Jump to content
HybridZ

Newly rebuilt engine with loud tapping noise even at idle?!!?!


JCan

Recommended Posts

Summary:  

 

After starting our rebuilt L28 the engine we heard an extremely loud tapping coming from the head.  Debugging the tapping required removing the head twice and going through the head with a fine tooth comb to determine the problem.  This thread documents the work we did in an effort to find the problem (of course the answer is at the end of the thread and in this summary).  

 

We found (and another member confirmed) the problem is caused by not dressing (slightly grinding) the rocker arm face that is in contact with the new lash pad such that the rocker arm touches the lash pad in the center of the cap.   The new lash caps are shaped like a U, the stock lash caps are shaped like an H.  The valve side of old rocker arms wear in the middle but not on the edges.  When you replace the old lash cap with a new flat lash cap, the rocker arm touches the lash cap at the edges and not the middle.  The reason the valves are noisy is due to the fact that you cant properly gap the rocker arm as caused by the rocker coming in contact with the lash pad at one edge and not in the center.  (This causes torque on the rocker arm when the cam depresses on the valve.  Take a look at pictures on page 3.

 

The engine sound can be found on page 2 and pictures of the rocker arms and lash caps can be found on page 3.

 

Enjoy reading the thread, if you have a problem with your valve train making lots of noise and you replaced the lash caps with new ones and did not grind the valve side of the rocker arm, then:

 

  1)  read page 3 of this thread

  2)  remove your rocker arms (be sure to number them along with the lash caps so you can replace them in the proper order)

  3)  slightly grind the valve side of your rocker arm such that you remove the high spots on the edges (make sure the rocker arm only touches the center of the lash cap)

  4)  reassemble, warm up your motor and carefully gap your valves

 

Please document your findings.

 

Best of luck - Jim

 

On to the original thread.

 

 

________________________________________________________________________

 

 

 

Hey Hybrid - Z team...  I'm stumped and would appreciate some suggestions.

 

I have just started my L28 and have a very loud tapping noise.  sounds like it is coming from the head, it also sounds like it is a very bad valve. but the folks that rebuilt the head are very good at what they do!

 

Background:    Engine - 1978 L28 with flat top pistons with a P90 head, milled 0.080, plus new valves and cam tower shims, compression set at 10:1, mild cam (head work done at Valley head service in LA.  Carburators and Electronic ignition are working great...  Just have to figure out what this tapping is...

 

I have done the following:

 

     - adjusted valve clearance

     - checked the oil pressure and ensured oil is flowing through the holes in the cam.  

     - I have removed spark plugs turned the engine with the valve cover removed.  (no noise, looks like the valve geometry is good)

     - I reinstalled the valve cover and checked compression and have 190 - 230 psi compression on each piston.

 

I am at a loss of what to do next,  It looks like I might have to remove the head?

 

Any suggestions would be appreciated.

 

Thanks

 

Jim

post-29130-0-57768500-1357169981_thumb.jpg

post-29130-0-87070300-1357170048_thumb.jpg

post-29130-0-60277800-1357170082_thumb.jpg

Edited by JCan
Link to comment
Share on other sites

  • Replies 118
  • Created
  • Last Reply

Top Posters In This Topic

Top Posters In This Topic

Posted Images

Thanks...  The lash caps are in place.  I'm really dumbfounded.   I measured piston to head space and got approx 0.027 in .  Maybe, just maybe the noise is the pistons crashing into the head?  either way, I have to remove the head to take a look....  2 days more of work!  

 

sigh...

 

Jim

Link to comment
Share on other sites

When I was a kid, my brother and I ended up assembling a 2.8L motor and inadvertantly dropped a washer in #6 cylinder.  It made a very loud noise.    This sounds more like a really bad valve job to me, however when I turn over the motor using the starter with no spark plugs, the valve train seems to work perfectly with no noises...

 

 

Tomorrow I take off the head.

 

Thanks for your support

 

Jim

Link to comment
Share on other sites

Your piston to head to clearance is fine, but the valve to piston clearance with cam at desired setting also matters. Usually checked with a pre-assembly.

 

Even so, the one time I missed timed a cam the tapping went away after a few revs when the valves made there own pockets in the pistons and slightly bent. Then it showed with comp check.

Link to comment
Share on other sites

update.

