Helicoil Repair to Cam Studs

[William2]

I would value a few opinions on carrying out helicoil repairs to cam cap studs. I have 2 that need doing, one of which is the taller stud for one of the rear cam cover attachments. I was wondering whether this could be attempted with the head in the car still using a 5/16" UNC helicoil repair kit. I realise that for starters all the cam gear would come out and the surrounding area would need to be sealed off to eliminate any ingress of swarfe.

[el-saturn]

...not only will you want to block off all of the nearby "passages" you can also use this special grease that will keep all of the "shavings" where you want them. i d0n't use it so I can't give you any details sandy

[john.p.clegg]

Why take all the cam gear off? Surely if you're ultra careful and make sure everything is masked off, just remove the cam cap...

John

[Craven]

Most common problem I?ve seen with Helicoils is in the accurate drilling of the new hole size, unless you make some arrangement (jig) to ensure the new hole is drilled perfectly upright etc it will result in disaster.
Cam caps have a positive location via the collar, so there is very little wriggle room for error. You will need to do the same with the tap, as others have said the swarf will be a problem.

[fattogatto]

I might suggest using Time-Serts instead of Helicoils. More accurate installation.

[el-saturn]

....................use only helicoils!! all of those inserts (the ones i considered) will result in new dowels with a larger id! do yr arithmetic and you'll soon see what i mean --- precision/location:X,Y,Z etc. are important and you can't do it with your wife holding the head and you using your black and decker drill!! sandy: just had a new helicoil fitted on 4982

[alan.barker]

imho i would remove the Head, it's the least of your worries. Get the holes for the Helicoils drilled using a Pillar Drill and use cutting lubricant, do the job correctly. I worked in the Aerospace Industry and all the threads in alloy parts were Helicoiled no problem from new. You need Helicoils 1.5 x dia of screw. Be careful to use a punch the correct dia to break tangs off and be sure the bottom thread of the Helicoil has not jumped.
If done correctly it will be stronger than original.
good luck with the repair
Alan

[el-saturn]

....that's exactly what i wanted to say ALAN, BECAUSE on a proper modern machine, your dowel gives the center of your new helicoil hole and new thread - that's within 1/100mm - it will include a bit of work and some new gaskets!! good luck sandy

[fattogatto]

For an alternative with some very nice additional features you might check here:

http://www.timesert.com/

Obviously, your mileage varies. I would never use a helicoil to repair a stripped spark plug hole. I have used these in situ more than once.

[StressCraxx]

+1 on the timeserts. I used them successfully years ago on a Volvo Turbo's exhaust studs They were the only thing that would not pull out from the thermal expansion.

[el-saturn]

.....i fully agree with you and I'd never use a helicoil as a plug thread substitute - but in the case of the camshaft cap threads there's not enough meat to fasten an insert properly ---- the insert has a flange/collar which a helicoil doesn't - sandy

[alan.barker]

+1
Alan

[rgh0]

This discussion has intrigued me as I have never attempted to put a cam cap helicoil in a head in situ in the car. Done it a couple of times in a head I am rebuilding where I can set the head up in a pillar drill to get the new hole central and vertical but i wondered if the tolerances was such you could do it in situ with hand tools. So for what its worth I had a look at it in more detail

I pulled out some parts and did some quick measurements as follows

Cam Cap Stud - 5/16 UNC
Thread OD - 7.70mm
Shank OD - 7.90mm

Cam Cap
Hole for stud ID - 8.80mm

Hollow Cam Cap Dowel
OD - 11.10mm
ID - 8.30mm

Helicoil - 5/16 UNC ( actually a "Recoil" which is the Australian made "Helicoil" equivalent)
OD - 10.0 mm
Tap Drill - 8.25mm
Tap OD - 9.80mm

The first thing you will see is you have a fair bit of clearance between the stud shank OD at 7.9mm and the hollow dowel ID at 8.3mm and the cam cap hole ID at 8.8mm. So you should be able to centre the helicoil tap drill OK particularly as the amount of material to be removed from a stripped thread hole is only around 0.5 mm diameter and precise central location is not critical

You would need to be careful and have good feel to drill the new diameter vertical and parallel with the existing stripped thread hole but the clearances are such that it appears possible to do. Once that is done, tapping and threading the hole and fitting the helicoil can be done without disturbing the hollow dowel location at the top of the hole and you should be able to fit a stud that goes up thought the dowel and camp cap given the clearances

Careful collection of all the swarf is critical using grease to trap it and sealing of the surround areas with taped on cloth or plastic sheet.

Personally I would pull the head to do it on the bench but it does not look impossible to do in situ. You could also make a drill guide jig that located in the hollow dowel diameter in the head to make drilling the tap diameter precisely vertical and central easier in situ with a hand drill. The helicoil tap drill is actually just fractionally smaller OD than the hollow dowel ID so you could use a spare hollow dowel as part of a guide to precisely locate it. Just remember you have a multi thousand dollar head so you don't want to stuff it up

It also looks possible to install a Timesert rather than an Helicoil if you felt the need as provided the OD of the top flange is less than the 11.10mm OD of the hollow dowel. If necessary you could turn down this top flange on the timesert to that diameter so it fitted inside the dowel location diameter.

cheers
Rohan

[el-saturn]

as always rohan - a great job BUT the problems i was looking at have to do with the dia. X (the collar: the special tool you need forces you to make new dowels etc.)! sandy

[rgh0]

Hi Sandy

I agree the diameter of the collar is the problem with using a Timesert. If it or its insertion tool is larger than than 11.1mm diameter hole for the hollow dowel and it cannot be turned down to be under that size then you cant use it. Helicoils have provied fine for me in this application the couple of times I have used them.

cheers
Rohan