not then you will have to feed the cam chain around the sprocket a link at a time until everything does.
- 25. If not, with the cam chain tensioned by holding it in one hand (this stops it jamming), rotate the engine until the 'T' mark on the flywheel aligns with the pointer on the side case.
- 26. Remove the screwdriver or bar holding cam chain!
- 27. Fit the cam sprocket to the cam chain so that the 'O' mark is away from the engine, in about what you estimate is the right position to align with the notch in the cylinder head when you fit the cylinder head.
- 28. It is best to do this with the engine on its side, chain side upwards, then the cam sprocket won't tend to fall back down inside the cylinder barrel cam chest.
- 29. Fit the three 9mm cam sprocket bolts and tighten.
- 30. Recheck the cam timing.
- 31. Fit the right and left cylinder covers.
- 32. Adjust the tappet clearance (this may not need to be done – but being as you are putting new parts on it is better to make sure that the gap is correct). The clearance is the same for both tappets at 0.002 in. (0.5 mm).
- 33. Rotate the engine a couple of times to make sure that it feels OK and that nothing is jamming it. Don't use excessive force, because if you need to then something has not been assembled correctly. This is to check that the valves are not hitting the piston – and that the timing is not out.
- 34. Have a break wash your hands - and have another fag! - Or coffee- cup a tea etc.. – or just chill for a while – you have now finished the hard bit – it is all plain sailing from here on in!
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- 35. Put a new NGK CR7HS plug in and tighten it up.
- 36. Rotate the engine a few more times, make sure you have no parts left over and that everything on the engine looks OK. This is to check that the spark plug is not hitting the piston.
- 37. Reconnect the spark plug cap.
- 38. Bolt your new 20mm manifold to your 20mm carb. – If you have bought a power filter attach this if not re attach the old air box – [personally I would always use a power filter – like a K&N or S&B type – but if you are using the old air-filter I would recommend taking the foam out – unless you will be riding you bike in really dust environments].
- 39. Unscrew the top of the new carb and take out the slide. Fit the accelerator cable in a similar fashion to the way you took the old one off.
- 40. Replace the slide in the carb – note: [the carb has a small spigot inside which aligns the slide – and a cut-out which should line up with the tick over screw on the carb].
- 41. Screw the top of the carb back on.
- 42. Attach petrol pipe to carb.
- 43. Refit front mudguard.
- 44. Switch petrol back on!
- 45. Kick bike over a couple of times with the engine switched off.
- 46. Switch engine on and kick bike up – it make take a couple of kicks – as the petrol needs to get into the new carb.
- 47. Wash hands and clean up work area.
Job done! – one 85cc kit fitted!