Higher Compression Pistons

[gog]

Does anyone know the feasibility of putting higher comp R100 pistons (9.5 CR, 93.96mm from 1976 onwards) into standard 1988 R100GS cylinders? Current cylinders and pistons are UK spec and so (i believe) are 8.2 CR.

Going on to bike with standard 1987 R80RT heads. Bit of a jumble I know.

Any problems with this I should look out for?

 

[Rob Farmer]

I have 9.1 Pistons in one of my 100gs. Works very well.

If you want to use 1000cc Pistons with your 80 cylinder heads ideally you will need to have the heads machined to clear the piston. This is a great setup because you get to keep the 32mm carbs. Richie Moore at moorespeed is the man to talk to. I have a pair of 800cc heads on my moorespeed powered gs.

 

[gog]

Thanks Rob. Do you mean you have 9.5:1 ones? Didnt see any 9.1 looking at duane ashermans spec site.

Im sure it was you that recommended richie moore to me last time I brought this up, asking about the suitability of the R80 heads on the stock 100 cylinders. Im going to run them as-is I think, if it will work, until I have the funds to have them machined properly by richie. Main concern is with piston heads hitting anything they shouldnt. Is the best way to do this to just put it all together and turn the engine over by hand?

 

[Rob Farmer]

I'm typing on an iPhone and then damn auto correct keeps kicking in.

I have 9.5:1 Pistons in one engine and approx 11:1 in the other engine. The lip of your piston will hit the head. I've never found Richie to be expensive. If you look at his prices he's generally a small amount more expensive than others but you are getting a car superior product and service

 

[gog]

I was hoping that the piston wouldnt hit the head, damn. Others have used the stock pistons with R80 heads with no machining without contact, only suffering a bit of sub-optimal squish. I take it the length difference with the 9.5 pistons over stock is just enough to make it hit?

I will email richie and see what he says. I cant afford it right now even if its very reasonable as ive already overspent on a motogadget m-unit, but I will run the stock pistons until i can shuffle away some funds. Thanks for the guidance.

 

[Rob Farmer]

If you look at the siebenrock 800 to 1000cc Pistons they have a small cutout in the lip of the piston so that it clears the head

 

[Beemerboff]

If you are careful and have a steady hand you can remove the required amount from the head with a Dremel or a air grinder, it isn't critical to the last 1000 th of a micron, dispite what the guys over on ADV say. Or is it one ten thousand of an inch ?, they are all precision engineers over there and like things pretty precise.
Finding a figure for the best squish clearance is not easy, around 60 thou seems to have worked OK for me so far, but that is not to say that there is not something better - 50 thou was the figure for Manx Nortons and tuned Velos, but they would be stripped ,cleaned decoked and checked regularly.

It would also depends on the age of the block, good late blocks are reputed to be plenty rigid, but some earlier blocks may not be, particularly if they have been overheated at some time, and a a bit more clearance might be advisable.

 

[gog]

thanks bb. to be honest though, altering the heads myself is a bit out of my league, I wouldnt even know where to start. I have the necessary tools, but how to work out exactly where to remove material? Its not out of the question if I can find or be given instruction though as these heads are not the best (stripped exhaust threads)

In all likelyhood though, they will go to richie moore. I have emailed him already.

 

[DrFarkoff]

Stick a base gasket in??

Same effect

LINKY

 

[gog]

Im starting to think i should just use the standard comp pistons that the cylinders came with!

Putting in different pistons to get higher compression, then lowering it again with a base gasket, for all I know ill end up with the same as I would have had with standard pistons and no gasket. Im not even sure if the gains are significant enough to compensate for all the farting about. I stripped the bike just to clean and service it, god knows how it turned into a full motogadget rewire, new 100cc heads, strip to frame to paint, its all got a bit out of hand

 

[Canuck]

The next question is... Since you started 'cleaning' your bike... Have you had any thought of adding another... Or has that pile of spares made you think... Well just just a few more things I could build another bike!

 

[gog]

Dana I have come too close to adding another for comfort. I tell myself its not a good idea, yet my ebay watch list has several on it at any given time, and have missed out on a few by close bids. I know its not a good idea, and my wife would have something to say about it, i have no room for any more nor can I afford it, but it just seems to be there in the back of my mind. Im trying to get her interested enough to sit her licence, and I already know what kind of bike *she* will be getting.

My parts stash isnt as big as needs be yet to put another together, but its creeping towards it. Your frame is still in my attic, im a crank case, heads, gearbox, carbs and wheels away from another complete frankenbike.

 

[Rob Farmer]

Well that's it then! You're so close you may as well build another one. After all it's a shame to have all of those bits sitting there doing nothing

 

[gog]

Its probably inevitable, though im buying nothing else until my bike is back on the road. Between the R80 and my land rover, i have lost count how many times ive said that.

 

[Canuck]

What size carbs would you need...

 

[gog]

32 bings, but shipping from Canada may be a fair whack and my funds are depleted just now! to be honest im in no rush to build a second bike but it's likely to happen naturally over time as the spares pile increases

 

[chrisshrops]

+1 for Richie nice fella and helpful