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Home > Technical > Any gearbox gurus out there?

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Shroppy Member Since: 25 Feb 2016 Location: Shropshire Posts: 866

Any gearbox gurus out there? Hoping someone may be able to confirm/counter my thoughts on the below please: I stripped a suffix K R380 yesterday which intend to use as the basis for my 127 gearbox. The box was essentially free but had been stored outside, upon opening it up, it appears as though the selector yoke bush (disco) disintegrated, got chewed up and the box was removed due to a very sloppy shift. I'm going to replace all the bearings and seals as a precaution but would appreciate thoughts on the following. Mainshaft Click image to enlarge This looks to be in good condition save for the splines which meet the LT230 input gear, whilst they are not damaged, it is rusty. I'm sure this will clean up easily with some scotchbrite but could I be at risk of accelerated wear? Layshaft Click image to enlarge Unfortunately it looks as though some moisture got into the box and there is some pitting to the layshaft. Am I correct to assume this is scrap? Somewhat annoying as a new one is £175+vat! Dog teeth Click image to enlarge Click image to enlarge The first photo shows the dog teeth on the mainshaft reverse gear and the second photo shows the dog teeth on the mainshaft 1st gear. Am I correct to think that the reverse gear should be replaced due to the chipped/worn teeth but the 1st gear is ok? Any help appreciated!  1985 127 V8 Build Thread Series 2 109" Series 1 80"

23rd Jan 2021 10:36am

v8bob Member Since: 14 Mar 2018 Location: Midlands Posts: 317

So, the rust, I would get a stanley blade, just the blade, and rub on the teeth rust to see if it just rubs off. If it doesn’t leave much more than a bruise or mark then use it. The “Dog teeth” I am guessing you are talking about the brass bit? Generally referred to as a blocking ring. If these are a bit worn on the pointy bit, as in flat instead of pointy, it may cause the selection of a gear to block/not go into gear. This being because you could hit and push on the flat, rather than being led into engaging. On the other side of the blocking ring it pushes onto a cone to slow the gear and speed match. If this is worn you get the chink, crunch noise going into gear. The brass is really the sacrificial part, should wear first. I don’t know specifically which gearbox. But generally reverse is a straight cut gear without a blocking ring. Hence if you try and engage when rolling you get the crunch. Hope that helps. Just looked at the pictures again. It does look like some of the steel teeth are chipped. In general this shouldn’t happen. But forcing a gear shift too quick or a worn blocking ring could cause the chips. This being because the brass cone hasn’t slowed the gear down. Oh yea, did a bit of google and yes, synchro on reverse - I didn’t know that, obviously. I presume I don’t own one, or maybe I always stop before selecting reverse and never checked. Last edited by v8bob on 23rd Jan 2021 6:46pm. Edited 1 time in total

23rd Jan 2021 5:16pm

ashtrans Site Sponsor Member Since: 08 Nov 2008 Location: Harpenden Posts: 257	Hi, mainshaft, OK to use Layshaft, rub with scotchbrite, should come off, if no pitting, OK to use Reverse, replace 1st, OK Dave Dave
23rd Jan 2021 6:11pm

Shroppy Member Since: 25 Feb 2016 Location: Shropshire Posts: 866	Hi both, thank you for your responses. Will add a reverse gear and potentially layshaft to my order thanks Dave. 1985 127 V8 Build Thread Series 2 109" Series 1 80"
23rd Jan 2021 6:25pm

Co1 Member Since: 19 Aug 2018 Location: North Yorkshire Posts: 3674	ashtrans wrote: Hi, mainshaft, OK to use Layshaft, rub with scotchbrite, should come off, if no pitting, OK to use Reverse, replace 1st, OK Dave You asked for a guru Shroppy, well you just got one!
23rd Jan 2021 6:33pm

jst Member Since: 14 Jan 2008 Location: Taunton Posts: 8024	a big for Dave sharing knowledge on here yet again. Cheers James 110 2012 XS Utility 130 2011 M57 bespoke Camper 90 2010 Hardtop 90 M57 1988 Hardtop
23rd Jan 2021 8:03pm

