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Scotch10
Borekit Bruiser



Joined: 16 Sep 2018
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PostPosted: 07:19 - 12 May 2022    Post subject: tapping Reply with quote

Hi I have a 1979 honda cb125t2. I have just replaced pistons, rings and put a brand new old stock cylinder head on. The head is built up with new valves springs seats and seals. I have put old camshaft, rockers, and camshaft holders back on but replaced tappet adjust screws. After all this I'm getting a tapping noise definitely from the head. Valve clearances double checked and set according to haynes,timing chain, guide and tensioner checked and are fine, valve timing double checked and fine, the head is correctly torqued.The tapping was not there before. Could it be the old camshaft bedding in to the new cylinder head otherwise I am at a loss. Any help greatly appreciated.
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bikenut
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PostPosted: 07:52 - 12 May 2022    Post subject: T Reply with quote

Check if piston crowns are "contacting" the head...to rule that out.

If you cleaned the piston crowns, look down plug holes and look for " clean" ring mark on piston crown edge...if piston crowns not clean, look for clean ring mark on piston edge...
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Scotch10
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PostPosted: 07:56 - 12 May 2022    Post subject: Re: T Reply with quote

bikenut wrote:
Check if piston crowns are "contacting" the head...to rule that out.

If you cleaned the piston crowns, look down plug holes and look for " clean" ring mark on piston crown edge...if piston crowns not clean, look for clean ring mark on piston edge...

I put new genuine piston and rings in.
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Islander
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PostPosted: 09:56 - 12 May 2022    Post subject: Reply with quote

Check your exhaust headers for leaks.
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MCN
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PostPosted: 10:49 - 12 May 2022    Post subject: Reply with quote

Only way for pistons to clatter the head is:
Worn big ends. (Piston connecting rod to crankshaft bearings)
Worn small ends. (Piston rod to piston bearings)
Massive layer of combustion deposits on crown/head.

I'd rule out the zost manifold leaks 1st.

We had a cnut put a main engine off-line for 10 weeks until a brand new cylinder head could be slow-boated into a rig in Iran.
Uncle MCS checked the Zost gaskets on that head. No gasket to be found. Shocked
New gasket fitted. Tappy noise now missing.

New head kept as spare. Laughing
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A100man
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Joined: 19 Aug 2013
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PostPosted: 12:29 - 12 May 2022    Post subject: Re: tapping Reply with quote

Scotch10 wrote:
The tapping was not there before. ..


Can you put the old head back?

Did you check the clearances (plastiguage) of the old cam in new head?

Any chance you could have stripped a cam-cap thread?
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stinkwheel
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Joined: 12 Jul 2004
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PostPosted: 12:45 - 12 May 2022    Post subject: Reply with quote

Are those heads not line-bored? So the camshaft caps go with the head? Many 70's and 80's jap bikes were.

Like A100man, my guess would be excessive play on the cam, only way to find out is to measure it with plastiguage.

Other classic small honda twin thing is loose camchain. The CCT spring isn't really strong enough to properly tension it in many cases. The done thing used to be to loosen the adjuster nut then nip it up again while pushing down on the tensioner slightly.
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jeffyjeff
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PostPosted: 13:35 - 12 May 2022    Post subject: Reply with quote

stinkwheel wrote:
Are those heads not line-bored? So the camshaft caps go with the head? Many 70's and 80's jap bikes were.

What he is saying is that the cam holders (caps) are not interchangeable between heads. The cam holders and head are matched, and can only be assembled in their original location and original orientation.
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Scotch10
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Joined: 16 Sep 2018
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PostPosted: 14:58 - 12 May 2022    Post subject: Reply with quote

MCN wrote:
Only way for pistons to clatter the head is:
Worn big ends. (Piston connecting rod to crankshaft bearings)
Worn small ends. (Piston rod to piston bearings)
Massive layer of combustion deposits on crown/head.

I'd rule out the zost manifold leaks 1st.

We had a cnut put a main engine off-line for 10 weeks until a brand new cylinder head could be slow-boated into a rig in Iran.
Uncle MCS checked the Zost gaskets on that head. No gasket to be found. Shocked
New gasket fitted. Tappy noise now missing.

New head kept as spare. Laughing

I have put old exhaust gaskets back on due to exhaust having to come back off for the clutch. Thanks for tip will let you know when I put the new ones on.
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Scotch10
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PostPosted: 15:01 - 12 May 2022    Post subject: Reply with quote

stinkwheel wrote:
Are those heads not line-bored? So the camshaft caps go with the head? Many 70's and 80's jap bikes were.

Like A100man, my guess would be excessive play on the cam, only way to find out is to measure it with plastiguage.

Other classic small honda twin thing is loose camchain. The CCT spring isn't really strong enough to properly tension it in many cases. The done thing used to be to loosen the adjuster nut then nip it up again while pushing down on the tensioner slightly.


I dont think these are or you wouldnt be able to buy new replacements
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bikenut
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Joined: 21 Nov 2011
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PostPosted: 18:59 - 12 May 2022    Post subject: T Reply with quote

You know of CMSNL ?

I though these have " cam shaft bushes" that the cam bearing surfaces, the outer ends, run in ?

Check CMSNL to see if they are still available...

A picture of the cam area might be good....
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stinkwheel
Bovine Proctologist



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PostPosted: 19:37 - 12 May 2022    Post subject: Re: T Reply with quote

bikenut wrote:
You know of CMSNL ?

I though these have " cam shaft bushes" that the cam bearing surfaces, the outer ends, run in ?

Check CMSNL to see if they are still available...

A picture of the cam area might be good....


Right you are, they're bushed. I thought they just ran in the head. One less thing to worry aboiut then.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.
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Scotch10
Borekit Bruiser



Joined: 16 Sep 2018
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PostPosted: 20:06 - 12 May 2022    Post subject: Re: T Reply with quote

bikenut wrote:
You know of CMSNL ?

I though these have " cam shaft bushes" that the cam bearing surfaces, the outer ends, run in ?

Check CMSNL to see if they are still available...

A picture of the cam area might be good....

Yep I know CMSNL got the new harness I needed from them. The cam and bush are fine checked with guage. The bike has only done a definite 6500 miles in its 43years I did this because valve seals needed doing so might aswell do pistons and rings, didnt need a rebore I already had the brand new head so thought why not
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jeffyjeff
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Joined: 02 May 2020
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PostPosted: 22:15 - 12 May 2022    Post subject: Reply with quote

Wonder if the old valve covers on the new head fit in such a way that the rockers are striking the valve covers? Could try running the engine sans valve covers. Tent a shop rag over the head to catch oil splatter.
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Scotch10
Borekit Bruiser



Joined: 16 Sep 2018
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PostPosted: 22:35 - 12 May 2022    Post subject: Reply with quote

jeffyjeff wrote:
Wonder if the old valve covers on the new head fit in such a way that the rockers are striking the valve covers? Could try running the engine sans valve covers. Tent a shop rag over the head to catch oil splatter.


Cheers but checked everything was identical before going ahead
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