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MG MGB Technical - 3-sync transmission rebuild
| I have just torn down 3 (yes three) early MGB transmissions and all 3 (yes all three) had major layshaft / laygear issues. One was out of a 67 with the larger layshaft and four bearings but it still had wear problems. Has anyone found a good solution to this problem? Thanks, Jim |
| JCR Royal |
| I am no expert, but have experienced this problem myself, both on MGB , Magnette and midget 3 synch boxes. I believe somewhere in the archives someone has written something about increasing the size of the layshaft needle bearings. Seems like a good idea. On my reconditioned early MGB gearbox I plan to use a magnetic drain plug to catch all the abrasive metal particles in the oil, and to use an easier flowing, synthetic oil. I use synthetic oil on my original Midget Mk 1 gearbox, and it works quite well. Tore |
| Tore |
| Thanks Tore, The larger layshaft does seem like the way to go. I wonder if that means using the larger shaft found in the latest version (67ish) gear box. I am using these boxes behind a warmed up 1800 so I need it to be as strong as possible. I know I could uprate to a 4-sync but I like the "personality" that the 3-sync gives the car. I have seen other transmissions that use bronze bushings in place of bearings for the laygear, in fact I believe that all of the other gears in this box use bronze bushes. Any thoughts? also I have heard some mention of using 1st motion rollers in place of the caged bearings, has anyone had any experience with this? I am in the planning stages of what to do with this thing so any input would be appreciated. Thanks, Jim |
| JCR Royal |
| I think that part of the problem is that people replace the bearings and the shaft but not the laygear. The worn laygear then accelerates the wear on the new parts. Brand new laygear are available - I would rebuild the latest box using new parts (laygear, bearings, shaft, 1st gear outer and reverse idler) and stay with the caged needle bearings - but beware, there are some out there that are a very poor fit, the cage being too small for the shaft. |
| Chris at Octarine Services |
| Hi Folks: I agree that almost all the major components shoud be replaced during a major overhaul. NOS gears, 2nd & 3rd gear bushings, ball bearings and thrust washer, etc; can always be found on ebay. I have always rebuilt this type of gearbox with the 22H1301 laygear and associated bearings and layshaft. The 3 synchro design goes back to the MGA and requires more attention on an 1800 MGB motor. If you need replacement parts contact Quantum Mechanics, they have almost everything you need. Good Luck: Rich Boris |
| Rich Boris |
| MGB Turbo Original is 3 pieces of needle roller bearing. Short spacer tube and mount ca 3,75 pieces of needle roller bearing, and the gearbox will hold. http://hem.passagen.se/larsragnar/ Page MGB & page MGB K-series |
| Ingemar |
| Then there is the option of changing to the 4 synchro gearbox! There is a great deal of information about this in the archives. Personally I think it is the best change I ever made to my MGB and no one really kbows. cheers |
| Ian Buckley |
| If you use the later 4 bearing laygear with new bearings and shaft you won't have much problem. The earlier 3 bearing gears are definitely more prone to wear, as well as a bitch to assemble. |
| Bill Spohn |
| Thanks for the help. I think that I will use the box with the larger shaft and the 4 bearing laygear. and replace the laygear and any other worn parts as Chris suggests. Chris, do you know where I can get a close ratio gear set reasonable? They run about $2500 here just for the gears. Thanks, Jim |
| JCR Royal |
| Jim, They are around 1000 UKP or 2000 USD here. |
| Chris at Octarine Services |
| Why would you fit a close ratio gear set if it is a road car ? I would advise against . |
| S Best |