Shroppy Member Since: 25 Feb 2016 Location: Shropshire Posts: 866

Co1 wrote: ashtrans wrote: Hi, mainshaft, OK to use Layshaft, rub with scotchbrite, should come off, if no pitting, OK to use Reverse, replace 1st, OK Dave You asked for a guru Shroppy, well you just got one! Quite! Just shows you how great the members of this forum are. Unfortunately I think this marks the end for my layshaft though! Click image to enlarge 1985 127 V8 Build Thread Series 2 109" Series 1 80"

23rd Jan 2021 8:15pm

ashtrans Site Sponsor Member Since: 08 Nov 2008 Location: Harpenden Posts: 257	Yes, can’t use that layshaft Dave
24th Jan 2021 1:21pm

Shroppy Member Since: 25 Feb 2016 Location: Shropshire Posts: 866

Thanks for your input thus far everyone, a genuine question for Dave: At what point does a gearbox rebuild become uneconomical and what are the alternatives? I've been through each gear and synchro hub assembly and have discounted all bar the layshaft 5th, layshaft reverse and reverse idler. All gears either had worn/chipped dog teeth or scored/damaged teeth. A few photos below showing what I mean; Click image to enlarge Click image to enlarge Click image to enlarge Click image to enlarge So I'm at a loss somewhat, whilst the box doesn't really owe me anything I'm not in the financial position to be able to replace almost all of the components and in buying a second box, there is no guarantee I won't be in the same position only more out of pocket.... Anyone know of a reasonably priced new R380?  1985 127 V8 Build Thread Series 2 109" Series 1 80"

24th Jan 2021 9:40pm

v8bob Member Since: 14 Mar 2018 Location: Midlands Posts: 317

A gearbox will work with many problems, but not always at its best. That is why I was saying more about the issues rather than saying replace or not. I have an XK120 box in the garage, I stripped this for a nice bloke that has about a hundred cars. I know him, and just listed all the parts that are a bit worn, along with how it will be if parts aren’t replaced. Because he has a hundred cars he doesn’t do many miles in them. But all the ones I have seen of his cars look immaculate. And they all can be just jumped in and started, once you get them out of the garage, they all drive fine. Basically what are you using the vehicle for? If you are going on an overland trip of 10,000 miles the parts you change will be different to having a reliable vehicle that you are happy to live with a few shall we say characteristics! If it has good bearings, and the teeth are all present it will drive if you can get it in the gears. What do you want?

24th Jan 2021 10:00pm

Shroppy Member Since: 25 Feb 2016 Location: Shropshire Posts: 866

A very good point Bob, this is where my perfectionism gets the better of me! I suppose my concern is that by replacing only say the layshaft and the gears with damaged teeth, I would be left with worn dog teeth and syncro hubs which I suspect will accelerate wear on the new gears that come into mesh with the old. That said, you could argue that everything should be replaced! As far as usage goes, I'd like to be able to go overlanding with this box (amongst other parts ) but with the world so connected these days I dare say a gearbox failure in Morroco is more easily overcome than it once might have been. 1985 127 V8 Build Thread Series 2 109" Series 1 80"

24th Jan 2021 10:15pm

v8bob Member Since: 14 Mar 2018 Location: Midlands Posts: 317

If you are doing some sand driving, then if you need a quick downshift, then you don’t want it to block when you rush it. Because you will be digging it out. So those chipped teeth can give you a gear miss. If you are just constantly driving on just unsurfaced roads mile after mile then would it matter if you missed a gear? Those burrs, you can just take them off. They will most likely build up again, because they built up before, and something caused it. Most likely a bit of slack and selector not pushing things up square. But hey, it will still work. Those, what look like grind marks on the teeth are probably where it has never meshed with anything. It should mesh about half way down the tooth on the radius part. The tips shouldn’t touch because that isn’t where the strength is. So, make sure the bearings are good, no debris anywhere, and don’t create any more debris in the box. Good clean oil and treat it gentle. It will then outlast you. Treat it hard, rush the shifts - causing wear and chips, progressively getting more problems and causing more wear. It will contaminate the oil and not last as long, normally because of bearing failures.

24th Jan 2021 10:33pm

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