 

I removed the head and have the following update.

 

Good news - there are no signs of the valves crashing into the pistons and there is no evidence the pistons are hitting the head.

 

I was able to take some measurements.

 

Using a metal straight edge and a feeler gauge.  I am measuring a piston to head clearance of 0.023.  My previous measurements (using a different head gasket) during assembly measured 0.027 inch.

 

Next steps.  Take the head back to valley head service and have them determine if they can find a problem with the valve train.  If they say it is ok then the only option I have is to assume the pistons are hitting the head and increase the gasket thickness from 0.050 to 0.070.

 

I took a stethescope and put it on the valve cover, it was loud.  When I put it on the head below the valve cover, it was much quieter. 

 

I am stumped.

 

Thanks for your inputs.  

 

Jim

Link to comment
Share on other sites

Mousetrap springs on backward or missing?  Lash clearance mis-interpreted as inch/millimeters or visa versa and now wayyyy too wide? grasping at straws now...

 

Man a picture would be worth a thousand words here... Also some audio would help us understand if its one cylinder or all of them (from frequency of noise).

Edited by z240
Link to comment
Share on other sites

Is the tapping frequency matched to enfine rpm - meaning do yopu get one tap per engine revolution?  Or do you get one tap every other engine revolution?  Or do you get 6 taps every engine revolution?  You see what I'm getting at...

Link to comment
Share on other sites

John. - I think it is one per revolution.

 

Update.

 

I made a mistake!  When I was cleaning the head with acetone I noticed a very small collision in cylinder no 3 (pictures  below)

 

It looks like a very small piece of sand was on the piston and crashed into the head and made a divot.  I have decided to place two head gaskets on top of each other and reassemble the engine and start it to see if the rattle goes away.  If it does, it means that my clearance between piston and head is too close.  If it does not, then the problem is probably the cam/rocker arm/ lashpad area.

 

 

 

 

Link to comment
Share on other sites

John. - I think it is one per revolution.

 

Update.

 

I made a mistake!  When I was cleaning the head with acetone I noticed a very small collision in cylinder no 3 (pictures  below)

 

It looks like a very small piece of sand was on the piston and crashed into the head and made a divot.  I have decided to place two head gaskets on top of each other and reassemble the engine and start it to see if the rattle goes away.  If it does, it means that my clearance between piston and head is too close.  If it does not, then the problem is probably the cam/rocker arm/ lashpad area.

 

 

 

 

post-29130-0-74237700-1357428500_thumb.jpg

post-29130-0-20248900-1357428539_thumb.jpg

Link to comment
Share on other sites

Okay.  I am reassembling the engine now with two head gaskets 2.4mm and will start the engine.  If it rattles, I will take video and audio.  I will also take more pictures.

 

Guys thanks!  I really appreciate the help!!!

 

Jim

Link to comment
Share on other sites

Alright... bad idea.  Of course there is no way I can add an extra 1.25mm under the head without stretching the new timing chain and chain guides....  I tried!

 

Regroup!  New plan.  I am going to purchase a custom head gasket which is 1.6mm (old gasket 1.2mm).  This should give me a piston to head gap of 0.035" (sorry for mixing units) instead of my current 0.024".  I found a shop close to me (Pasadena CA) Gasket Works USA LLC.  

 

Unfortunately, I will have to wait a week for the new gasket.  

 

Thanks for your support I really appreciate it.

 

Jim

Link to comment
Share on other sites

Using a metal straight edge and a feeler gauge.  I am measuring a piston to head clearance of 0.023.  My previous measurements (using a different head gasket) during assembly measured 0.027 inch.

Did you just stick a straight edge on to an uncompressed gasket on the block and use a feeler gauge to measure the gap betwen the straight edge and psiton top?  I think a stock gasket compresses to 1.25 mm (.049").  If so, you might recalculate to figure out what the real clearance is.  Can't tell what your method was.

Edited by NewZed
Link to comment
Share on other sites

NewZed

 

When I removed my head, I used the compressed gasket (The thickness was 0.050" as measured with calipers).

 

I used a metal straight edge and spanned it across the gasket/piston such that the edge was down and the flat part was perpendicular to the piston surface.  then I slipped a feeler gauge between the piston and the straight edge. 

 

FYI - Jim

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.


×
×
  • Create New...