I'll qoute the Stu's book pg 140.

"In high mileage M103 engines, it is not uncommon for the cam bearing caps bolt threads to strip. " Remove loose bearing cap bolts and remove cap. Drill damaged thread , retap per the manufacture's instruction. Helicoil or timeserts.

I think the "tubes" you are refering to in a cylinder head are alignment guides. So, yes they should be snug to slightly loose-if we are talking about the same part.

Sorry about the class information yesterday- it was wrong. For metric threads would be 4H5H and 5H.

They use a drill template for head bolt threads in the block of older aluminum V8 engines. Most cases, it's just you being careful- remember you don't want any chips left in the engine and the hole needs to be straight. Drill size is based on material size- so for example a 8mmX1.0 or 8mmX1.25 helicoil in aluminum would be a 21/64, but ideal is 8.25 mm and 8.3 mm respectively.

Not difficult to do and most places charge $25-50 per insert. The drilling is not hard- it's just slighly bigger than the major diameter of the old threads. Then if you've got a countersink put a champher on hole. Then put the tap in a tap handle(don't try and do it free hand!). Tap away per my previous comments.

Michael

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Michael McGuire
83 300d
01 vw A4 TDI
66 Chevy Corsa
68 GMC V6 w/oD
86 300E