Topic: Please help me make sense

[Phoenix]

In the process of putting helicoils in the eight holes for the cam caps.  The holes for the studs went in fine. I can't get the back holes to bite.  Put pressure on in and can't get past the first thread.  The front holes cut like better.  Also, the stud hole for the left-hand cap goes all the way thru.  Any ideas as to how I can get the front four drilled.

[madmtnmotors]

You may have created more problems for yourself than you bargained for. The cam caps are indeed held by studs towards the front of the engine and bolts towards the rear of the engine, if we are talking about a CB750. All four of the stud holes are "bottomless" in that they pass all the way through into the void above the spark plug well below. These studs usually require thread sealer to prevent oil seepage past these threads. By putting helicoils in place you now have two sets of threads, both the outer and inner threads of the helicoil repair. Others may be able to chime in about how they have successfully sealed these threads.

Now as far as the bolt locations towards the rear...

These bolt locations SHOULD NOT pass all the way through the head into the void below. There is not much material at the bottom of these bolt hole locations and if you mix up the tower center bolt(s) with the bearing cap bolt(s) you can easily "punch out" the bottom of these holes into the open cavity below. If you drill too deep trying to effect your helicoil "repair" you can drill into the cavity below. The problem with this is the oil feed to the cam bearings passes through the cam cap bolt bore in the cam tower. Look into this bolt bore of the cam tower and you can see the oil passage holes of which I speak, only at the rear bolt locations, not the front stud locations. If you "punch through" the bottom of the bolt hole locations in the head you will have high pressure oil behind these threads and these can leak profusely. I don't know if both the inner and outer threads of the helicoil could be effectively sealed if the bottom of the bolt hole has been compromised. If you have managed to avoid breeching the bottom of these four bolt holes so far, then you may well need a "bottoming tap" in order for the repair threads to insert properly. Otherwise, the traditionally "tapered tap" for the repair threads likely will not cut deep enough to get the repair threads inserted properly.

Best of luck. For the rerasons I've described above I have avoided helicoils at these locations if at all possible.

[Don R]

 Cycle X puts Helicoils in all of the 6mm threads.  He uses a special deep Helicoil to make sure they get enough thread. 

[madmtnmotors]

Cycle X puts Helicoils in all of the 6mm threads.  He uses a special deep Helicoil to make sure they get enough thread. 

From what I understand, they helicoil the 6mm bolts into the cylinder block and maybe the center tower bolts. The cam caps are a whole different animal.