Using R80 heads on 1000cc jugs-and update on DonW

[DonW]

First off- what a treat to have this site up and running. I've been on and off over the years (maybe 8-9 years), and it's been a great resource. I even think I've been able to contribute a few times.

My question this time is about the rumored possibility of using 800cc heads on 1000cc jugs. On the yahoo ST forum this is common advice, so folks can add displacement, and use their existing heads. I think I read that the 800cc heads don't actually raise compression, but make a nice squish band combustion chamber. Any one done this? Advice? And, specifically, if I do this would I use 1000cc head gaskets or 800cc head gaskets?

A quick update on me-

Still have one of my original R80ST's, which now has 1000cc jugs (and heads) and a R100GS front end. It was a frame up restoration about 4 years ago, which many on this Board helped with. I bought a 'barn find' R100GS last spring and have competed another frame up restoration, but still working out kink.

I have also bought, rebuilt or parted out, and sold off about 10 airheads (mostly R65's) in the last 5-6 years. Keeps my motorcycle slush fund liquid. Oh, and I went to the darkside and bought 2 R1150RTP (CHP surplus) 2 years ago- parted out one and am riding the #$!@@! out of the other... So, three running bikes right now and a wife that wonders why I even want one...

Hope all is well with all of you-

Don

[Ken in Oklahoma]

I have a '76 R75/6 and some 1000cc nikisil jugs with pistons. And I'm comtemplating a union. So I'm watching to see what y'all have to say to Don.



Ken

[Roy Gavin]

Adding 1000cc barrells to a 800 cc bike works fine , but some of that is because you are running a 1000cc bike on 800 gearing.
I have 1000 barrells on my twin plug G/S , and even with the gearing advantage there isnt a lot of difference to my 1000cc GS. Both go verry well!.

I000 pistons generally have a flatter dome than 800 ones , so the increase in cr isnt as much as you might expect.

BMW advise against setting the squish less than 80 thou, and Udo Gietl found that less than 70 thou on his race bikes was asking for trouble - Googlke - King of the boomers- for the full story, and if you stick to these sort of gaps there wont be a huge difference anyway.

[Ken in Oklahoma]

Well Don, you're not getting too much response so far. Where the hell is gspd? I know he would have a lot to say. But until he, or somebody else knowledgable comes along I'll fill up a bit of space on the page.


I have a couple concerns about swapping 1,000 cc jugs onto a R75 or R80 using the original heads.

First I would worry about the compression ratio ending up too high. I would do the hot rodding approach of cc'ing both heads and see how much volume is there. If the volume of a 1000cc head is pretty close to the R80 or r75 head I would be OK on that point. But if the volume of the smaller heads was much smaller than the 1,000 cc heads I would be concerned about pre-ignition. My fix would be to fit some spacers between the block and the base of the cylinder to bring the compression down. I think these are still made so it shouldn't be difficult to find them.

Secondly I would be concerned if the R75 or R80 head had a sharp edge on it which was covered up by the cylinder but due to the larger bore might now be exposed to combustion. I would be conderned about a hot spot causing pre-ignition. The fix I imagine would be to dress the sharp edge down, rounding it off and polishing it.

As a matter of course I would check the piston to valve head clearance and piston to head clearance just to make sure all was OK there.

For jetting I would start out by modifying the 32mm Bings to the jetting specified for a R100/7. I figure that would get a person into the ballpark.

I'm sure some people would caution is that the gearing of the final drive might feel a bit short for the greater power of a 1,000cc engine. I don't think it would bother me all that much, and if it does a couple hundred dollars should buy a taller final drive.

Actually, what we both really need is to get your friend John Falconer to respond here. I betcha he would have the answers.



Ken

[Roy Gavin]

Not much point in CC ing the heads - you gotta do the combustion chamber volume with the heads on, as the piston domes are different.

I have also bored a 75/7 up to 900cc - I didnt change the carbs except for adding springs over the pistons and it works fine, but I altered the exhaust and air box too.

Bings are a crude old carb and work OK if they are anywhere close, and there is not a lot of good information out there on optimum settings.

Which is a pity - there is at least 10% more power lurking in there if you can get the intake, exhaust and carb tune working together.

[Ross]

I have 94 1000cc barrels on a 80's R80. I also have my R1000 gearbox fitted.

[gspd]

[quote="Ken in Oklahoma"]Well Don, you're not getting too much response so far. Where the hell is gspd? I know he would have a lot to say. But until he, or somebody else knowledgable comes along I'll fill up a bit of space on the page.

I answered this one via email a while back.
my answer was along the lines of...
if you have 90 to 180 psi compression (and gas, and spark),
and the parts have enough clearance so they don't smash into each other,
IT WILL RUN!
How well and for how long is a mystery to anyone who has not tried the exact same setup.

Improper squish chambers, poor flow patterns, hot spots, etc will obviously affect any engines' performance,
but basically you won't know until you try it. There is NO book for this one.

[DonW]

Thanks All, I'm stalled on this but may get back to it this weekend. Here are a few points-

(Yes, thanks gspd for your earlier email. Always good advice. Unfortunately, with the changeover to the new phorum, I can't retrieve any of my old messages...)

I'm pretty sure the modern (post '81) 800cc and 1000cc heads are made from the same casting, and then machined differently.

I have swapped 1000cc jugs (with the appropriate 1000cc heads) several times to my R80ST (removing the 800cc jugs and heads) and the carb jetting always worked out to be the SAME.

Not sure about R75 jugs/heads as those are from a different era then my engines, so no info there.

I've heard that the volume of stock 800cc heads and 1000cc heads is essentially the same, hence no change in compression ratio, but if I do this I'll cc the combustion chamber to check. Seems counter intuitive, but looking at them, the 800cc chambers look deeper, and the 1000cc chambers have a smoother transition at the outside. Could be...

My BIGGEST concern is what head gasket to use, but now that I've mocked it up on the bench, I'm pretty sure I need to use the 1000cc gaskets- the 800cc gaskets extend into the combustion chamber and the little metal ring around the edge seems to protrude completely into the chamber. With the 1000cc gaskets it is flush with the jugs and leaves more of a squish band for the heads.

I've heard so many rumors that this works, I'm very willing to try it...

Don

[DonW]

Update- I answered (with John Falconer's able assistance) my own original question this weekend- Using 800cc head gaskets on 1000cc barrels WILL NOT WORK. Piston will slam in to the edge of the smaller 800cc gasket.

Also did a rough "volume" test on an 800cc head and a 1000cc head. Sitting on the bench, they both hold within a few cc's of the same volume of water.

All for now-

Don

[DonW]

Well, I'm finally getting to this. Pulled the jugs on my GS last weekend- the rings have an end gap of .095" and the valves don't look so good. (The new rings have end gap of about .024)

So, I've fitted my perfect (new valves, guides, seats, and dual plugged) 800cc heads on the 1000cc jugs, and everything fits (used modeling clay and turned engine over by hand to make sure valves clear, and piston doesn't hit the head. It will create a weird "squish" band- sort of a little wedge, but it looks like at least .040 clearance.

Any last suggestions before I put things together for real, and fire it up?