curious thumping in rear end

[ging]

Ging's husband at the keyboard again.

We got the GT on the road today and as mentioned before I was sitting on the passenger side floorboard and "GING" was driving. I notice that every time the drivetrain 'loaded' (instant of acceleration or coast) there was a hollow, metalic thump in the rear end that I heard in the rear end and felt in my levi's. I investigated a little when we got it back in the yard. What I found may be normal. Just need the advise of someone who has had their hands on a good shaft and a bad one. I am able to twist the shaft (I'm estimating) maybe 10 degrees. If I twist with enough force I get the same 'thump' in the differential. I have no play in the central 'hanger' bearing and no fore and aft movement (at least not enough to alert me or cause suspicion). The center console and shift boot are not yet installed so I may be hearing more that I would normally. Also, my butt would normally be on a padded seat and not resting on floorboard (that is connected in someway to everything in the car that moves). Additionally, everything that moves between the radiator and the differential was removed and reinstalled to facilitate the crank replacement. Reinstalled per the FSM.
SOOOOO... I guess the question is... Is this shaft play normal? Is the noise not unusual? Am I just being too paranoid after such an invasive repair.

any commentary is welcome

thanks , dan

[nobody]

That noise is usually associated with the turque tube and the bushings or doghnut in it. More likely the bushingd or stops from past experience.

[jeff denton]

Dan, did you inspect the differential at all? It is so incredibly commonly overlooked when servicing cars. How many people ever drain the rear end and inspect the old lube?
Trucks have a magnet on the fill plug. If the rear end is going you know it by seeing "junk" on the magnet every time it is checked, which better be once a week on a rig that runs hard every day and night!
I tried once to find a magnetic plug to fit the GT. No luck, so I put a magnet in the cover. I suggest everybody do this! It is just too simple to pop the diff cover off and check it. If all is good, oil is clean, feel free to reuse the oil.

[ging]

thanks guys, I'm familiar with the magnetic drain plugs (what a clever and simple engineering trick). I'll check the oil in the a.m. The PO rebuilt the whole car, top to bottom and front to back, and addressed alot of the common problems assosciated with GT's (wire harness, light buckets ...). Apparently, though, not alot of attention to detail. We purchased the car 250 miles after the rebuild. 750 miles later we needed to replace the crank and that is where the story starts.
Long story short...I'm assuming the diff had been serviced along with everything else. Of course, I cannot say it was done correctly. I normally would not suspect bad or failing parts so soon. Just wondering if I'm being overly paranoid with every noise. Don't want to tear something apart just to learn how it makes that normal noise.
I know some rotation in the drive shaft is normal, I just don't know how much would be considered excessive.

[GTJIM]

The Opel diff is flexibly mounted so that everything can move just a wee bit when power goes on and the suspension moves. The mounting set up at the front of the torque tube accommodates a happy little hop during operation.
The drive shaft extension inside the torque tube can oscillate as the power goes on or off and connects to the pinion vis a barrel shaped spline which likewise allows considerable movement. The only thing keeping this extension back in contact with the pinion is the spring between the gearbox tailshaft and the yoke at the front end of the driveshaft which is bolted to the pinion extension. So check that the spring is there - if it is not, then the whole drive shaft/pinion extension can slam backward and forwards.

Also check that all the rubber elements at the front of the torque tube, including the 'dough nut' and the rubber bumpers on the outside of the torque tube are in place and not deteriorated. If they are broken or missing then the "happy little hop" turns into an "unhappy thump" ; especially upon acceleration.

[Gordy]

I had my Opel GT up at a Lube Pros for them to add trans lube and just happened to ask them to check the rear end as well. When they stuck their finger in, it came out like mud! They sucked out the old grease and refilled with new. They actually did it twice since my fluid was so bad. All this cost about $29. I bet there are a lot of Opels that have 30 year old fluid in the rear ends!

[swiss]

When I bought my '69 GT MANY years ago I got it with about $500 off the going price because it wouldn't idle below about 2k-2.5k rpms and when you gave it gas it "thumped" hard each time you got on the gas. I took it home and looked at the drive tube and found that the rubber donut and the other pieces were worn out. I went down to the Buick dealer (kind of neat that back then you could buy parts at a dealership) and got new rubber pieces that were in stock. I put them in and the noises were magically gone! Then I went to work looking at the throttle and found that when the "Race Shop" that had worked on the engine had changed the cam and installed the Weber, they had reassembled the throttle linkage without ANY of the bushings in the linkage mounts. So I built some bushings from grommets and metal sleeves and put everything back together. With some adjusting on the idle screw, the idle came down to about 800-850 rpms and this was with the "racing cam" that the "shop" had blamed on the bad idle. Took me a weekend to fix this stuff.

Swiss

[tekenaar]

When the rubber donut inside the front of the torque tube ages and /or gets brittle and cracks in the rubber, it can no longer control the torque loading of the driveshaft under either accelerating or decelerating conditions resulting in the "thump" you hear. If it looks bad enough, replace donut and the bumper and two screw-in side mounts of the torque tube.

