r80 to r100 using standard parts

[frank1200]

a friend of mine is building an all singing all dancing r100pd
he had gone for high compression pistons and twin plug heads
he has offered to sell me the standard pistons and barrels from his r100gspd
i have 2 r80 bikes a 93 r80gs and a 83 r80st that i am going to convert to a g/s
my question is will the 1000cc pistons and barrels fit and will it make any difference
i know the 800 is very smooth but would not mind the extra power some times to
what do the experts think

 

[VAL. H.]

a friend of mine is building an all singing all dancing r100pd
he had gone for high compression pistons and twin plug heads
he has offered to sell me the standard pistons and barrels from his r100gspd
i have 2 r80 bikes a 93 r80gs and a 83 r80st that i am going to convert to a g/s
my question is will the 1000cc pistons and barrels fit and will it make any difference
i know the 800 is very smooth but would not mind the extra power some times to
what do the experts think

Pleeease don't do that The ST's are rare enough. There's very few around now, because people keep converting them into g/s's it's so sad I think.
Especially as the st is still reputed to be the best handling BM of it's day.

Convert it to 1000cc if you want to. I'm sure the barrels fit straight on, though I'm sure someone will be along to confirm this later. But please don't destroy yet another st. They're such lovely little bikes.


Val.

 

[Kenny]

Yup, barrels will go straight on

 

[Rob Farmer]

You will need a pair of 40mm carbs to fit though and you may well find you struggle to get your feet under them. 1000cc with 32mm is a better option on yours it will run smoother at the bottom and mid ranges as well.

 

[Beemerboff]

The extra power with the G/S s lower weight make for a nippy little bike.

The Haynes manual has all the information on valve sizes, carb settings etc and you will find that there is not a lot that requires changing .
The G/S collector box seems to work OK even although it is reputed not to, but a air box with two large snorkels instead of one small and one large is an improvement, despite some 1000cc bikes having the odd snorkels.

One point that doesn't get a lot of coverage is piston to head clearance .

If you have the skill to machine the head and set the squish clearance between 40 and 50 thou you will get the full benefit of the change.

Ir not you want at least 80 thou clearance.

What you want to avoid is having dead air in a gap between 50 and 80 thou, which can cause combustion problems .

My squish band 900 cc bored out R75/7 was faster than my recently acquired R80 G/S with 1000 cc barrels until I sorted things out, and now the G/S has the edge - it picks the front wheel up on the throttle only without too much trying.

 

[(RIP) Proff]

Pleeease don't do that The ST's are rare enough. There's very few around now, because people keep converting them into g/s's it's so sad I think.
Especially as the st is still reputed to be the best handling BM of it's day.

Convert it to 1000cc if you want to. I'm sure the barrels fit straight on, though I'm sure someone will be along to confirm this later. But please don't destroy yet another st. They're such lovely little bikes.


Val.

I'm with Val 100% go find a 80 paralever to fuck about with, leave ST's alone..
Remember they ain't making them anymore

 

[Rob Farmer]

I'm with Val 100% go find a 80 paralever to fuck about with

That's made my day

 

[Oggyr80]

Don't do it.

It's a terrible idea sell them to me instead. If you decide not to I'd be happy to make an offer, so pm if things change.

 

[frank1200]

ok so the consensis is leave the st alone and feck with the 93 paralever r80gs
so if i put the pistons and barells on the para will i still need to change the carbs,will i still need to get the heads done as well,
what would i get the carbs off or should i just ring motorworks.
have a good engineering shop about 6 miles away which rebuilds car and truck engines so he should be able to sort heads if i tell him what i want

 

[frank1200]

I'm with Val 100% go find a 80 paralever to fuck about with, leave ST's alone..
Remember they ain't making them anymore

thats the question am i fecking about and should i leave well enough alone and leave the 800 engines along as they are quite smooth and not that slow

 

[fayeslane]

thats the question am i fecking about and should i leave well enough alone and leave the 800 engines along as they are quite smooth and not that slow

Can anyone quantify the power increase? I'm thinking in terms of power gained versus time spent rooting around doing it,fine tuning and expense eg:

Significant improvement
Marginal improvement
No bloody difference

It seems straight forward but my experience is that when you pull something apart there is always extra costs eg if the heads are off might as well get the valves reseated, oh and a set of rings, pushrod tube seals, associated gaskets etc etc

If the consensus is only marginal improvement then the question is " is it all worthwhile?", might be better to get a professional tune up done on the 800.

 

[(RIP) pastyman]

For what its worth. What do you want to use the bike for. ?
If you want masses of power there are plenty of powerful bikes out there which will save you a lot of messing about and expense. There are better bikes for doing off road, and lighter too. An 800 or 100 gs will go better than most sports bikes on twisty poorly surfaced C and unclassified roads ( and there are plenty of those after this winter. Touring should not be a "thrash" so standard bike is fine, Im not sure I see the point in getting more and more power for the sake of it, how many can use the power of so many modern sports bikes? just look at the back tyres on most. If its just an engineering project... well fair enough.
Improved riding skills will add more performance than most engine mods anyway

 

[Kenny]

For what its worth. What do you want to use the bike for. ?
If you want masses of power there are plenty of powerful bikes out there which will save you a lot of messing about and expense. There are better bikes for doing off road, and lighter too. An 800 or 100 gs will go better than most sports bikes on twisty poorly surfaced C and unclassified roads ( and there are plenty of those after this winter. Touring should not be a "thrash" so standard bike is fine, Im not sure I see the point in getting more and more power for the sake of it, how many can use the power of so many modern sports bikes? just look at the back tyres on most. If its just an engineering project... well fair enough.
Improved riding skills will add more performance than most engine mods anyway

This is just about the most perfect answer for airhead power mod increases ..... but we all like to play with our bikes, eh?

Frank...no need to change carbs or squish clearances etc etc...just bung the barrels and pistons on and then see how she runs. I've stuck RS barrels etc on and didn't need to alter anything and the beast flew !! Put the barrels on and then see what needs adjusting... you'll be surprised at just how much bullshit ( nicely said though ) is written about airheads....IMHO the suspension being sorted makes the biggest difference for Blighty's roads.
If I want speed..I jump on my old CBR900RR-R Urban Tiger

 

[fayeslane]

 

[Beemerboff]

It isnt much trouble to check the piston head clearance when the bike is apart.

If you get it right you can get 29 % more power and 35% more torque - get it wrong and you could wreck the engine

Some combinations of factory parts do work without any modification, some dont, and I cant really see the logic in suggesting that because someone else's parts worked well you shouldn't check the parts that you have.

I had two completely different experiences with my R75/7 and R80 G/S , one combination of parts was checked prior to fitting and was spot without any adjustment, the other, which was fitted by the PO without checking , was a vibratory slug until the clearance was corrected.