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MG MGB Technical - Rear Suspension Bushes / Pads
I am replacing the rear suspension rubber bushes and pads on my MGB Roadster with Polyurethane alternatives. Can anyone tell me what the correct tightening torques are for the u bolts and shackle pins etc ?
|Torque figures are only given where it varies from the 'normal' figure for that size, typically 6-7 ft.lb. for 1/4" thread, 19 for 5/16" and 30 for 3/8", oiled threads, Nylocs will add a bit to that.|
For U-bolts the rubber at least compresses gradually, meaning there is a very slow torque rise, as well as Nylocs being the usual nuts adding their torque. They tighten gradually then it suddenly rises very rapidly, and that is where I stop. Even so a month or so later on rechecking I could tighten them another turn or two without much effort, although I hadn't experienced rear-wheel steering which is the usual symptom of loose U-bolts. Recheck them periodically until you find they aren't moving an more.
Shackles are different, they (and front eyes) tighten onto a shoulder, not rubber, so should tighten very rapidly, not gradually.
|As Paul noted, the rear shackle bolts are "shouldered" and the front eye bolts pass through a steel spacer. Thus, the bushings cannot be over compressed because of the limits of the bolts/nuts. The U-bolts are a different matter. If your old U-bolts are still in place, make a mental note of how many threads are beyond the nyloc bolts and assume the new polyurethane are similar thickness, the new U-bolts are same size and that achieving the same amount of threads should be close to start with. Without any restriction like the shoulders on the shackle bolts, you can make up your U-bolts until the polyurethane pads are squeezed out of shape. Tighten the U-bolts by alternating diagonally between the nuts until the pads are snug, but not "oozing" out of their holders. There is no applicable torque value for the U-bolts, your visual inspection and a test drive is the only measurement. Don't use the special grease on the pads, but follow the directions for applying it to the bushings.|
|And presumably you are not disturbing, or removing the springs? This can be the biggest problem as the front eye bolt can seize in the old rubber bushing and sleeve.I agree with Rick, count the threads that show on the 'u' bolts, and use new nyloc nuts. Mike|
Thanks for all of your replies, I will take on board what you say. Unfortunately I will not be able to count the threads that show on the u bolts as I have already done the change.
It has been my winter project, the rear suspension on my roadster was looking tired so I thought that I would replace the springs with parabolics, the dampers with GAZ telescopics and the rubber bushes and pads with polyurethane. It all came apart without too much difficulty. When it came to putting it back together I managed to get the springs on with new bushes but when it came to tightening the u bolts I found that I couldn't register any real torque, the pads were just compressing.Hence the reason for this thread.The other problem I have now is that the rear ride height (which was previously sitting too low) is now riding too high. So high in fact that I am going to fit lowering blocks. The MG owners club spares dept who supplied the parabolic springs for my 1977 roadster said that the springs will take time to settle down but with a rear height of 17 inches (between brake drum centre and bottom of chrome strip) and front height of 14.5 inches it looks wrong. Hopefully they will be correct and the ride height will drop back when the car has completed a few thousand miles.
|When I mentioned counting the exposed threads, I did not mean to imply that there is a correct exposed thread count fall all MGBs. I only meant it as a point at which you might assume that you were close to correct for your car, then adjust from there. Mostly, it's helpful to make sure that the same amount of threads are exposed on all to assure axle alignment and snugness. I've performed similar renovation of my 80 Limited Edition. Like your experience, I had to add the lower blocks. The rear still measures about a half inch higher, but the car is still in the shop and hasn't been driven yet. Check with Moss Motors about the lowering kit. They recently extended a sale and might be cheaper for a moment. Otherwise, they're about 90 dollars USD. Also, I assume you followed the instructions to let the car down with the rear shackle and front eye bolts slightly loose before and tighten totally only when under normal weight of the car. I'm putting the transmission mounts back on my car today and will take a moment to look for the exposed thread count of the replacement U-bolts that came with the lowering kit and post it for you. I should have a photo on my camera as well. I think it's about a half inch. I found it helpful to use an extra nut (non-Nyloc) to thread onto each of the U-bolts to inspect for same length. I didn't leave it on, just used it for measurement.|
|Phil - the parabolics are supplied with three or four (can't remember without going out to its garage to look at my car, and it's raining!) metal spacers as replacements for the 'missing' leaves. Where have you put them in relation to the parabolics?|
If you've put them underneath the ride will be too high. I've just fitted them to my ex-r/b roadster, and I specifically asked the MGOC for 'chrome' parabolics. The car now sits about level front to rear.
The image attached was taken immediately after fitting. Sorry it's a bit dark. The height possibly looks more than it is because the camera was at hub-centre height. I've still got some 'adjustment' left, but thought I'd wait to see if they settle down a little.
I have already ordered the lowering kit from the MG owners club spares dept. and it should be arriving tomorrow. Hopefully I will be able to get them fitted this weekend. I will slacken the eye bolts and shackles again when I fit the lowering blocks and re-tighten when the car is off the axle stands. I don't know if the new u bolts that come with the kit will be the same length as those supplied by Moss so the exposed thread count may not be relevant. I think I will just need to ensure that I tighten the nyloc nuts evenly and not over compress the pads and see how it feels on the road.
The Parabolics as they arrived from the MGOC spares dept came with the 3 spacers secured below the springs. I assumed that as I had ordered parabolics for a rubber bumpered roadster that the springs would come ready for installation and so I attempted to install as was. I found that it was impossible to fit the bump straps and the back end was way too high. I contacted the MGOC to confirm that they had sent the correct springs and they told me to reposition the spacers as required. I moved all (3 off) spacers to the top of the springs and tried again. This time I was able to connect the bump straps and the back end was lower but still too high. I was a bit annoyed at having to do the job twice and still finding that the car did not sit correctly. That is where I am now, hopefully I will find it acceptable when the lowering kit is installed.
Am I right in assuming that you chose chrome bumper parabolics for you car because you had already lowered the front suspension as part of a rubber bumper to chrome bumper conversion ?
Yes, the car is lowered (I don't think the front is ever quite as low as an original chrome car?). The rear springs had the lowering kit fitted - I also replaced the bottom leaf with a spacer to weaken the springing and thus lower the car even further. This made the ride to soft and put the car on the bump stops too often.
I presumed the parabolics for a r/b car would be designed to retain the original r/b height, hence my specifying chrome. (They are marked '100' if my memory serves.) I also felt this would be a good way to ditch the lowering blocks.
The car now looks, to my mind, about right. I also went the Gaz route, front and rear, because I felt that the front dampers, especially, started to tire when using the car on bumpy roads in the hot weather we get here in the summer.
It may be 'the king's new clothes', but I am much happier with the way the car feels.
I have also removed the rear a/r bar.
This thread was discussed between 20/02/2011 and 21/02/2011
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