| With the engine that is in my car it really likes 2500-6000rpm. It will pull from 1500 but really gives you a kick in the pants at 2500. I also am running an over drive transmission which I do not need! The car humms along quite happily at 70mph and around 4000rpm with plenty of pedal left in 4th gear and no overdrive. With a close ratio I could lower the rear end ratio to maybe 4:11ish (I'd have to do the math) and have great accelleration with much smaller gaps. this would enable me to keep the car up on the cam with less effort and use the overdrive to put me at the same crusing rpm I'm at now. In fact The car currently has no problem taking off and pulling 2nd gear so I could probably get away with the 3:9 that is in it.Am I totally off base on this? Thanks, Jim |
| JCR Royal |
| 4000 rpm is very high for cruising at 70mph representing only 17.5mph/1000rpm. The use of overdrive would cut this by about 15% giving a speed of over 20mph/1000revs or only 3400rpm at 70. This represents a much more relaxed cruise less noise, engine wear and of course improved economy. I think you do need the overdrive. In my opinion for a road car, lowering the diff ratio would go completely against this, neither do I see much sense in raising 2nd gear ratio just to lower it again with an alternative diff. Allbeit it does give you the closer ratios but arguably not necessary or even desirable for road use. 6000rpm wow,you are a noisy b---er. It's gone right off the cam by that time anyway. |
| Iain MacKintosh |
| Jim- Here are the calculations for the 4-syncro transmission- Road Speed in MPH w/ 4.875:1 rear axle @: 2,000 RPM 3,000 RPM 4,000 RPM 5,000 RPM 6,000 RPM 3rd: 21 31 42 52 62 3rd Overdrive: 27 41 54 68 81 4th: 29 43 57 72 86 4th Overdrive: 35 53 70 88 105 Road Speed in MPH w/ 4.55:1 rear axle @: 2,000 RPM 3,000 RPM 4,000 RPM 5,000 RPM 6,000 RPM 3rd: 22 33 44 56 67 3rd Overdrive: 29 43 58 72 87 4th: 31 46 61 77 92 4th Overdrive: 38 58 77 96 115 Road Speed in MPH w/ 4.3:1 rear axle @: 2,000 RPM 3,000 RPM 4,000 RPM 5,000 RPM 6,000 RPM 3rd: 30 45 60 74 89 3rd Overdrive: 31 46 61 77 92 4th: 33 49 65 82 98 4th Overdrive: 41 61 81 103 122 Road Speed in MPH w/ 4.1:1 rear axle @: 2,000 RPM 3,000 RPM 4,000 RPM 5,000 RPM 6,000 RPM 3rd: 25 37 49 62 74 3rd Overdrive: 32 48 64 80 96 4th: 34 51 66 85 102 4th Overdrive: 43 64 85 107 128 Road Speed in MPH w/ 3.909:1 rearaxle @: 2,000 RPM 3,000 RPM 4,000 RPM 5,000 RPM 6,000 RPM 3rd: 26 39 52 65 78 3rd Overdrive: 33 50 67 84 101 4th: 36 54 72 89 107 4thOverdrive: 45 67 89 112 134 Road Speed in MPH w/ 3.7:1 rear axle @: 2,000 RPM 3,000 RPM 4,000 RPM 5,000 RPM 6,000 RPM 3rd: 27 41 55 68 82 3rd Overdrive: 36 53 71 89 107 4th: 38 57 76 95 113 4th Overdrive: 46 69 92 116 139 Road Speed in MPH w/ 3.07:1 rear axle @: 2,000 RPM 3,000 RPM 4,000 RPM 5,000 RPM 6,000 RPM 3rd: 33 49 66 82 99 3rd Overdrive: 43 64 87 107 128 4th: 45 68 91 114 137 4th Overdrive: 56 83 111 139 167 |
| Steve S. |
| Jim- Hmm....... Looks like you'll have to do a Copy & Paste, then use your space bar to get the chart properly aligned! |
| Steve S. |
| Ian, That depends on what cam your using. This engine is still pulling right into the red. I guess you would actually have to ride with me to believe me but the crusing @3900-4000 is not all that bad, This engine really doesn't mind it. But while accelerating I have to get it up to about 5000ish before shifting to get max accelleration. I forgot to mention that I plan to use this car for spirited road use and track days so It isn't your everyday B although I do drive it to work quite often. Steve, Thanks for the numbers. My 70 @4000 wasn't too far off. Also thanks for the engine article you snt me a while back lots of good info. Thanks, Jim |
| JCR Royal |
| You are going to be exceeding the recomended piston speed for longevity. From memeory you can run a B up to 3700 rpm indefinetly with no harm done , above that you will get rapid bore wear .I have to say that my balanced and cammed engine is almost as happy to rev as modern short stroke 16 valve , however I have chosen tyres that keep give the 22 mph/100rpm it was desinged for with the o/d in . A lower back axle ratio is a good idea with a close ratio gearbox, preventing the converation between 2 MGA enthuaiasts I know who had swapped cars for fun when the CR one got stuck beacaus first was too high on a humpback bridge "slip the clutch" "I am slipping the clutch!" |
| S Best |
