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General Category => General Discussion => Topic started by: yackee on April 16, 2025, 12:29:18 PM
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Hi all,
I am installing the Delrin plastic intake manifolds on my '76 T3 to install newly purchased PHF carbs. The manifolds take M6 bolts, but the cylinders on the T3 are drilled for M8.
I've been advised to use M8-M6 reducing threat inserts. I have a pack of stainless steel ones. You use a flathead screwdriver to screw them in.
I was wondering if anyone else has done this and in particular whether I should be thinking about red loctiting the reducers in or just install them dry.
Thanks for any thoughts.
PS the Delrin manifolds used to come in an M8 version but that no longer seems available. I used to have a set, but sold them long ago when I realized that they didn't work for the PHB carbs that I had mounted at the time.
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I’d put them in dry. As a general rule loctite around plastics and composites is a bad idea. Delrin might be ok but I’ve seen some expensive parts like canopy’s on aircraft destroyed. The loctite causes hydrogen embrittlement in them so they craze and crack . It may be ok but not worth the risk.
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Lock tight will definitely cause lexan plastic to crack. Not sure about the type of plastic your application is however I would era on the safe side and not use it.
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The thread inserts would be going into the alloy cylinder heads, not the manifolds. I wouldn't use red Loctite, blue should suffice. Anti-seize on the bolts going into them.
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Thanks guys, I got the reducing inserts and will blue-loc-tite them into the cylinder. This looks like a perfect solution. Once I get it together and running I will update the post.
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You don't have enough material in the manifold to ream the holes to clear 8 mm screws? That would be my first choice.
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You don't have enough material in the manifold to ream the holes to clear 8 mm screws? That would be my first choice.
The headache is while there MAY be just enough meat to enlarge to 8mm, you then lose the little 6mm metal sleeves in the hole. Am aware of plenty of folks that thought they could get away with it, but they squished the Delrin plastic and cracked it off. Some have probably pulled it off, but from my experience, the success to failure rate isn't real good. You only seem to hear about the failures I realize, but take it for what it's worth.
While there is a possibility that there were some created with 8mm holes, I've never seen or heard of them. Keep in mind, these were conceived and made in Europe, (Moto Spezial), where the only Guzzis that used PHF/PHH carbs had 6mm threads in the heads. In the US, the only round fin bike that used 6mm threaded heads, was the 850 LeMans. You don't see 6mm until the square fin heads. The manufacturers/suppliers of the manifolds weren't figuring on, or caring about anyone trying to mount them to heads with 8mm threads it would seem. If they were out there over the last 30 years, I wish I would have found them, I could have sold mega tons of them
T d F
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Yes, this is exactly the problem. The manifolds come with brass rivet-type washer things in the holes for the manifold bolts, and it looks close to impossible to drill them out without destroying them. I think it would risk the plastic cracking.
I definitely mounted an 8mm version some years ago, but I don't know if it's made any more.