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So I've decided to put the 750 head on

  • Thread starter Thread starter Anonymous
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Anonymous

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I've decided that I'll put the 78gs750 head on my 79 GS 1000 engine. I know this will boost compression a little, but as long as the piston crown won't contact the head I think I'm alright. I just don't have the time to dismount the 1000 head, rebuild it and mount it back on especially since this is all going to be done outside in the parking lot. Has anyone tried this? am I missing something? I'm going to use the 750 cams because the teeth are a little shorter, set up to run with the idler sprocket. Ofcourse I'm first gonna check the minor diameter of the sprocket to make sure that they are the same as the 1000 (so the timing doesn't naturally walk off slowly). Once I have the head off I'm going to start rebuilding it. I think I'll take my time and work slowly and run the 750 head most of the summer, unless one of you gives me a big heads up on why I shouldn't. This way I can port and polish the head, lap all the valves, replace all the valve seals and springs, and any questionable valves, check for true in the cylinder head and mill if neccessary, cut slots in the cams for degreeing, ect ect. Any advise guys? I really rely on you guys for the gs info, my tech has only general info on GSs
 
Be careful!

Be careful!

I went and measured all the important dimensions on a 1000 & a 750 head. One area that may cause trouble is contact between the piston crown & the cylinder head. There is a 5mm difference in bore, so the combustion chambers on the 750 head are smaller in diameter. If you decide to do the swap, you should "clay" your pistons to check clearances. Claying means coating the tops of your pistons with a thin layer of pliable clay, reassembling the engine, turning it over slowly, tearing it down again to measure the clay depth where valves / combustion chambers have left impressions. This is tedious to say the least, but it beats piston to chamber contact at speed.
Another concern is compression ratio. Thre is such a thing as TOO high<g> A stock 750 has an 8.7/1 compression ratio. Wiseco piston claims to raise the CR to 10.25/1 with their 844 cc big bore kit.
Going to 997 cc would put you up in the 12.75/1 range. That puts a lot of stress on the rest of the motor.
I know it's spring & the road is calling, but it might be better to find a 1000 head to use for the summer. It would be easier and safer.
 
I'm also willing to bet the wiseco kit has a larger dome though, causing compression to be raised by increasing the surface area, because Wiseco has a couple of applications where the displacement is the same, but compression ratios change. I will try the clayin thing, It'll be tedious to say the least, but may save my rear in the end... I'm a little concerned about introducing a foreign object like clay into the system. Any thoughts?
 
Mainly use the clay in the valve pockets on the pistons to make sure you have clearance. Play-doh or Silly Putty will work. Put a little in the corners of the combustion chamber too. This way you can make sure the piston won't strike anything.
 
Sounds good enough, I'll be putting the 750 head on tomorrow, if everything goes well and I don't have a holed piston. I'll let you guys know what kind of PSI I'm getting afterwards for compression, and we can see if it's too high. Hey, maybe this could be a performance enhancing mod. It'll definately enhance the performance of my bike, from nothin to somethin

~Ben
 
Well, I got the head off, after much swearing and 2 broken fins and I borke the seal on the base gasket (I'm going to try to re-seal it and replace it when I get the 1000 head set up for performance) and it doesn't look that bad. The pistons are ofcourse black with carbon, and the valves aren't broken or holed, the intake held fluid, but the exhaust didn't even try to hold, so it looks like I'll be replacing the exhaust valve, once I get it ordered. Also I need some advice on Cam degreeing. If I want to set it to 106 degrees, does that mean the intake is opening SOONER or Later, same for the exhaust. I don't have adjustible cams but I have limited access to a mill, so I can drill a hole, or a groove. I want to get this head good and ready to go in excellent shape before I put it back on. Whats a good way for cleaning up the carbon? I was thinking about a wire-brush drill bit.
 
A couple more caveats- The 1000 pistons probably have larger domes too!
Sorry to hear about the busted fins. Did you try cranking the engine over by hand with the bolts loose and the cams out? I found this loosens the head right up.
That base gasket will probably leak. Do yourself a big favor and replace it.
Use a plastic or softer metal scraper for de carbonizing the piston crowns. Be careful - Try to keep the loose crud from getting in the rings. This will be safer with the cylinder block and pistons off the engine. Both Clymer & Haynes manuals reccommend this procedure.
Clean everything twice!
Degreeing the cams properly requires adjustable cam sprockets. If you don't have them, WAIT. Don't break out the degree wheel until you get a set.
 