| really, I had never heard that above 3700 is a bad thing. I have had this engine apart many times and there has been no wear to speak of in the last 20K miles (about 4 seasons of use for me). I have made many changes in the evolution of this engine but I have never changed the rings or pistons. I will have to retink my position if this is a fact. Can you please tell me where you heard this? BTW this is my first MGB transmission rebuild EVER and I abuse them pretty bad. I have to take my hat off to the engineers my car is about to celebrate it's 40th birthday in Feb and it is still an absolute ball to drive. It's 40th birthday present is already in progress. It's getting a full restoration. All the way down and all the way back in 7 months (maybe) Thanks, Jim |
| JCR Royal |
| I've heard the "calculated piston speed" stuff before and only put a certain amount of weight on it. MG themselves said (and I'm paraphrasing) that engine speed was irrelevant and only the proper supply of clean lubricant was the controlling factor for engine life. Hondas run silly rpm for tens of thousands of miles from the factory as do nearly all motorcycles. Yes, the bore/stroke ratio and rod ratios are different but with modern piston/ring materials, modern lubricants and closer tolerances on rebuilds I am certain you can turn higher than 3700 rpm for long periods of time with no discernable loss of service life. JCR's lookin' fer fun! I'd say his experience with numerous tear-downs and wear indicators on his application speaks pretty well to the way he runs his motors. Mike! |
| mike! |
| I will dig up the piston speed stuff , of course as materials get better so the maximum piston speed allowable goes up. Re motorcycles , sure you can get your ZX10 on the cam at 10000 amd scream it up to 14 ( well I could if I was 40 years younger , sigh) but because of its short stroke it will not have a higher piston speed than a 1600cc Fiat .This is the reason that only bore was specified in the pre war UK horsepower tax, once you fix piston speed and bore, there is no need to define the stroke . |
| S Best |
| I remember reading the piston speed calculations before on this board. I found them very interesting and it would be great to see them reproduced here again. They made a lot of sense to me. cheers |
| Ian Buckley |
| S BEST It would be even better if the piston speed calculations were put on a new thread so they are easier to find in the archives? |
| Ian Buckley |
| There is not much to the sums on this , every time the crank goes round the piston goes up and down the bore . With the 3 1/2" stroke of the B series this gives you 7" per rev. So at 4000 rpm the piston is travelling at an average speed of 2333 feet per minute . The hard part is the experimental data derived in engine test cells , and I will try to remember where I got this |
| S Best |
| Jim, I couldn’t agree more but as it turns out this is not an ordinary road going car and you ARE a noisy bu---r ! Now I would have to agree with Steve and always felt that engines would be happiest between 3 / 4,000 rpm. Everything has a finite life and if this happens to be x million revolutions the faster you rev it then the quicker it will get there, the more you flog it in the lower gears the shorter will be the mileages between overhauls although the total revs will be approx the same. As the revs rise out of balance forces will increase within the motor and this reduces life albeit if the engine is well balanced then these forces will be reduced. I agree that oils have improved dramatically over the years but we are still using 50 year old technology and it does not take kindly to continuous high revs in the same way as a modern motor does and these have the added advantage of new materials, more accurate manufacture and better internal design which in itself reduces internal stresses. So for MG to have said that provided an adequate supply of clean oil is provided no other factor will affect the life of the engine is complete rubbish. Having said all that I can still see where you are coming from so you obviously want to preserve the low speed performance but I still feel it would be detrimental to lower the diff ratio in the interests of relaxed cruising. Only you can make the decision whether or not to go for a c/r gear set which on the face of it would seem to be OK bearing in mind the already low ratios but the price tag seems to put this into question. |
