Big Bore Tick

[taotaolivin]

I recently installed an 80cc big bore kit on my scooter. It came with a new cylinder, piston, head and gaskets, but not a new rocker arm assembly. Once I installed, I found out the new head was 64mm but the original rocker arm assembly was for a 69mm head. So I put the original head (69mm) and rocker arm assembly on and didn't use the new head (64mm) that came with the kit. Now I hear a ticking noise when I run the moped.

I have the gap set at .04 of an inch, should I make it smaller like .02?

Or do I need to order a 64 mm rocker assembly to fit the new head and install the new head and rocker?

[FrankenMech]

The gap should be .004" (inch).

[taotaolivin]

Sorry I meant .004, forgot a 0.

[90GTVert]

Try 0.002" intake, 0.003" exhaust and see how that sounds. It's always going to make some noise.

[FrankenMech]

Get used to some noise. These are not hydraulic valves. When set with very small clearances you are more likely to burn a valve without constant attention to the valve clearance which equals more maintenance. Tight valves mean the engine is harder to start due to lower compression at low speeds. I like to hear some valve noise, an extra .001" tight will only produce a negligible increase in performance detectable only by a very sensitive engine dynomometer.

[GrumpyUnk]

Frank, the smaller the clearance, the longer the valve will be open. If larger, the valve will not start to move as the clearance must be closed as the lobe rotates. It will also close sooner, as the clearance will start to open up sooner, allowing the valve to close. You get max duration with a closer tolerance clearance.
My personal take is to check the clearance more often, but most OHV/OHC designs will tend to increase their clearance with wear. The moving parts in most cases wear more quickly than the valve seat material, leading to more clearance. If the engines were flatheads, the clearance would close up with wear, as the seat wears more than the cam lobe and tappet. OHV have more 'bits' wearing, so apparently open up even as the seat wears.
I could be wrong, but figure I'll just check as I like some tappy-tap, but not such that I sounds as if it is falling apart.
I do find it difficult to get consistent clearance, and have to remind myself to adjust, rotate the engine a couple turns, and check it again. The 'if the next size doesn't fit, I will be very close' method seems to be a good idea. If you want .002", use a .002" feeler and set the clearance. Then try to fit a .003" feeler. If it fits, you know your clearance is a bit larger than desired. If it won't fit, you know you are very close, especially if the .002" fits nicely. One thing not mentioned is that excessive clearance can allow for more 'impact' wear, when the rocker impacts with the valve stem. I have seen rockers that have a depression worn into the contact area that is the same diameter as the valve stem, figuring it was caused by too-large clearance allowing the rocker to tap on the valve stem too much. I blab too much...
tom

[FrankenMech]

Oct 14, 2020 13:44:10 GMT -5 GrumpyUnk said:

Frank, the smaller the clearance, the longer the valve will be open. If larger, the valve will not start to move as the clearance must be closed as the lobe rotates. It will also close sooner, as the clearance will start to open up sooner, allowing the valve to close. You get max duration with a closer tolerance clearance.
My personal take is to check the clearance more often, but most OHV/OHC designs will tend to increase their clearance with wear. The moving parts in most cases wear more quickly than the valve seat material, leading to more clearance. If the engines were flatheads, the clearance would close up with wear, as the seat wears more than the cam lobe and tappet. OHV have more 'bits' wearing, so apparently open up even as the seat wears.
I could be wrong, but figure I'll just check as I like some tappy-tap, but not such that I sounds as if it is falling apart.
I do find it difficult to get consistent clearance, and have to remind myself to adjust, rotate the engine a couple turns, and check it again. The 'if the next size doesn't fit, I will be very close' method seems to be a good idea. If you want .002", use a .002" feeler and set the clearance. Then try to fit a .003" feeler. If it fits, you know your clearance is a bit larger than desired. If it won't fit, you know you are very close, especially if the .002" fits nicely. One thing not mentioned is that excessive clearance can allow for more 'impact' wear, when the rocker impacts with the valve stem. I have seen rockers that have a depression worn into the contact area that is the same diameter as the valve stem, figuring it was caused by too-large clearance allowing the rocker to tap on the valve stem too much. I blab too much...
tom

I realize the "smaller the clearance, the longer the valve will be open" but the difference in performance is very very small for a .001" difference just to get less valve noise. My memory is fuzzy but I think cam durations are figured at .004" lift because the flow under that is inconsequential.

I too like some valve noise, when it is gone it is time to check the clearance again.

[taotaolivin]

I also installed an A9 cam, would this make an difference on the suggested rocker arm gap?

[FrankenMech]

Oct 14, 2020 19:40:37 GMT -5 taotaolivin said:

I also installed an A9 cam, would this make an difference on the suggested rocker arm gap?

I don't believe so.

[taotaolivin]

Wish I could attach a video so you could hear the sound.

[FrankenMech]

Did you check the piston to valve clearance?

[taotaolivin]

I don't know how to check the piston to valve clearance.