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Author: Subject: Motorbike engine rebuild

posted on 18/3/14 at 09:09 PM Reply With Quote
Motorbike engine rebuild

Hey guys,

I'm about to start rebuilding an old motorbike engine (1970s CB360) and I am after some advice on a few things. I've never done anything to this level before so just want to make sure what I do is right before getting started.

Cylinders: I am going to be running standard size pistons and rings, so cylinders wont be bored, but should I hone them out just to clean them up? I've seen people running a drill fitted hone through, is this a wise thing to do?

Valves: I am going to oull the valves out and give them a refresh. Currently they have a good coating of carbon on the underside. So theyll need a good cleaning. Whats the best way of cleaning up valves without damaging them? Ill also need to clean up the top of the cylinder (where the valves seat). So hows best to clean this as well?

I take it I'll then need to lap the valves so a good seal again? Any tips and pointers on doing this right first time?

Piston: There is a bit of deposit on the crown of the pistons, hows best to clean this without damaging anything?

Cheers guys!

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posted on 19/3/14 at 12:02 AM Reply With Quote
Cool bike squire,

If you are replacing rings, yes get them honed properly. its cheap and allows the rings to bed properly. Its actually quite a skill/important to get right.
If you aren't replacing the rings (you really should while its apart) then a clean up with green abrasive pad will be ok.
Pistons can usually be reused but worth measuring them to spec.

After honing clean the bores like a mad man until you see no contamination. A little oil on kitchen towel is a good indicator of this, rub on the bores and the towel should still be clean.

Do you have access to a pillar drill ? these are very handy for cleaning valves. careful use of fine wire wool (not on valve contact area) will have them cleaned in no time

Oven cleaner will help soften carbon from the head/piston tops as will a WD40 soaked rag.

Definitely replace the valve guide seals as these go hard over time.

I still like lapping valves by hand (wooden stick with a sucker on the end) this is pretty tedious on 16 valve engines but won't take long on the CB.
This way you take the minimum off and get a perfect seal.
You can buy small double ended tubs of valve grinding paste, one end medium, the other fine.
Just use a little at a time, cleaning and checking your work until you get a complete even seal.

For the cost (20-30) may be worth getting the head lightly skimmed, the slight bump in compression will do the old girl good

Apologies if you already know most of above

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posted on 19/3/14 at 09:26 AM Reply With Quote
Some great info Super,

Appreciate all of the advice. Never done a full engine build myself so great to hear some important steps.

I am replacing the rings so I will try honing the bores. Is this something I can do myself? Have you got any experience of the hand drill mounted honers?

I was planning on re-using the piston, Will give them a good check for condition.

I have bought a full gasket kit, which has valve stem seals so they will be getting replaced.

Yeah i want to give lapping by hand a go. Seen it before and it looks very satisfying, and yes hopefully my 4 valves won't be too tedious (I do love the simplicity of these old engines!)

Is there anything specifically i need to look for/avoid when buying the grinding paste? Can you check the seal without the valve spring in place? Or do you need to lap them, fit the valve springs, check the seal, and if it leaks, take it apart and try again? (sorry is this is a stupid question!)

Cheers again.

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posted on 19/3/14 at 09:37 AM Reply With Quote
Have you actually checked that the bore is in good shape, i.e. not significantly worn, no lip etc?
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posted on 19/3/14 at 09:56 AM Reply With Quote
Bore looks in good shape, No lip, so unusual marks or gouges along the length,
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posted on 19/3/14 at 06:38 PM Reply With Quote
valves - stick em in drillpress and use wet n dry on the bottom and top of the valce head AVOIDING touching the seating face; same for the stem BUT NOT the part that goes in the guides.

oven cleaner on the pistons and toothbrush - may beed several goes. for stubborn depositis use a blunt knife but be careful of not gouging the piston top, assuming they are alloy which they most likely will be.

Visit China. Meet the child that made your trainers.

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posted on 19/3/14 at 07:10 PM Reply With Quote
Thanks for the advice, will have a go over the next week.
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posted on 2/4/14 at 12:00 PM Reply With Quote
I would get them honed professionally, it cost very little and is vital for bedding the rings in/future oil control. The angle and depth of the cross hatch really are critical.
A pro will also check everything is within spec.
You could argue for a basic engine is it important, personally I think it is.
I've seen countless engines with bad blow by from poor ring sealing, mainly due to bad honing techniques and the use of synthetic oils.
Some oils are so advanced these days, that it is almost too good to allow initial ring/bore wear for correct sealing.
I would also use a mineral oil for running in before changing to a bike specific semi synthetic

With the valve seats, engineers blue will show the areas that seal, or you can get a good visual (decent lighting + magnifying glass are helpful) when you have a uniform flat grey seat area, after lapping the valves, with no spots or uneven areas. Just go slowly, start with the medium paste and check it every minute or so, cleaning the paste off and checking the valve and the seat. You will see any dark spots on that first run. I only use the medium for the first couple of passes before switching to the fine stuff.
Don't worry you don't need to reassemble the valve's/springs to check the seal !

You could also tidy up the ports while its in pieces (do this before you lap the valves in) CAREFULLY blend the valve seat into the port wall, keeping well away from the sealing part of the seat. A small nick will ruin it !. Remove any casting flash and match up the inlet rubbers to the ports. On most mass produced bike from this era the valve seat/port area can be improved upon and will give you a more efficient end product

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posted on 2/4/14 at 12:16 PM Reply With Quote
thanks for the advice supersport
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