Either way, while you have torque tube hanging down a bit by its loosened bracket (support the nose of the rear end center section while doing this, BTW), cut six 3.5" sections from an old V-belt and push these in pairs into the three slots on the outside of the torque tube bracket. This is an old trick Opel racers used in their Rallying heyday.

[ging]

Thanks everyone for all the tips and advice.
I did some more tinkering today and didn't see anything that I thought was unusual (other that the amount of twist, which I haven't determined as unusual). I did not remove anything or drain the diff yet, just poked around. I was honored with permission from the wife to take the GT for a spin around the block. COOOL. Actually sitting on a padded set made a significant difference. The thumping I had noticed when riding on the floor is not even noticeable. Right now I'm going to assume I was overly paranoid over every noise I heard. There are several other items to tweak so I'll concentrate on them for now and do a more thorough investigation on the rear end later.

BTW tekenaar,the "V-Belt" trick? I saw a thread recently about that. Is that trick just basically installing little bumpers around the guts of the bearing so it doesn't bounce around inside the hanger or is there another purpose. I'm sure there are some pics somewhere around here... just haven't looked yet.

Thanks again for everyone's advise and help
dan aka... Mr. GING

[tekenaar]

Originally Posted by ging Thanks everyone for all the tips and advice. I did some more tinkering today and . . . do a more thorough investigation on the rear end later. BTW tekenaar,the "V-Belt" trick? I saw a thread recently about that. Is that trick just basically installing little bumpers around the guts of the bearing so it doesn't bounce around inside the hanger or is there another purpose. I'm sure there are some pics somewhere around here... just haven't looked yet. Thanks again for everyone's advise and help dan aka... Mr. GING

Exactomundo!

My illustration of my previous post to yours wasn't clear?

[soybean]

Originally Posted by Gordy I had my Opel GT up at a Lube Pros for them to add trans lube and just happened to ask them to check the rear end as well. When they stuck their finger in, it came out like mud! They sucked out the old grease and refilled with new. They actually did it twice since my fluid was so bad. All this cost about $29. I bet there are a lot of Opels that have 30 year old fluid in the rear ends!

Wouldn't it be easier to do the same thing using post 1 of this thread http://www.opelgt.com/forums/showthr...ial+drain+plug. The question I have, is there a way to "flush" the rear end, or would doing so destroy the seals?

Jeff, how did you secure the magnet in the cover?
Thanks, Jarrell

[jeff denton]

I just placed the magnet in the cover, thus, it is held in place by magnetism. It isn't going anywhere.
When building my race car the cover was off a lot and I had some rags stuffed in it, while the axles were out I put rags in the bearings, too. But I know some trash got in the diff, once the rag even caught on fire from welding sparks... Plus I welded the spider gears to the side gears while the unit was in the diff. housing. So I needed to do a little cleanup...
When I got it running and cam broke in we ran the car in gear with the tires off, had 30 wt oil in the trans and diff. Drained that and put in 80-90. After practice session #1 I pulled the diff cover and there was metal on the magnet. We haven't checked it since, it only ran practice session #2, but as I have some 4.22 gears coming it will be apart soon.
That's how I flushed mine. I wouldn't put any mystery gook in there, I don't trust what it could do to the seals.
Same with the transmission! Put a magnet in there, too. I will do this, but first I will make a "deep pan" like Rally Bob shows in another thread.

[RallyBob]

Originally Posted by jeff denton We haven't checked it since, it only ran practice session #2, but as I have some 4.22 gears coming it will be apart soon.

I just realized something Jeff....are you aware that you need to use a different carrier to fit 4.22 gears in the rear axle? Opel 3.18 and 3.44 ring and pinions use one type of carrier, while 3.67, 3.89, 4.22, 4.30, 4.75, 5.13, and 5.28 ring and pinions use another carrier style. Manta Rallye models with 4-speed transmissions came stock with 3.67 gears/carrier and will drop right in a 1971-1973 Opel GT rear axle housing.

Alternatively, you can make a 4 mm thick pinion gear spacer to use the 4.22 gears with your stock 3.44 carrier.

Bob

[jeff denton]

Stan was right on top of it! He's sending the ring gear attached to the proper carrier, welded spiders and all! What a guy, huh?
Thanks for thinking of me, it's good to know I have expert help available!
Come time to actually install the gears I may need some pointers, I've done Chevy 12 bolts and zillions of big trucks but I know there's just gotta be at least one thing Opel had to do weird there, too!

[soybean]

Thanks Jeff for the answer, and thanks RallyBob for the tip on the Rallye gears/carrier. I knew storing those rear ends for 20+ years would come in handy some day. Jarrell

[jeff denton]

Do you have some extra rear ends, Stungunner?

[soybean]

Jeff, Most of the rear ends (2) I stored were for the Manta series, (1 Rallye, 1 regular). I have 1 rear end in a Gt junker that I cut the front end off for Gene. I have another that's still in a 75 Manta, (Non Rallye) and know where 2 more are. Jarrell

[BQS4]

I have a junker GT here that still has the rear end under it, it's up for grabs.
Gene