74 GT750 Valve guide seals

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Bern
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74 GT750 Valve guide seals

Post by Bern » Mon May 07, 2012 10:01 am

Looks like the failing exhaust valve guide seal on my vertical cylinder has gotten worse so I need to take some action. I would like to try and do this without dropping the engine. Is this doable? Other than ensuring that the piston is on the top of the compression or exhaust stroke so the valve doesn't drop into the cylinder, are there any other tips?

Thanks,
Bern

wdietz186
888
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Re: 74 GT750 Valve guide seals

Post by wdietz186 » Wed May 09, 2012 6:37 pm

To do it you will need to block the valve from dropping into the cyl. I've heard of people using rope,tubing etc. stuffed into the cyl.,or using air pressure to hold the valve shut. Fashioning a spring compressor using a rod in the rocker spindle hole and a pad to compress the spring wouldn't be too hard to fab up. But! More than likely it has a worn out valve guide and a new seal would be a temporary bandaid. You need to check the stem for play by turning the engine until the valve is about 1/2 open and pry[gently] front to back on the spring and see if you have discernable play,much over a few thou. of an inch [004" max. I think] indicates a worn out guide.

Bern
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Re: 74 GT750 Valve guide seals

Post by Bern » Thu May 10, 2012 4:17 am

wdietz186 wrote:To do it you will need to block the valve from dropping into the cyl. I've heard of people using rope,tubing etc. stuffed into the cyl.,or using air pressure to hold the valve shut. Fashioning a spring compressor using a rod in the rocker spindle hole and a pad to compress the spring wouldn't be too hard to fab up. But! More than likely it has a worn out valve guide and a new seal would be a temporary bandaid. You need to check the stem for play by turning the engine until the valve is about 1/2 open and pry[gently] front to back on the spring and see if you have discernable play,much over a few thou. of an inch [004" max. I think] indicates a worn out guide.
Thanks WDietz

I was hoping to do just the seals but I think it makes sense to do the valve guides at the same time. I guess it's pop the heads off time. I've seen the rope trick, compressed air seems like a fair bit of effort.

I'll get the cylinders measured to see if they're within tolerance while I'm at it.

Cheers,
Bernie

wdietz186
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Re: 74 GT750 Valve guide seals

Post by wdietz186 » Thu May 10, 2012 6:23 pm

Beware,'Tis a slippery slope you are about to descend. It will probably go beyond valves and guides.Those hi comp pistons you didn't know you wanted will end up being ordered,and while the engine is out the powdercoat for the frame will just pop up,the stainless spokes will appear,the painter will tell you only about two weeks ,the new wiring harness will appear in your dreams..........

I've been down that road

radecal
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Re: 74 GT750 Valve guide seals

Post by radecal » Fri May 11, 2012 12:57 am

Agree with wdietz186!!!!!!
You start with a quick tune and then " I'll just check the shims quickly" and then to "maybe the rings need a quick measure" and before you know it, you have a frame on the floor, engine parts on the bench and hole in your wallet you could drive the Titanic through.
Probably more scary is the old " Hmm! Seeing as I am going to do this bike, I might as well do the other one at the same time as there will be synergy in the production line and I will have both bikes done much quicker than if I do them one at a time!".
I have the photographic proof unfortunately.
Regards,
RAD

Bern
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Re: 74 GT750 Valve guide seals

Post by Bern » Fri May 11, 2012 3:09 pm

Well the exhaust guides were shot, the intakes were still OK. The exhaust valve guide on the vertical cylinder literally fell out once I took off the valve spring??? :shock: I noticed that there were two oversized valve guides available from bevelheaven, +.002 and +.004. I'm thinking I would need to go with the +.004 on the vertical and would go with +.002 for the rest ,or should I take it to a machine shop and have them tell me? The heads are off so I might as well replace all of the guides right?

I did a quick test with a dry erase marker on the valve seats and the current valves (turning them 1/8 of a turn)and they appear to be seating OK. without doing a proper measurement. I will hold off on any other repairs at the moment.

I noticed that there was a lot of play in the bearing for the bevel gear that drives the cam, more on the vertical cylinder than the horizontal. Is this normal. I couldn't find these bearings on the site. Anyone know where I can source a replacement?

