Do I need to replace Crank Seals? Pistons?

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Olafskii
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Do I need to replace Crank Seals? Pistons?

Post by Olafskii »

First of all, sorry if this subject has been answered on this Forum. However, I haven't been able to find enough information to satisfy this question and my searches haven't yielded good results.

My bike is a 1972 GT750J. When I bought it from the original owner a couple of years ago, he started it and his nephew rode it up and down the lane to demostrate it. I haven't run it since but when I spray layup oil in it, and crank the engine, it tries to run. The bike has 10,000 miles on the clock and is completely unmodified other than a set of ugly 1970s style ape hanger bars that were dealer installed (the guy still had reciepts from the 1970s to prove it) The engine has never been apart or out of the frame. I'm working on a full restoration.

I changed the crankcase oil after I got the bike. The oil came out milky white meaning it had water contamination in it.

My understanding is that a radiator leak-stop (like Bars Leaks) might seal up the cooling galleries. But apparently the 'J' leaks easily at the barrel to crankcase joint. This problem was later fixed with an o-ring in the cooling gallery joint.

My question: should I plan to pull the barrel off to fix this leak. I have no idea how long the bike sat like this with the contaminated oil so it's possible that there's rust in the engine. While I have thinks apart should I plan to replace the crank seals? Crank Bearings? Pistons? If I replace the pistons, should I rebuild with the old style pistons that have the connecting rod 'fixed' at the big ends or should I rebuild with the newer style pistons that have the connecting rods fixed at the wrist pins?

These questions are important because if I'm going to order parts I want to try to get as many in one shot as possible. This saves in shipping cost.

Thoughts?

Olaf
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tz375
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Re: Do I need to replace Crank Seals? Pistons?

Post by tz375 »

The milky oil is a sign of water in the oil as you suspected and is not a good sign.

Some may argue that you could do a crank leak down test and it that's OK, then leave the crank alone. I feel that if the motor is coming out and cases split to see if there's any water damage, you may as well replace the crank seals while it's all open.

The pistons are probably OK to use after they are cleaned up and that makes for one less set of decisions. If the pistons are cracked or otherwise unusable, I would suggest a replacement later style crank and later style pistons. I think I have seen ever combo of mix and match with old and new rods on the one crank, new rods and old pistons and old rods and new pistons. None of those are ideal though.

Old style cranks are OK for the sort of miles most of us do, but for the two Allans that do serious miles on their bikes, maybe better to go all later style. So if the pistons are dead, see what you can get and if you can only get new style, pick up a replacement crank with later style rods and get it rebuilt. Old style cranks are not only different in the big end design, the crank webs are also different, so try to avoid new rods on old style webs. The problem is that the rods are likely to rub and over time that will cause failure.

Once it is all torn down, you could also fit a later style clutch center bearing or even later type gear set. That reminds me that I should sort mine out and throw some up on flea bay.

Good luck and please post lots of pictures as you go along.
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Re: Do I need to replace Crank Seals? Pistons?

Post by Vintageman »

later OEM stuff usually better, check all service bulletins that apply, change all items in crank if you have the coin. yes, rust is probably what makes changing most all necessary and for long term reliability if that is important to you (or the next person). That engine takes a little effort to work on so getting it right first time saves a significant second time effort.
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Olafskii
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Joined: Sun Dec 18, 2016 6:15 pm
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Suzuki 2-Strokes: 72 GT750, 74 GT750, 75 T500, 76T500, 09 DR650
Location: Northern British Columbia

Re: Do I need to replace Crank Seals? Pistons?

Post by Olafskii »

Thanks for the detailed replies. As a start I'll clean the carbs and run the engine, ride a few miles on a new batch of oil to try things out. I want to avoid tearing into the engine if things are generally OK.

Keep the group posted.

Olaf
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