| Iain MacKintosh |
| I've been thinking about this a bit this morning and I believe that may be an oversimplification. During the rotation of the crank the piston start at TDC and accelerates rapidly reaching peak velocity in the center of the downstroke and then slowing until it reaches BDC where it stops and reverses it's motion, the same happens on it's way up. This may be why stroke is so important in this equation, The longer the stroke the faster the PEAK velocity at a given RPM. My thinking is that the piston not only has to move faster but it has to do it in an area that is not spread evenly across the whole bore but more concentrated in the center. Does this hold any water or should I just shut up now. Jim |
| JCR Royal |
| Jim, I take your point but I think its more to do with the average piston speed rather that the maximum at any one point in time. Obviously the average piston speed of an engine having a 3" stroke will be 50% greater than that of a short stroke motor with only a 2" stroke. The maximum piston speed will also be much greater. The other point worth remembering is that the total distance travelled by the piston per rev is greater and so in say 50,000 miles the long stroke piston will have been subjected to the percentage of increased wear bearing in mind the distance travelled. Travel this distance at considerably higher revs and the wear factor goes up again. |
| Iain MacKintosh |
| Some rapid research reveals that low tech engines like the B series would not have max piston speeds of about 3000 to 3500 feet per minute, while modern cars might be 5000.These numbers lead to the 5000 to 5500 rpm red and yellow lines on our cars . Mean piston speed is a rule of thumb since what really kills the engine are considerations such as the piston accleration at TDC. It's very interesting to hear that modern oils seem to prevent scuffing of cylinder walls even at constant RPM of 4000 and above. I always keep the cars speed down on motorways (any car) as drag ( and so fule consumed)is proportional to the square of speed , and any correlation between increased cruising speed and lower journey times on our crowded roads is very weak . At other times , well sports cars are built to be fun. |
| S Best |
| What has really changed in the last 60 years with respect to engine materials and performance? Fuel and spark management are biggies (no effect on engine stresses other than minimizing knock and those nasties). Air management in the head, swirl and chamber design biggies but again, they are "non-moving" parts. Reduction of parasitic losses are nice with overhead cams and "space-age" coatings. This brings us down to the innards. Alloys have been tweaked, manufacturing methods have been massaged and design flaws eliminated but in all, the bits all are pretty much the same. Ditto for the other pieces (no comment on Indian or Chinese parts). Sure, the massively under-square B-series engine is not a high RPM screamer, but when you rebuild do you find 60 year old pistons and boxes of NOS parts or do you source new items "hot off the shelf" ? Economies of scale pretty much ensure that your MGB pistons (especially in oversizes) are going to be the same material as the new pistons the company makes for the big dogs. The theories of gas control (re: the rings) have developed so that it is understood that high-tension fat rings (or five of them for heaven's sake) are not necessary or desireable. Your machineist may not like to but a good one has the ability to give you very close tolerances. The engine can be balanced within a gnat's posterior of "perfect." All this leads to much better "damage control" within the old girl than was available from the factory no matter how careful they were with shimming the mills with cigarette paper every morning. This is why strict "piston speed" limits are less of a consideration to me than the other factors described above. Sorry, I just get all worked up when I hear people being berated for operating their pride and joy in contradiction to some "absolute" limit. Mike! |
