Who here has rebuilt a motor with a holed piston?

[MWGS400]

I recently purchased a '78 GS400 and then holed a piston 2 months later. Now, its looking like my only options are either to replace the motor or go for a rebuild.

My question is; who here has experience with this situation? Have any of you gone for the rebuild? If so, how did you go about doing the rebuild? Did you get away with a top end rebuild or did you go for a full rebuild. This is my first bike and I paid $850 for it so I'd like to go with my cheapest option here.

Or, is a replacement motor the way to go? How easy/difficult would it be to find a GS400 motor?

Thanks, MG

[chuck hahn]

Your gonna have to pull the head and the jugs. Do a valve job ( recut the seats and new valve stem seals) . Check the valves seating surfaces per the service manual.

Remove the circlip at the end of the wrist pin and tap it out...replace the piston and reinsert the pin and clip. Good time to haone the cylinders and new rings. Now the top end is new for the next 60,000 miles IF you keep up on the valve adjustments and regular oil changes.

[chuck hahn]

Manuals found here.

http://members.dslextreme.com/users/bikecliff/

[chef1366]

I have
I had piston melting out of my valves
I only reused the cases and transmission.
You should clean out the cases to rid the motor of bits and pieces of piston at the least.
I'm sure the crank and bottom end may be ok. Check the head by removing the valves and looking at the lip of the valve and the seat.
The cylinder is more than likely toast.

[chuck hahn]

Hopefully he will be a lucky one and its just a little pinhole and no real damage anywhere else. Fingers crossed.

[BiPolarBear]

It sounds formidable, until you do a couple of them, and then it's not so bad. Chuck Hahn and chef 1366 has obviously done them -- their descriptions are spot-on.

Remember, back in the Glorious Sixties bikes were frequently the only transportation many blokes had. If you read some of the magazines of the period, it was as common as changing tyres.

When you do your first, go slow, and make a checklist. Also, put brown craft paper on a small table, and label the different areas where the bits you remove are placed as they come off. (I usually label it as I go, so that reassembly in reverse order is easier. I'm in my Later-Day Fifties, and I need all of the memory aids I can get!).

Take PLENTY of pictures, too -- in this digital age, it is a benefit that those who grew up in the age of chemical photography could only dream about. And isn't it a bit troubling how old fashioned 'snaps' and Polaroids are beginning to look as dated as Daguerreotypes?

If you read the manuals, visualize the procedure, go slow, ask some of the true experts here (and I'm NOT one!) any questions as you go, and don't let yourself get frustrated, you'll do fine, I'm certain. And there's a rare pleasure in riding a motor that you've rebuilt yourself.

[chuck hahn]

Little tip to get the wrist pin out relatively easy.

The aluminum piston will absorb heat and expand faster than the wrist pin. So..heating it good with a propane torch for 3 or 4 minutes will expand the piston and make the wrist pin looser.

[chuck hahn]

Second tip....DO NOT grab that hot piston!!!!!! LOL

[MWGS400]

Thanks for all the advice/information guys. It's good to know its not as uncommon as I initially thought.

A little about the me and bike: This is my first bike and I purchased it 2ish months ago and rode the crap out of it for the next month and a half. I don't know much about bike and everything I have learned, was learned int he past 2 months. I have tools and have worked on Jeeps for years so I'm not afraid of digging in.

When I bought it, I bought a Clymer manual and messed with the carbs and I got it running pretty well. The previous owner said he cleaned the carbs and changed the oil not long before it was sold. I was on the highway cruising 70ish when this happened and had to get a tow home and it sat for a few days until I mustered the courage to dig into it.

The hole is no pin hole; its about the size of a dime.

I have the whole top end pulled apart already and I took the head and the cylinders to a local guy who does bike work (Salt City Builds in SLC, UT if anyone is familiar). He said the valves look pretty bad and would need to be re-worked and his recommendation was to find a new head if I went that route. He said the cylinders aren't bad, no gouging or scratches in the walls, and would likely only need a hone. He mentioned I'd have to do a few flushes of oil to get the gritty, melted aluminum debris out and even then I still could have destroyed bearings and other stuff in the bottom end. His final recommendation was to find a replacement motor.