Here we go.. :roll: :roll:

radecal
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Re: 74 GT750 Valve guide seals

Post by radecal » Sat May 12, 2012 2:27 am

Hi Bern,
If you are going to do one, do the others so at least you have a good set-up and are not worrying when the others will go down the track and having to pull it apart again to do them.
If you are not sure, take it to someone who is familiar with Dukes and has the right gear to do the job. Might cost more but it will be right.
Normally replace the valves with the guide so don't worry about the seating as it will need to be checked when the valves are finished.
As for the bearings, get the numbers off them and any good bearing shop will be able to supply them to you.
Not sure I have heard any issues with the vertical bearing going before the horizontal but others may chime in.
Lots of things possible to cause one bearing to go before the other.
Regards,
RAD

wdietz186
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Re: 74 GT750 Valve guide seals

Post by wdietz186 » Sat May 12, 2012 5:55 am

Bern, The bearing on the top of the shaft is designed to allow for some misalignment and the shaft will move a fair amount. The important thing is when everything is in place and the bevel gears mesh properly. To work properly the guide does need to be an interference fit in the head and the hole straight and true. The .004" will probably do the job but an accurate measurement of the hole in the head is essential.Guides of larger dia. can be obtained. The valves should be measured carefully and it's usual to replace them rather than reuse. The valve seats will need to be recut as new guides are rarely concentric with the old ones O.E. Ducati valve guides rarely had the bore parallel to the outer diameter.It is also worthwhile to check the rocker spindles and bushings as these things tend to wear pretty quickly.

Bill

Bern
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Re: 74 GT750 Valve guide seals

Post by Bern » Sat May 12, 2012 10:53 am

what do you think the interference should be, about 0.002 over the inside diameter of the valve guide holes in the head?

So new guides, valves and valve seats at minimum. Potentially do the bearings, new pistons and bore out the cylinder if they are not in spec.

Thanks,
B

machten
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Re: 74 GT750 Valve guide seals

Post by machten » Sun May 13, 2012 2:26 am

Just backing up what wdietz said. The bearings are dual row self alinging bearings that are meant to have some angular "play".

I used SKF 1203 ETN9 bearings when I last replaced them. Numerous bearing manuafacturers supply equivalents - any bearing shop should be able to supply them.

From the SKF web site:
It has two rows of balls and a common concave sphered raceway in the outer ring. The bearing is consequently self-aligning and insensitive to angular misalignments of the shaft relative to the housing. It is particularly suitable for applications where considerable shaft deflections or misalignment are to be expected. Additionally, the self-aligning ball bearing has the lowest friction of all rolling bearings, which enables it to run cooler even at high speeds.
If your "play" is up and down, that is a different issue, but it may not be the bearing, it may be bad shimming.

Kev

Bern
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Re: 74 GT750 Valve guide seals

Post by Bern » Sat May 19, 2012 5:05 am

Thanks for all the input. :-D The heads and barrels are in the hands of a competent bevel mechanic now. The guides in the head appear to have been home made and someone didn't get the interference fit quite right on a couple of them.

So the work being done;

new guides,
new valves,
machine the current seats,
new pistons 1st oversize,
machine the barrels,

Is it worth going with a higher compression piston?
Any changes you would recommend to the carbs if I go that route?

Thanks,
Bern

wdietz186
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Re: 74 GT750 Valve guide seals

Post by wdietz186 » Sat May 19, 2012 8:31 am

Bern, Yes higher compression[like the 750 Sport] is worth the expense.It really livens up the bike and if you use premium fuel you shouldn't have any problems with pinging/detonation. Syd's Cycles sell a nice set. See, I told you it was a slippery slope!

Bill

Bern
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Re: 74 GT750 Valve guide seals

Post by Bern » Sat May 19, 2012 8:44 am

Ha!! It sure is..
:lol:
With 30k miles new pistons are not unexpected though. I'm pretty sure Steve, sells a set here. I need 80mm first over.

Bern
Parallel Twin
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Joined: Thu Jun 10, 2010 6:29 am
Location: Toronto, On Canada

Re: 74 GT750 Valve guide seals

Post by Bern » Sun Aug 19, 2012 9:47 am

machten wrote:Just backing up what wdietz said. The bearings are dual row self alinging bearings that are meant to have some angular "play".

I used SKF 1203 ETN9 bearings when I last replaced them. Numerous bearing manuafacturers supply equivalents - any bearing shop should be able to supply them.

From the SKF web site:
It has two rows of balls and a common concave sphered raceway in the outer ring. The bearing is consequently self-aligning and insensitive to angular misalignments of the shaft relative to the housing. It is particularly suitable for applications where considerable shaft deflections or misalignment are to be expected. Additionally, the self-aligning ball bearing has the lowest friction of all rolling bearings, which enables it to run cooler even at high speeds.
If your "play" is up and down, that is a different issue, but it may not be the bearing, it may be bad shimming.

Kev

Thanks for the part number. While I've got it apart I'll replace them both, one has significantly more lateral play. The lash is fine but I'll be careful to shim correctly when I put in the new gasket. It looks like I have to take the bevel gear off of the cam first before I will be able to pull the tower gear out.

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