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Hello. I have a friend who has just put new synchro's in a 750 tunnel box and it is not operating as it should. When operating the gears the synchro's don't work efficiently, especially on down shifting. We are using 90 grade non EP oil, the clutch is good. what else could it be? He purchased new synchro's , do they need lapped in with the gears? Does anyone have any other insight?
Thank-you
 

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I've always had mixed reactions on these here GB's. I'd heard kudos about how they were the best, smoothest GB's ( "shift like butter" ) on the planet when they work. I could never imagine how they could ever be better than a good split case. I had Claus Menzel rebuild mine in a '57 Sprint and it either needed a break-in period or it was what it was .. The synchros worked fine but the throws to me were tight.. almost "agricultural" compared to a "thumb and one finger " throw on a split case. I'd give it a chance for a while before I got too excited about curing what ails you about it.

I had never driven one before mine so I had no baseline to compare what a "like butter" shift should feel like. Give it a chance with some miles under it before you chase the rabbit down it's hole, is the best advice I can give. Perhaps those who have better experiences will chime in. It might be you only need to just evaluate how the linkage is adjusted.
 

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I've rebuilt tunnel cases for years. And have one in my 58 spider veloce. Great shifting box. On one of my recent rebuilds I used some new synchros. I found they have much greater clearence over NOS ones. 90th up on the gear cone vs. 60th for NOS ones. I wondered if this extra clearence would cause shifting problems because the synchro hub would be farther away from the gear engaging teeth with the repops. If this is the case then maybe waiting for the synchro to wear down is the way to go. However, be careful, you dont want to round off the gear engaging teeth in the process. So careful and slow shifting at first.

Also make sure you are using a GL 1 gear oil. GL 4 or 5 will not work. tractor supply sells GL 1 in two gallon jugs for about $20.

Reed
 

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I have trouble adding pictures. Send me an email and I will provide pictures of the clearence differences. RWCearley at gmail.com

Reed
 

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750 gearbox

Reed
Thanks looks like i'll remove the gearbox and have another attempt. the old synchros had a gap of 0.9 mm new ones 1.8 mm so will try and bed in. I have tried driving for 2 hours shifting at regular times can go up through the gears but down shifting is not possible unles nearly stopped. The synchros do not seem to slow down the gear cluster.
I have been stopped push the clutch and try and select a gear the gear cluster is not slowing when try any gear. when i wait until it stops spinning all gears are easy. Clutch has been checked adjusted and not hanging up

Clues other than gearbox out and lap synchros and cones to get closer fit and check contact with engineers blue
 

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I generally try rebuilding with original synchros for this reason. But with the original supply drying up, its repops or nothing. I suspect the problem is the synchro is slowing down the gear prior to the internal teeth of the hub fully engaging the teeth on the gear. This would account for the hard shifting as described.
 

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One more idea on problems in shifting tunnel cases. If the hub is worn that will also cause problems in shifting. Forks worn badly or hub is too loose on its slide. Common problem with 60+ yr old trans.

Reed
 

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These are the pictures that Reed wanted to load earlier, I've re-sized them suitable for the BB format

Pic 1 - Original NOS tunnel case syncro - note the 60" (1.5mm) clearance between the syncro teeth & the dog teeth on the gear

Pic 2 - New repop/re manufactured syncro - note the 90" (2.3mm) clearance between the syncro teeth & the dog teeth on the gear

Pic 3 - New repop syncro on top of the cone

Ciao
Greig
 

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As Angus knows, (and Greig who has been extremely helpful) we have just been through exactly the same process, using parts from the same supplier. 25 hours to remove, strip, rebuild and replace the box, it's frustrating to say the least!:frown2:
 

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kiwiangus

I will remove box strip and carefully inpect to find the problem and advise of findings
I suspect the angle of the cone and synchro are not the same some fine fetteling
 

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I suspect the angle of the cone and synchro are not the same some fine fetteling
Thats what I have been thinking.

I have built 2 of these in the last several months. The first one came out fine. I put new bearings and synchro's in it. But I had to get the synchro's from several different places as most of the parts suppliers were out of them and would have them in about 4 weeks. This works as it should.

The second one I built was after the new stock came in. All the synchro's came from the same parts supplier. So I believe the one I got were of recent production. Its doing the same as the original poster describes.

I have built a few of these over the years and have never had this problem.

So I'm leaning the same direction as Kiwiangus.
 

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@RWCearley

Can you PM me with when you bought the repo ring?
@kiwiangus

Can you also send me a PM on when you got the new rings? Also have you taken your back apart?

I believe the problems are from the latest batch of rings that were made. As I built 2 of these gearboxes about 3 months apart and know I end up with 2 sets from different production runs.

I will post some pictures tonight of a set of rings I just got in. It think I have an idea of what the problem is.
 

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I purchased the synchros approx 2 yrs ago. I recently sent someone to the vendor (in Germany) who purchased a full set of these synchro rings. He reported back that the tunnel case shifts smoothly with the new rings.
 

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I purchased the synchros approx 2 yrs ago. I recently sent someone to the vendor (in Germany) who purchased a full set of these synchro rings. He reported back that the tunnel case shifts smoothly with the new rings.
Was that the NOS ones or the repop ones?

Which vendor was it?

Thanks
 

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Heres what I feel is the problem. Is the slag for the machining of the vertical grooves. To correct the problem the slag will have to be filed down and the top of the groove opened up. Also the the fine horizontal grooves will have to be opened up in the valley of the groove where it meets the vertical groove.

First 2 pictures are of the grooves on new ones I got about 3 weeks ago.

3rd picture is an original and the repro with flat teeth.

Last to are of a old repro and an original showing what the grooves should look like.
 

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Has anyone tried filing the burrs of of the groove edges to see if they work correctly?
 

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Has anyone tried filing the burrs of of the groove edges to see if they work correctly?
I have removed the gearbox deburred all the synchros reassembled and had no improvement i am considdering refitting old synchros a 2 of them were passable as im in New Zealand the owner will suffer double declutching until winter
 

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I have removed the gearbox deburred all the synchros reassembled and had no improvement i am considdering refitting old synchros a 2 of them were passable as im in New Zealand the owner will suffer double declutching until winter
I talked to Classic Alfa as they have completely removed the pages for the 750 series synchro's. Tom said. My understanding is the cone angle is wrong. Tom also said they would take them back and refund the money spent. He even told me they would take back the ones I had installed in the one I did.

He said they were looking into having them made there in England but it would probably be 6 plus months before they have them.

Which would make sense. As you still have the problem after deburring them. Also in talking with Larry Jr. he filed the burrs down and found that they some of them were also egg shaped and wobbled on the gear.
 
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