Does this sound correct/reasonable? I'm not sure finding a kick-start 77/78 GS400 motor will be easy.

One thing that has me worried is the holed piston doesn't move freely on the wrist pin. It's pretty tough to turn by hand. How bad could that be? Would this require a new rod, which would require taking apart the bottom end?

If I were to rebuild this thing, right now, my plan would be Ebay parts.
I'd purchase: a used head, NOS pistons and rings, necessary gaskets and other miscellaneous parts. Id have the cylinders re-honed and I'd flush the bottom end best I could. Does this sound like a decent plan of attack?

On the off chance, are there any motors that directly swap into these GS400's? There is a '80 GS450 motor for sale locally but I'd really like to keep the kick-start feature

Ill update this thread with some pictures soon. I really love this bike and have some big plans for it so I'd love to not give up on'er.

Thanks again, MG

[tatu]

I would rebuild. Cheaper and easier, then you know what you've got rather than an unknown engine which might leave you again where you are now.
You will definitely have to clean out as much of the bottom end of ally/carbon debris as possible, drop off what crankcase covers you can and flush the engine out with paraffin/diesel or Kerosene. Then once you've got it all rebuilt you'll have to address the cause as to why the piston detonated in the first place, most likely causes will be weak fuel/air mix, ignition timing and possibly wrong spark plugs.

[chuck hahn]

bottom end is probabkly fine..those roller bearings can damned near be literally filled with sand and survive. Why not just split the cases and wash the crap out with some kerosene.

[Nessism]

GS400's are cool. Roller bearing crank, unlike the 450. Sucks that it crapped out. Was it detonating under load? It almost certainly was, even if you didn't notice. Some times that happens on modded bikes, or poorly maintained bikes (not sure on this bike though). Pretty common when people chop of the mufflers or dump the airbox, then the bike runs lean and holes a piston. Sucking air from the intake boots or exhaust can cause that too.

I'd go ahead and split the cases so you can peak inside and make sure there are no surprises lurking. Pull the valves out of the head too and look at the faces and seats. GS engines are really tough, and many times all you need is to hone the cylinders and lap the valves. Worst case is find a good used head, or just the valves. OEM gaskets are common on ebay so that's working in your favor. Don't be tempted to use aftermarket gaskets or you will get leaks.

Good luck and hope you get it sorted.

[BiPolarBear]

Second tip....DO NOT grab that hot piston!!!!!! LOL

A friend of mine (and a fellow scientist) has a teeshirt that he wears in the lab that says, "Hot Glassware Looks Identical To Cold Glassware!"

But then, he's a chemist. I'm a physicist, and mine says, "Physicists Do Things In The Lab That Would Be A Felony In Your Garage!"

[gearhead13]

If the head really is toast, then get a replacement. Any used head will need cleaning, seats ground and seals replaced too. Make sure any head you buy comes with cam caps. They are matched to the head. If your head only needs a couple valves and a valve job I would fix that head. BUT, I have done and can do all that myself, WAAAAY cheaper than paying someone else to do it.
Take a good look at the plug from the other cylinder, white color with speckles means detonation. Make sure to fix that or you will be doing it all over again. Check for availability of new pistons and rings, might be hard to find for that old of a bike.
About a hundred per piston and rings new, ouch! Get new wrist pins and make sure they turn ok in the rod ends.

[Steel Toed Tank]

(Salt City Builds in SLC, UT if anyone is familiar). He said the valves look pretty bad and would need to be re-worked and his recommendation was to find a new head if I went that route. He said the cylinders aren't bad, no gouging or scratches in the walls, and would likely only need a hone. He mentioned I'd have to do a few flushes of oil to get the gritty, melted aluminum debris out and even then I still could have destroyed bearings and other stuff in the bottom end. His final recommendation was to find a replacement motor.

This advice about getting a new motor comes from a guy that looks at the things from a cost per hour point of view.

He was looking at 'what it's going to cost you' when he gave that advice.

Often times the amount of work that goes into rebuilding and engine is way more than swapping out that engine for another.

The advice from everyone on here is from people who turn all their own wrenches and clean and tune all their own carbs.

All it's going to take from you is a little reading and searching on the forum and the cost of parts, and an effort to make the repairs.

It really doesn't get any easier or cheaper than that.