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GT750J Engine Renewal--work begins

Posted: Thu Jan 03, 2019 11:06 pm
by Olafskii
I got the J engine on the bench finally (after figuring out and developing some tools on my 'L' practice engine).

I checked pressure stats to evaluate condition before tearing everything apart.
Cylinder compression (R-L): 110, 122, 122.
  • Engine cooling system pressure test: 8 psi & holding for 6 hours. This is a real head scratcher because when draining the gearbox oil there was visible glycol in it. This is after 2 years of dry storage with the cooling system full. The engine had been turned over every now and then but not run. Ideas anyone?
    Crankcase pressure (R-L): up to 2 psi and not holding, 8 psi (this is the max) and not holding, up to 5 psi and not holding.
(BTW--I did plug the SRIS valves)
IMG_0940.jpg
All the leaks were internal so the crankshaft needs new seals.
So I'm at peace now with my plans to open up then engine. I'd hate to do this if everything was good.

Re: GT750J Engine Renewal--work begins

Posted: Sun Jan 06, 2019 10:57 pm
by Olafskii
Over the weekend I got the cylinder head off. Here's how it went:

-The washers under the sleeve nuts were not the sealed type that were fitted to later GT750s. (this is a relatively early 'J'--serial number 16172). The head looks like it's never been off--the previous owner bought the bike in 1975 and didn't do any work on the bike.

-3 of the stud holes had glycol coolant in them meaning that the cylinder base gasket is shot. This may also be one of the reasons that glycol found its way into the gearcase during storage.

-2 of the 11 studs were seized to the sleeve nuts and unthreaded from the block

-all 6 of the 8mm capscrews came out OK with no seizing.

-the 8mm stud that's in the head sits in an aluminum or austenitic stainless steel insert (i.e. it's not magnetic) that's threaded into the barrel. The insert appears to have been installed at the factory maybe to correct a casting flaw. There's a bunch of RTV type sealant under the head gasket around the stud.

-the head came off with some light sideways tapping from by plastic tipped mallet.

-There were some black carbonaceous grubles accumulated inside the cooling galleries. I was able to dig it out with my plastic scraper and sucked a bunch of it out with my vacuum cleaner.

Re: GT750J Engine Renewal--work begins

Posted: Mon Jan 07, 2019 2:04 am
by Splant
Great pics / descriptions ,love to follow your rebuild please keep it going.

Re: GT750J Engine Renewal--work begins

Posted: Mon Jan 07, 2019 5:54 am
by GTandcbr
Agreed great pics. If you are not aware the big challenge will be removing the jugs/barrels as they are notoriously difficult to separate. ..

Re: GT750J Engine Renewal--work begins

Posted: Mon Jan 07, 2019 9:27 am
by Splant
I remember a h1 barrel that wouldn't budge i drilled through the bottom of the barrels on one cylinder where the studs were to see if the penetrating oil was getting down there, and eventually had to drill through the stud completely on one corner ,I guess that wouldn't work on a watercooled engine.

Re: GT750J Engine Renewal--work begins

Posted: Mon Jan 07, 2019 10:49 am
by GTandcbr
You can get a puller and some guys have made their own
viewtopic.php?f=2&t=6323&p=166731&hilit=Puller#p166731" onclick="window.open(this.href);return false;