Topic: CB750 Tapping Noise from Top End after Rebuild

[KickstandDan]

Are you sure you replaced and tightened the conrod bearing bolts properly? So its not the pistons hitting the cilinder?

Verstuurd vanaf mijn SM-A536B met Tapatalk

I am confident that the lower end was all torqued properly. The noise sounds very high up on the engine. Today is the day that I reinstall the headers and test it out to see if the noise returns or if the issue was exhaust leak out the header to flange connection. I will keep you updated and write a concluding post if the issue is resolved.

[bryanj]

I read somewhere that some pattern exhaust did not fit 100% without trimming the pipe that goes inside the spigot

[KickstandDan]

I reassembled the headers and exhaust system and fired it up and I still hear the noise. I replaced the copper crush gasket and applied sealant and I still hear the noise. I will run it more tomorrow and see if the noise persists. Next I will open the valve cover.

[KickstandDan]

I would like to add too that I have use the delkavic 4 into 1 headers and I had no issues. My guess is that the noise has something to do with the top end. I will use the stethoscope and try and figure out where the noise is coming from before removing the valve cover.

[scottly]

For the third time, use a length of rubber hose as a stethoscope to locate an exhaust leak. Please post a pic of your exhaust where it fits onto the head. Also, is the exhaust supported at the back, or is it hanging off the head?

[KickstandDan]

I explain this each time but here we go. The original exhaust leak was large enough that smoke was visibly coming out of the exhaust headed. This is not a small leak. I removed the headers and sealed things up properly like delkavic advises. I had forgot sealant went reinstalling the headers. Now with sealant the noise is still there. With everything properly sealed the issue is not an exhaust leak. The leak is gone…

Also, I have used the delkavic 4 into 1 system without any issues in the past so that is why I think the issue is now not related to the exhaust.

The exhaust is supported in the back and is NOT hanging.

[KickstandDan]

Why did you use the Phillip head screws for the towers? I have replaced the originals with 12.9 Allen's that are longer for complete thread engagement. I don't think those screws would cause e a knock but.....😄
 Personally I don't see the need for all the oil holes....it's not necessary. Also never had problems with the 5mm screws holding the shafts...a solution looking for a problem IMO.
 As somebody else said turn it over slowly...maybe a valve is bent a bit and hanging in the guide. And change those screws out for the correct hardware.

Mystery Tapping Noise Solved — Stretched Cam Tower Bolts

Wanted to post a quick update since many of you helped troubleshoot this. I had been chasing a tapping noise on the left side of the engine despite checking valve lash, oil flow, exhaust leaks, and carb sync. Someone had previously mentioned the bolts I used to secure the cam towers weren’t stock, and turns out they were right.

Both exhaust-side center cam tower bolts had stretched—one so badly I could remove it by hand. The bolt nearly broke off when removing it, that's how stretched it was. My guess is the cam tower was rocking slightly with each revolution, causing the rhythmic tapping noise. I’ve got high-strength replacements arriving tomorrow, and I’m confident that’ll fix it.

Lesson learned: don’t skimp on cam tower bolts. Appreciate the help, everyone.

[CycleRanger]

Wow, glad you caught that!

[Don R]

 Check your other cam tower bolts and threads, the cam cap studs, bolts and aluminum threads would have been stressed by the moving towers. It's pretty common to need thread repair in that area.

[PeWe]

All those bolts and nuts are often overtightened.
Even Honda manual has very optimistic torque for those M6 threads in aluminium.
I tighten 8-9Nm despite thread inserts.
The M5 screws holding shafts, 5Nm.

I use a torque wrench for all screws, bolts an nuts.

The Honda stock bolts holding the towers are rigid and cost some as new.

[KickstandDan]

Do you think it would be a good idea to remove the top end components and retorque the head? I have a mls head gasket so I have heard you don’t need to retorque that. While I am here I can check on the status of the other threads too. Just curious.

The 4 cam tower bolts have heli-coils added to give the bolts more threads.

I called Ken at Cycle-X and he told to remove the cam towers to make sure the head pucks didn't shift and will now cause a leak.

[Dr. Frankenstein]

This might be a silly suggestion, since I don't have a 750, but on my CB500 and CB550 I had a similar tapping; I re-did the valve lash and adjusted the cam chain adjuster and that helped a lot. Just an idea...

[KickstandDan]

Big thanks to everyone who chimed in on this thread. I finally found the cause of the tapping noise, it ended up being the shaft stopper bolt in the cam tower. I had used a lower-grade bolt, and it stretched under the stress of the upgraded valve springs and cam.

Ken at Cycle X pointed me to some ARP replacements he uses for high-performance builds. If you’re running stiffer springs or a more aggressive cam, I’d highly recommend swapping to these for peace of mind. These might be overkill but peace of mind is priceless.

Find these at Summit Racing:
• 660-1010 – 65mm M6x1.0 ARP bolt (shaft stopper)
• 660-1008 – 55mm M6x1.0 ARP bolt (cam retainer)
• 300-8390 – 6mm ARP nut (cam retainer)

[Don R]

 It's an awesome thread when the problem is discovered and we have all learned something from it. Often threads just go dark and we never know what the problem was.
  Thanks for continuing to ask and share, Dan. I just spent some $$ on stainless mufflers and pipe at Summit but in my next order I'll add some of this hardware. In my basket already, W0W a few more nuts and I can get free shipping.

[Oly]

+1  Great thread and excellent follow-thru.  Tis what the this forum is all about. 

[MRieck]

Big thanks to everyone who chimed in on this thread. I finally found the cause of the tapping noise, it ended up being the shaft stopper bolt in the cam tower. I had used a lower-grade bolt, and it stretched under the stress of the upgraded valve springs and cam.

Ken at Cycle X pointed me to some ARP replacements he uses for high-performance builds. If you’re running stiffer springs or a more aggressive cam, I’d highly recommend swapping to these for peace of mind. These might be overkill but peace of mind is priceless.

Find these at Summit Racing:
• 660-1010 – 65mm M6x1.0 ARP bolt (shaft stopper)
• 660-1008 – 55mm M6x1.0 ARP bolt (cam retainer)
• 300-8390 – 6mm ARP nut (cam retainer)

I mentioned using the longest 12.9 Allen bolts you can fit for complete thread engagement. You can trim them if necessary. Putting 12mm length Helicoils in the cam tower hold down bolt holes is a good idea too.
 ARP bolts are expensive and 12.9 hardware isn't bad at all in regard to price. I use a company called Bel Metrics....very good folks with very quick shipping.

[KickstandDan]

Quick update: I installed the ARP bolts and nuts from Summit Racing and they fit great. Just a heads up, don't use the washers that come with the bolts or the heads will hit the underside of the valve cover. The bolt heads are already flanged, so there's no issue running them without washers. Everything cleared fine and the engine noise is gone.

Also want to agree with the comment about 12.9 grade bolts. Those will definitely do the job. The ARP hardware is probably overkill, but it gave me peace of mind with the upgraded springs and cam.