| mike! |
| Thanks to all for your comments, Mike, That is exactly what I was thinking. The only original parts left on this particular engine are the block, crank, rods and the core of the head. Everything else has been tweeked to try to make it just a bit more fun. I keep the original engine stashed in the far reaches of my barn so if I ever want to drive her "the way god intended" I'll pull it out and install it. But until then I'll keep trying to kill this motor which I have not even come close to doing yet. I am not looking for longevity here, I actually enjoy trying new things and the R&D process that goes along with it. I actually had a small amount of excitment when the tranny went bang! It allowed my to dig into somthing I had never done before but I am serching for that better mousetrap so it doesn't happen again. Thanks again Jim |
| JCR Royal |
| Jim- Ah, we're so much alike! Just have to create a better mousetrap, right? Cambridge Motorsport offers helically-cut close-ratio gearsets for the three-synchro transmission in helically-cut gearsets, that use the more desirable larger diameter (.668") layshaft. The ratios are: 1st- 2.450:1, 2nd- 1.620:1, 3rd- 1.268:1, 4th- 1.000:1 This makes for the following ratio gaps and engine speed changes when shifting: @ 3,250 RPM @ 5,500 RPM Drops: To: Drops: To: 1st-2nd .830 1,016 RPM 2,234 RPM 1,863 RPM 3,637 RPM 2nd-3rd .352 652 RPM 2,598 RPM 1,195 RPM 4,305 RPM 3rd-4th .286 687 RPM 2,563 RPM 1,162 RPM 4,813 RPM The available rear axle crown &pinion gearsets produce the following results: Road Speed in MPH w/ 4.875:1 rear axle @: 2,000 RPM 3,000 RPM 4,000 RPM 5,000 RPM 6,000 RPM 1st: 12 18 23 29 35 2nd: 18 27 35 44 53 3rd: 23 34 45 57 68 3rd Overdrive (D): 28 43 57 71 85 4th: 29 43 57 72 86 4th Overdrive (D): 33 50 67 84 100 Road Speed in MPH w/ 4.55:1 rear axle @: 2,000 RPM 3,000 RPM 4,000 RPM 5,000 RPM 6,000 RPM 1st: 13 19 25 31 38 2nd: 19 28 38 47 57 3rd: 24 36 48 61 73 3rd Overdrive (D): 30 46 61 76 91 4th: 31 46 61 77 92 4th Overdrive (D): 38 57 77 96 115 Road Speed in MPH w/ 4.3:1 rear axle @: 2,000 RPM 3,000 RPM 4,000 RPM 5,000 RPM 6,000 RPM 1st: 13 20 27 33 46 2nd: 20 30 40 50 60 3rd: 26 38 51 64 77 3rd Overdrive (D): 32 48 64 80 97 4th: 33 49 65 82 98 4th Overdrive (D): 41 61 81 101 122 Road Speed in MPH w/ 4.1:1 rear axle @: 2,000 RPM 3,000 RPM 4,000 RPM 5,000 RPM 6,000 RPM 1st: 14 21 28 35 42 2nd: 21 32 42 53 63 3rd: 27 40 54 67 81 3rd Overdrive (D): 34 51 68 84 101 4th: 34 51 66 85 102 4th Overdrive (D): 43 64 85 106 128 Road Speed in MPH w/ 3.909:1 rearaxle @: 2,000 RPM 3,000 RPM 4,000 RPM 5,000 RPM 6,000 RPM 1st: 15 22 29 37 44 2nd: 22 33 44 55 66 3rd: 28 42 56 71 85 3rd Overdrive (D): 35 53 71 89 106 4th: 36 54 72 89 107 4th Overdrive (D): 45 67 89 112 134 Road Speed in MPH w/ 3.7:1 rear axle @: 2,000 RPM 3,000 RPM 4,000 RPM 5,000 RPM 6,000 RPM 1st: 15 23 31 39 46 2nd: 23 35 47 58 70 3rd: 30 45 60 75 89 3rd Overdrive (D): 37 56 75 94 112 4th: 38 57 76 95 113 4th Overdrive (D): 47 71 94 118 141 Road Speed in MPH w/ 3.07:1 rear axle @: 2,000 RPM 3,000 RPM 4,000 RPM 5,000 RPM 6,000 RPM 1st: 19 28 37 46 56 2nd: 28 42 56 70 84 3rd: 36 54 72 90 108 3rd Overdrive (D): 45 68 90 113 135 4th: 45 68 91 114 137 4th Overdrive (D): 57 85 114 142 170 You can do a Copy & Paste, then space everything out so you'll have nice, useful charts. |
| Steve S. |
| Steve, All I can say is WOW. You just saved me about 3 hours time. It looks like I can use the MGA ratio (4.55:1 right) and still have about the same crusing speed when using the overdrive. The close ratio should enable me to stay on top of the cam and the low rear end gearing would make standing starts and acceleration better. I'm going to have to give this some thought. Thanks again for doing the math. Jim |
| JCR Royal |
| I know that racers like the 3-synchro gearbox because its lighter than the 4-Synchro gearbox, but which is stronger, the late 3-Synchro gearbox with the thicker shaft and 4 bearings, or the 4-Synchro gearbox? |
| Greg Hayes |
This thread was discussed between 13/12/2004 and 17/12/2004
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