New Threads !

A common problem with early 1970’s Suzuki’s is having the exhaust bolt thread strip that hold the exhaust flanges (and therefore the exhaust pipes) to the cylinder head. This is due to the soft aluminium alloy just wearing away over time with the repeated removal and installation of the exhaust pipes for cleaning or maintenance. An option to avoid this wear is to install studs to replace the bolts, and I know a few folks who have done that on their bikes, but I prefer the factory look. Being stripped is bad, but making it worse is that often what will happen once the original thread is gone, owners will open up the hole to the next bolt  size (10×1.25mm) and just put in a larger bolt. Of course, once that one strips – and it’s just a matter of time – then you are truly stuffed. In the past, your only real option at this point would have been to find someone who could TIG weld more metal back in, and then drill and tap a new threaded hole and to do this would generally require the engine be removed from the frame as the working space is quite limited.

For the ‘first time’ failure mode, installing a Heli-coil thread repair insert is generally what most shops will do – this is usually an easy, quick, long-lasting, relatively low-cost solution if you are lucky – the ‘luck’ part mostly has to do with whether you can access the thread to do the install without having to pull the engine – in some cases on a motorcycle it just isn’t possible.. The inserts themselves are about $1 each (in the US), and are made of a hard stainless steel wire which has been coiled into a spring, and which you just thread into a re-tapped hole.

But what about the case where the hole has already been opened up to a larger size and has now been stripped again ? Or for that matter is a stripped Heli-coil ? I suspect you have a better chance of winning the lottery, but I seem to be blessed with bad luck when it comes to exhaust bolts so I’ve included this as a possibility. For this issue TIG has been pretty much the only option – till the folks at Time-Sert came up with their Big-Sert product line. These are oversize thread inserts – plugs actually – and are really clever, but also more expensive than Heli-coils. Of course, if your other only option is to pay for someone to do some speciality aluminium welding, then it starts to look like a bargain !

In preparation for having my 1973 GT750 cylinders re-bored and bead blasted, I decided to first tidy up the exhaust ports which had a couple of stripped threads – one was suitable for a Heli-coil repair as it was just a damaged 8×1.25 mm thread and so fairly small. The second one was a small crater of a hole that you could almost drop a 10mm bolt through without it touching, so I decided to try the Big-Sert and see how well it worked. Ten minutes later, as can be seen in the lower right of the photo, I had a brand new, very nice looking 8×1.25 mm threaded hole and was very pleased with the result ! The inserts are available in various lengths – for this size of bolt between 8 and 20 mm in-depth. I had the ‘stock’ length ones of 11.7 mm, as well as some longer ones at 16.2 mm, and opted to install the longer ones just to give more contact area on the bolt given it was after all an exhaust port being used to secure the exhaust pipe, and so subject to extremes of temperature and vibration.

As with Heli-coils, you typically buy a kit for each size of thread you are repairing. The Big-Sert kits contain all the bits and pieces you need to do the installation: drill bit, tap, reamer, install tool and inserts (of course you have to supply your own electric drill and tap handles). Unlike Heli-coils, Big-Serts are threaded plugs with a cam-lock built-in to stop the insert from moving once in place.  It’s a nice design and ideal for this sort of repair – but they are about twice the price of Heli-coils (if you shop around). I bought mine from ToolsQwik in the US – very helpful and easy to deal with, good prices and fast delivery. After including the postage cost, and currency exchange (there is no duty charged to bring these into Canada – just GST which you’d have to pay anyway), I still saved about 30% over what I was quoted for the same items here in Calgary, so as always it does pay to shop around.

I will be dropping the cylinders off at a shop south of Calgary in High River called RPM Services next week after Christmas, to be bored to first over size. I already have the pistons, rings, gudgeon pins etc., so it now looks like I will be starting to re-assemble the engine sometime in January. In the meantime, I still have to finish the wiring on the GT750 and sort out the fuel tank and a few other odds and ends on the GT500 I’m also working on, so I’m not short of things to do !

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