YAMAHA DT175 MX
Ralph Ferrand sorts the suspension out this month.
Yup, it’s time for Ralph to sort the suspenders on his mud-plugging Yamaha.
This month I’ll be turning my attention to the DT’S suspension. The shock is like no other I have previously encountered, being pretty much twice the length of a traditional shock, disappearing up inside the top frame rail with its top mount being just behind the headstock. It has nitrogen gas and oil separated in a mono-tube construction by a floating piston, thereby preventing ‘airation’ (mixing of oil and gas). Luckily for me, despite the serious abuse this poor bike had suffered over the last 35 years, the shock seemed to be in perfect working order, which is just as well since, according to the manual, it is not serviceable and I have very grave doubts about the availability of a replacement. Mechanically it may have been in good shape, but pretty it was not, so I removed the spring using my suspension press and masked up the main body in preparation for a bit of rattle canning. First up was the pre-load adjuster ring. This was originally zinc plated, but it was not removable, so paint was the only practical solution. I usually apply paint outdoors, but as this was only a small job, I sprayed it in front of the extractor fan. Once the primer was dry it was followed up with some wheel silver. After that coat was hard, I masked over the preload adjuster and removed the masking from the shock body and ‘keyed up’ the original coating with some wet and dry paper. I didn’t get too anal about the paint as it was never going to be seen by anyone save my good, though critical, self. The original label will be irreplaceable, and call me sad, but while it’s in far from perfect condition, I rather like it as a historic feature. So I masked it up. Before any paint was administered, the surfaces were thoroughly cleaned with panel wipe (preparatory cleaner) on clean paper wipes. Once free from any grease or other allied detritus that might lessen the grip of the paint, I primed it and then finished it with PJ1 Fast Black. As soon as the paint had
dried, I removed all the masking. It’s important to remove the masks before the paint has gone really hard, to lessen the likelihood of chipping the edge of the paint. I cleaned up the cast aluminium piston rod eye with a satin mop on the polishing machine, which gave it a pretty original finish. As the masking on the piston rod had been there for a good few days, once removed I got rid of any residual glue with IPA (Isopropyl Alcohol) to stop it fouling the seal. Next up was the front telescopic forks which were in a typically deplorable condition. Underneath the faded ‘sporty red’ gaiters the fork stanchions were deeply pitted, where the chrome had given way over the years and the moisture had attacked the steel underneath. Once removed from the bike I clamped the stanchions in the vice and undid the top fork cap, removed the spring and poured the ancient oil together with some stagnant water out for responsible disposal. As with most forks of this age the next job is the remove the damper rod bolt from the bottom of the leg. For many forks there is a special factory tool that is inserted through the top of the stanchion to hold the damper while you undo the bolt. If the tool is unavailable or you simply don’t have one, I find that you can often shock them undone, with an air impact wrench. It doesn’t hurt to replace the spring and top fork cap temporarily to add a bit of resistance to the damper. Even a cheap Chinese gun can work. One mistake many people make though is to use a thin diameter air lead e.g. the cheap curly recoil type. You may have a perfectly respectable three horsepower compressor, but if the air-lead or airline is too thin, then the impact wrench won’t get enough air flow so will not give the required performance. With this particular application you want all the power you can get. I refilled the air tank on the
compressor to ensure that when I pressed the trigger on my uber-powerful air impact wrench, all the hell and damnation of all the horses of the apocalypse would rain down upon that bolt and have it flying out before it knew what had happened. It’s not often that you need massive torque and speed instantly in motorcycle fettling, but this is one situation where there’s no such thing as too much! Doing it back up, of course, is a far more subtle affair, involving the torque wrench. With this fork, I then clamped the stanchion in the vice again, having carefully stashed the damper bolt, and drew the lower fork leg off the stanchion. I then re-removed the top cap and spring. I usually rely on We moto to supply me good quality Italian-made pattern stanchions, but sadly they didn’t have any for this bike and genuine ones are no longer available, which left me only one option – get them re-chromed and ground. This is often said to be the best option, but is usually more expensive than the trade price for pattern legs and involves a fair wait. I sent them off as soon as they were degreased and got on with restoring the rest of the components. These are very simple forks, but at the top of the slider, the seals are unusual, at least compared to the road bikes that are my normal fare. First off a steel cover was removed which exposed the dust seal as well as a plethora of vile detritus. The dust seal was easy enough to remove, as was the seal circlip. The fork seals themselves were a different story altogether! Fork seals are not just rubber – they also have a steel cage in them. As the years go by moisture gets between the steel of the seal and the aluminium fork leg and acts as an electrolyte, setting up a galvanic circuit, which effectively welds the two parts together. This is bi-metal corrosion. Because I had recently seen a set of DT forks destroyed by some numpty with a screwdriver, trying to get the seals out, I was doubly careful with mine. I gingerly started with an appropriate tool from my seal removal kit, but it was stuck tight. I sprayed some ACF50 around the seals in both legs and left them for a day. ACF50 has some very clever anti-corrosive properties as well as being a very thin fluid, making it act as a penetrating oil. I decided not to take any chances and warmed the leg up with my plumber’s propane torch; not too hot but hot enough that all the oil started burning, so obviously I flicked on the extractor fan! Using the seal removal tool again, it finally came out with no more fuss. After the forks were stripped, all the components were thoroughly cleaned and degreased. The sliders had plenty of corrosion pitting, so were never going to be mint, but I gave them a good session on the polishing mops, which made them look far better. The chrome on the fork caps was pretty buggered and while I could have got them re-chromed, I have yet to find a reliable chrome plating shop and so try to avoid getting parts re-chromed, if at all possible. These parts are no longer available, but I did find a New Old Stock pair on ebay in Thailand. As with engine work, suspension components must be kept scrupulously clean. I firstly reassembled the damper
components into the newly chromed stanchions and then carefully refitted the fork lowers. The quality of the seal on the bolt that secures the damper in place is of paramount importance. It uses a copper washer, which should always be replaced: there is nothing quite as irritating as reassembling the front-end of a bike only to discover that the forks are leaking from the damper bolt! All parts must be clinically degreased and I tend to use brake cleaner in a chemical bottle for this task with an airline blower for thorough drying. If you do use an airline blow gun, do ensure that you wear safety glasses or goggles; you’ll never find a NOS eyeball! I then applied a coating of silicone RTV under the bolt head, fitted the copper washer and then coated the bottom of that with RTV as well. I then carefully administered thread lock to the end of the bolt threads to lock it in the damper. I tend to wizz the bolts in quickly with my 3/8in drive electric impact wrench and then torque them to the correct torque setting with a torque wrench. The drain plugs were refitted with new sealing washers and again tightened to the prescribed torque setting. Fitting the new fork seals must be done using some care, as the seal lips are easily damaged. I always cover the top of the stanchion with some plastic sheet, usually from some packaging lying about, which protects the seal from any sharp edges. I always lubricate the inside of the seal with some fork oil to make its passage down the stanchion easier. For the sake of the seal and stanchion the seal should be carefully driven into position with a suitable fork seal driver. I used a universal seal driver which did the job admirably. The new spring clip was fitted along with a new dust seal. I could not get any new metal covers so had to repaint the originals. When refilling forks, I prefer to set the oil height with a fork oil height setting tool, but as Mr Yamaha does not specify an ‘air height’ (the distance of the fully compressed fork leg from the top of the stanchion, with spring removed, to the oil level) I had no option but to measure in the fluid with a suspension syringe. Once I had measured in the specified volume of oil, I pumped the fork to get the oil around the damper, replaced the spring and finally fitted the shiny new fork tops. Job done! Next month, the motor is rebuilt.