Welcome back, everyone!

My last post started to build a picture as to what was wrong with the lemon of a motorbike I bought. Having identified the issue I thought it would be a straight forward task to remedy. The detailed saga below seems to suggest otherwise…

To summarise – The head gasket was badly blowing and the bike had reduced power (see video below) – In order to fix I had to eliminate the leak. There are a few ways to do this depending on whether you are lucky or whether you are me…

Attempt 1 – The first fix can be done with the engine in the frame and was my first port of call. Looking at the sketch below, the cylinder head is held down with 4 bolts to the top of the cylinder block. The video shows the exhaust gas leaking from this joint. The cylinder head gasket provides the seal between these two points in the case of my engine, the cylinder head wasn’t tight enough.

Bolts 15 & 16 secure the cylinder head to the block crushing the head-gasket (13) in between

After stripping the bike apart (surprisingly simple and much more enjoyable than the XS750, which was covered in bird shit and 10 years worth of filth) I was met with the 4 bolts in question. With my newly acquired/borrowed torque wrench I set about the simple task of tightening the bolts. A helpful hint from a wiser mechanical person than me (thanks Matt) suggested that this can be a common problem on single cylinders due to them wanting to rattle themselves to bits. Unfortunately, I was unable to tighten two of the bolts to the correct torque setting…the bolt was acting like it had been stripped….tight and spinning but not getting to the required torque….bollocks.

The engine had to come out….thanks to the now well-studied Clymers manual this was actually surprisingly a simple task with only a few more mounting bolts requiring removal. A swift herculean movement and the engine was on the bench…ignore that other engine in the pictures…

Little & Large, XS750 engine on the right

Stripping the engine down is interesting. especially considering I was part way through rebuilding the XS750 engine when I had to do this one. with having only 1 cylinder compared to 3, things were a bit simpler….and smaller. Best of all I didn’t herniate my lumbar spine lifting it onto the bench.

Investigation shows that the threads were completely buggered (technical term). See the below gallery for a summary. They had been repaired previously. The original threads had obviously suffered damage before, so the repair, in this case, was to use a Helicoil. Damage to the original threads was likely somebody over-tightening the original cylinder head bolts causing the threads to strip….doh…USE A BLOODY TORQUE WRENCH!

A Helicoil is a sacrificial thread that is run to replace the one that has been damaged…to do this, you drill out the existing thread, then tap a new slightly larger hole and screw in the Helicoil. You then should be able to use the same size bolts as before (preferably new ones). The premise is that as you tighten the bolt into the Helicoil, the outer Helicoil thread bites in and tightens. Due to this thread being slightly larger, it should be able to get to the torque required.

Helicoil…simplified

The pictures show that this repair was poorly carried out by a previous owner. The Helicoil thread was poorly placed and spinning itself, meaning the bolt would never get to the required torque…I reckon one of the previous owners knew about this and quickly bodged it back together to sell on. I went back to the person who sold me and asked (nicely…honest) whether this was a known issue…
After several hours of waterboarding, his answer was still that he didn’t know about it…oh well. Another reason that got me thinking that this was a bodge job was that whatever piece of shit human that had this engine apart previously had decided to skimp out on replacing a dodgy timing chain tensioner. Interestingly, the TT600 had a self-adjusting timing chain tensioner, what wasn’t so interesting was that when I tried to remove it, I obviously altered the status quo and the spring unit decided to dismantle itself in the engine…..COCK. My heart literally sank as I heard a rattle of an unknown tiny component make it’s way to the bottom of the crankcase…

Luckily I have the most helpful tool known to man for butterfingered humans such as myself….the telescopic magnet! If you haven’t got one…go buy one! After several minutes of aimlessly fumbling around inside depths of the engine, I got lucky (sounds familiar…or insert some other poorly referenced sex joke of your choosing). The other half of the tensioner was recovered, confirmed by cross-checking with a diagram online. I went straight to ebay to buy a manual tensioner instead…with no nasty springs.

Manual timing chain tensioner – much simpler….tighten the bolt until the timing chain slack is taken up….easily adjusted and no chance of it spewing its insides out into the engine.

With the cylinder block and head apart, I took it to a local machine shop to see if anything could be done in terms of repair. They were hopeful that they could use another type of threaded insert (called a Timesert) to make the repair. They recommended changing all the threads as they were all looking a bit tired…okay.jpeg. They also cleaned the cylinder block. 4 x Timeserts and £100 later I had my cylinder head back. I had ordered some new bolts from fowlers and picked up a replacement cylinder head gasket and started the reassembly process…..

Reassembly was fairly straight forward until I got to the cylinder head bolts. Three managed to tighten to the correct torque….one would not get there at all. After phoning the machine shop, they reckoned it the bolt could have been bottoming out…and that this repair was particularly difficult as they didn’t have much material to work with…or some bollocks. I used another washer to raise the bolt slightly which did allow me to get a slightly higher torque but not what the manual recommended. So I got everything back together, bike in the frame and it started 2nd kick! Amazement!