I have very serious doubts as to whether a 750 head will work on a 1000, if it fits on, and the pistons do manage to clear it (doubt it), it will raise the compression way to high to run normal fuels, if the starter is even able to even turn it over.
I have a 1mm over bore on my 1000 and the increase in compression is noticeable, I can no-longer run regular gas, I have to run super to keep detonation to reasonably low levels.
the 750 combustion chambers are allot smaller, you may end up with enough compression to run on diesel fuel with out any ignition system other than the compression!
 
Okay, so are we talking the 750 head won't work at all? It sounded like from previous posts I was going to be able to run the 750 head on premium, but judging from the responses I'm getting here there is no chance, and I'd rather save the 20 dollars in gas and the labor in getting the 750 head if there is no chance. Anyone know of an aftermarket avenue to get a valve/valves for the 79 GS1000 because they're 50 dollars dealer, add in a 65 dollar head gasket, whatever the base gasket is gonna run, a honer, a wrist pin puller, cleaning supplies, a valve compressor, valve seals and we're closer to 200 dollars I don't have. Ofcourse I could always try and reuse the base gasket, and if that fails deal with it later, but Summer finally arrived, not to mention I have to get this bike out of here in a couple of weeks when I move! HELP!
 
Try Sudco for valves. They have a list of applications on their website. If they can't help check out Manley. They can make valves for you from blanks.
 
Sudco does list valves for 79 GS1000s. Go to their website, click on products, then valve products, look on that page for a link to valve applications. They carry Vesrah valves.
 
Aftermarket parts are always cheaper. I was able to find an entire gasket set for less than the price of a factory head gasket. Cutting corners is never a good idea when working on a bike. Save up the $ & do it right the first time. If you don't, odds are you'll be doing the work again.
 
Billy Ricks said:
Sudco does list valves for 79 GS1000s. Go to their website, click on products, then valve products, look on that page for a link to valve applications. They carry Vesrah valves.
Click to expand...

hmm... how can I tell what the model of my engine is from the engine code? There are two different possible exhaust valve applications, and unfortunately this is not the original engine...
 
If fitting the 750 head is only a temporary measure, try it with two head gaskets to keep the valves clear of the piston and keep the compression ratio within limits. The cam chain tensioner will easily cope with the slightly taller set-up.


Kim
 
Good point Kim, I forgot about that! I talked to a guy who road his KTM Adventure through some 3rd world desert countries and he had run 2 base gaskets. That allowed him to use regular octane fuel.

Steve
 
I give up! I've been thinking about everything that has gone wrong with the 1000 engine. So today I'm retrieving my spare 750 engine and I'm going to mount that and then plit the cases on the 1000 and redo it from ground up the right way.
 
GSbuilder said:
I give up! I've been thinking about everything that has gone wrong with the 1000 engine. So today I'm retrieving my spare 750 engine and I'm going to mount that and then plit the cases on the 1000 and redo it from ground up the right way.
Click to expand...

That doesn't sound like giving up to me. It sounds like taking the challenge head on. Sometimes we have to keep making the same decision until we make it the right way round.

Kim
 
GSbuilder said:
I give up! I've been thinking about everything that has gone wrong with the 1000 engine. So today I'm retrieving my spare 750 engine and I'm going to mount that and then plit the cases on the 1000 and redo it from ground up the right way.
Click to expand...
That is the right thing to do. Somtimes if you cut corners and try to save money all it does it bite you in the Butt later
 
Well, disassembly of the 750 motor went well, I only have 5 bolts to extract from the head... oh, and a drill bit that snapped off while trying to extract one of the bolts. I had to take the head off to replace the head gasket, which failed 50 miles after installation and re-torqueing did no good, so I called up Vesrah and they sent me a replacement for free. This was one of the reasons the engine came off the bike in the first place, along with 2 of the 5 bolts that I'm now extracting having been snapped off previously... that made no sense. 2 of the bolts broke off last fall while removing the head to install the new gasket. There. I've taken almost everything off the 1000 engine I can before finally removing the barrels and the case. I still need to disconnect the clutch cable and chain, drain the oil, and remove the 2 working engine mounts. I have to do this all a little at a time ducking whenever a cop goes by because I am doing this in a University parking lot. I took a break because my drill battery died and I can't extract anything without my drill. Another benefit of removing the 750 head is now maybe the engine will weigh as much or less than me. I'm a scrawny little guy and have troubles lifting 200 lbs even just 8 inches, after all, I'm a whole 140lbs 6ft tall when I'm heavy. (University food does not inspire eating) Well, this has been a long enough break I guess, back to work! I'll keep you posted.
 
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