R100 78 to 83 Head Swap

[Smallwheelsgood]

Hello all. I have an 83 R100 RS with severely recessed valve seats. I also have a partially dismantled (but complete) 78 model with good unleaded heads, which I would like to use as replacements. Are the two compatable, and if so which set of pushrods should I use? Any information would be much appreciated.

[Beater]

Originally Posted by Smallwheelsgood

Hello all. I have an 83 R100 RS with severely recessed valve seats. I also have a partially dismantled (but complete) 78 model with good unleaded heads, which I would like to use as replacements. Are the two compatable, and if so which set of pushrods should I use? Any information would be much appreciated.

1. The 78 heads are actually better heads than the 83's.
2. They will fit fine.
3. The push rods should be the same length, however the 81 and on's were a bit stouter ... I would use them. But it doesn't matter if you ride like a regular guy. If you are a 'bat out of hell' kind of guy, then the push rods become an issue ...

Either way ... the 78 heads came from an R100, or an R100RT or RS?

[Smallwheelsgood]

Either way ... the 78 heads came from an R100, or an R100RT or RS?[/QUOTE]

They are from an RS. Can I assume that the pushrod tubes are also interchangeable?

[Beater]

Originally Posted by Smallwheelsgood

They are from an RS. Can I assume that the pushrod tubes are also interchangeable?

That's good. Then you should get similar performance.

Yes, the pushrod tubes and seals are the same.

As a matter of fact, If you have a 78 RS then I would swap the pistons, cylinders and heads to your 83. The 77 and 78 RS's were high compression. Couple that with the lighter flywheel (on your 83), and now we're talkin' serious fun.

[Smallwheelsgood]

Originally Posted by Beater

That's good. Then you should get similar performance.

Yes, the pushrod tubes and seals are the same.

As a matter of fact, If you have a 78 RS then I would swap the pistons, cylinders and heads to your 83. The 77 and 78 RS's were high compression. Couple that with the lighter flywheel (on your 83), and now we're talkin' serious fun.

Any issues with fitting early pistons to later rods?

[Hawk Medicine]

Originally Posted by Smallwheelsgood

Any issues with fitting early pistons to later rods?

No.

[Smallwheelsgood]

Many thanks guys.

[supershaft]

Glad we cleared up what model '78 they came off of. There are small port and big port '78 R100 heads. You have the big ports. Now we need to figure out if you have "squish band" heads? Some early '78 heads are still squish band. If they are squish band, they don't really match up with your '83 pistons. '78 heads are better? First off are they early squish band '78 heads or later non-squish band '78 heads (big diff). Either way, it is still completely debatable. They are basically identical to your '83 heads if your '78 heads are not squish band. '78 pistons will fit on your '83 rods no problem but the rings will need to be changed for your '83 nikisil cylinders. Plus the piston might not fit the nikisil bores. You never know until you measure them. Despite all the lore about the squish band heads, I would hope they are non-squish band heads. I think they work just as well if not better when the compression is bumped up real high with dual plugging.

[Smallwheelsgood]

I was actually intending to swap the entire head/barrel/piston assemblies to maintain compatability. The 78 motor had been professionally rebuilt, and was as sweet as a nut until I front ended the bike, hence the availability of parts. Notwithstanding the squish band question, do the 83 Nikasil barrels offer sufficient advantage over the 78 plain bore barrels to warrant trying to mix components?

[supershaft]

After a lot of miles with nikisil I would never go back. But then again I would never go back to a dual row timing chain after running a single row and I would especially never go back to an earlier clutch after running a late model clutch but that's just me.

On the other hand, I wouldn't want to waste a fresh top end!

[disston]

The advantage of Nikasil is the cylinders last longer. With the iron cylinders you would have to replace the rings once. Next time it needed oversize pistons so the barrels got bored. Got to change the rings another time, once and it was time again to bore for a second over size. You were doing variations of this every 50-80 K miles.

Along comes Nikasil. Change the rings every 80K. Change the pistons every two or three ring changes. Never bore the barrels. Never go to an oversize piston. If you wore out the pistons you could put new same size in.

There is no performance advantage to Nikasil.

The figures I used are my own and based on my personal experience. I'm sure these figures could be made more accurate by somebody who has a bit more experience than me. That would be a lot of people. My experience is rather limited for such a talkative guy.

What you probably want to do is use the '78 heads, which you need, with the pistons and barrels of the '78. If you try to fit the '78 pistons in the '83 barrels you will probably have fitment problems and maybe valve clearance problems if the '78 stuff is "squish" band. You may be able to use the '83 rockers but I'm not sure. Do use the '83 push rods. Since you have it this much apart pull the lifters out and check their condition. Check out Anton's site for info about which rockers will work; http://largiader.com/tech/rockers/

Or the '78 heads will work with the '83 barrels and pistons? I think so but don't know much about this squish stuff. There is a laborious way to check if it all fits. Often done by racers putting together high comprssion motors or people mixing and matching a lot of parts. The deal involves placing pieces of clay on top of the pistons and bolting everything up. The engine is then turned threw several times and disassembled. The thickness of the clay is checked in the valve pockets. This tells how much clearance there is when the engine is cold. After a factor for expansion and safety is added the engine can be built with confidence or a different set of parts used. Sometimes the pistons might get machining at this point to deepen the valve pockets.

Just use the pistons that came with those heads and the barrels that came with those pistons.

[Smallwheelsgood]

Straight swap it is then. Apart from the Nikasil advantage it sounds like i'm going to get the best from both model years. Many thanks for all your advice.

[supershaft]

Nikisil cylinders last a lot longer and they perform like new up until almost the end. That is a huge advantage over iron liners. Changing rings in nikisil bores is to be avoided at all costs. Rings have a very hard time seating in well worn nikisil cylinders. I just replaced mine with 101,000 miles because, as is often the case, one of my pistons compression ring grooves was beginning to wear. Someone who isn't as picky will get a lot more happy miles out of my old setup.

There are huge performance gains with nikisil. Namely less weight and better thermodynamics. Win/win.

Mismatched squish bands will often work without contact. The real issue is any increased or decreased tendency to ping. In my experience they are much more likely to ping with mismatched pistons and heads. The much easier and much more accurately measurable way to check running clearances is to use solder. The clearance isn't only for heat expansion but also for component stretch.

[chasbmw]

I think that the 78 heads have 40mm pipes, rather than the 38 mm pipes fitted to the the 83.

Charles

[supershaft]

'78 small port heads never had 40mm headers. In this country none of the '78 big port heads had 40mm headers stock. Here in the US only some of the early '77 big ports that came on RS's had 40mm.