Perfect I think the bike will be running really goo….oh shit never mind:

BOLLOCKS….still not working

The leak was still present, particularly from the area I couldn’t get the correct torque. Frustrated and wanting to set the bike on fire I went straight to the Ducati website and ordered a new Scrambler Desert Sled……I wish….well I was very close…

I did, however, go straight to eBay and manage to source a newer cylinder block with threads in good condition (this would come back to haunt me). Anyone who has used eBay a lot will know about the “make offer” button. I made a cheeky offer on a cylinder block in Germany and before I knew it, he had accepted and it was in my garage. I also got rid of the old one which was now a paperweight…

Just kidding… I may turn it into a lamp…this is remotorcycled after all!

Ok…we shall take a time out at this point to play spot the difference in the cylinder blocks…

For the more eagle eyed readers in my audience you would notice:

  • that the new one on the right is cleaner
  • there are only 5 mounting holes around the cam chain vs 6 on the new block
  • the locating holes at the middle of the cylinder bore are 3 mm wider apart

I did not notice this difference… I was completely unaware that Yamaha decided to change the design of the cylinder block (and subsequently the cylinder head and gasket). Annoyingly this was the only time I had the two of them side by side and I wasn’t looking for those changes. This will be obvious later on…

So I sent this block and head to the machine shop to give it the once over. Whilst they were at it they would skim the head (put it into a mill and remove any high spots) to make sure it was all level ready for reassembling, just in case this was a contributing factor to the leak. Another £100 lighter and the head was back in my possession and it looked great. They also gave me measurements they had taken of the old and new block…to be honest they failed to notice the difference between the two blocks too….just saying 😉

The measurements showed there was a difference between the bore size of the cylinder, which is fine but it meant more expense was needed in the form of piston rings. The new cylinder was standard size whereas the old one was +20 or 2 hundredths of an inch over stand size (2nd over).

Annoyingly I cross-checked the measurements of the piston with the manual using my trusty Amazon digital vernier gauge, but I had performed this whilst the piston was still on the bike…and was unable to get the calipers right over where I needed to be (unknown at the time). What made this worse was the fact that the diameter I measured ended up being exactly within the range for a standard size piston (which I needed for the new block), so I ordered piston rings to fit this.

Disaster struck (again….seeing a theme here) when I went to try and fit the piston into the cylinder and it didn’t fit…obviously because dumbass here had measured it wrong….what I needed was a standard piston…which was confirmed when I removed the piston and measured it properly! So off to eBay AGAIN to spend another £96 on getting something that will work. Luckily this piston came with new rings also.

Luckily this time, the cylinder block slid over nicely (see below).

Finally a bit of luck

Unbeknown to myself at this stage that the stud pattern was different on the two blocks, I pressed ahead with getting the cylinder head ready by fitting new mounting bolts as well as changing out the exhaust studs. Anyone who has had experience changing out exhaust studs knows they can be temperamental little shits that can break with little to no warning. I usually wouldn’t change them but guess what, someone had kindly bent them for me making the exhaust removal/fitment challenging. I used a handy stud extractor tool, which worked a treat….although there was still a heart in mouth moment until they started moving.

So before reassembly…I had to buy ANOTHER new head gasket (as the previous leak had caused a bit of damage to the already new one). I picked up a new old stock (NOS) on eBay (who were now starting to offer me a line of credit) which was perfect for the stud pattern on top of the block…which was strange because I ordered one for my older engine…a spot of luck?…

I started to reassemble the engine and all went fine until I discovered that the block and cylinder head would not mate up….gutted….

At this point contemplating setting the whole bike on fire, and getting back to Steven at Ducati for that new scrambler I was pretty low. Luckily I managed to do some digging online and found a gentleman called David Lambeth who I ended up having an hour-long phone call with about all things Yamaha scrambler. He proved to be immensely helpful and if it wasn’t for this chat I would probably still be in despair lost in the minefield of engine variations. Thank you David.

Armed with a new shopping list, I went to find the correct Cylinder head for this era of the bike….annoyingly they all seemed to be anywhere but the UK. Several cheeky bids to an Italian bike breaker and I am now in possession of the correct cylinder head (and £130 lighter….).

Having checked all the parts mate up successfully I am confident that i have all the parts I need now…please jebus let this work.

I have spoken to a local vapor blaster to clean up the top end, as you can see the rocker cover has a slight discolouration compared to the other two parts. I am also going to look at painting the lower part of the engine before reassembly…so those are the next tasks before reassembly.

Conclusion

This repair has been an absolute saga filled with soaring highs and crippling lows (sounds like my relationship with food….or a heroin addiction). But I know a lot of the issues that have happened have been as a result of ME…not the bike. Sure the damaged cylinder block is tough luck as well as a few other issues….but:

  • Could I have noticed the signs to avoid buying this bike in the first place? Yeah perhaps these issues may have been present
  • Could I have read more on the various engine types to save me the expense and heartache of buying multiple components? Yes, instead of rushing in…
  • Could I have learned how to measure pistons correctly? Yes

All of these things have been great learning’s that I never would have experienced if everything had gone to plan. The silver lining of this whole experience is that I now know where I went wrong and what to do again in the future. I have learned shitloads about the practicalities of engines, not just the theory and for this, I couldn’t be happier. That’s what all this is about.

Onwards and upwards,

Thanks for reading

Struan

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