Dumb Question - I need an education

[Dave Faria]

I've got a very nice R1150R engine and want to convert it to a R1150RT engine. The only difference between the two engines is the pistons and the heads. I think I've worked out the problem with the heads. My question is what is the physical difference between the R1150R piston and the R1150RT piston??? I know the compression on the R1150R engine is lower than the RT engine but, I don't know if this is due to the valve cams or the pistons or both. My R1150R engine only has 6.6k so the rings probably have not even seated for the R1150R pistons.

 

Is it possible to reuse the R1150R pistons in the RT engine???

 

Tell me what you know about the project.

 

Thanks

[ShovelStrokeEd]

I don't think there is any difference in the cams and cam timing is not used in the compression ratio calculation anyway.

 

Just throw the R engine in there, I doubt you will notice the difference, if, in fact, there is any. I'm too lazy to look up the specifications and too old to care.

[dan cata]

To compare the parts, just check theire OEM part codes on realoem.com and if they have the same code, it's the same product.

 

Dan.

[plext]

I don't specifically know in this case, but would be somewhat surprised if BMW went to the lengths of building two different bottom ends for these things. Doesn't make a lot of sense from a manufacturing economy standpoint.

 

I would suspect that the difference would be in the different heads assuming they use the same thickness gasket. CC'ing them would reveal all and is easy enough to do.

[Trullion]

The RS and RT 1100 had a higher compression ratio than the GS and R, producing 90bhp against the 80bhp of the latter. How they reduced the compression ratio (the RS was the first oilhead) I don't know, but I would imagine it would be by using a piston with a lower crown as they all use the same head gaskets and don't have a gasket at the cylinder base. Compression ratio has nothing to do with cam timing (at least in terms of how it can be varied); it is simply the ratio between swept volume and the combustion space volume. As all 1100s have the same bore and stroke either the piston top is shallower on the R and GS or the combustion chamber shape in the heads is different.

 

Over here many people put the air filter tubes from a GS onto their RS or RT because they improve the mid-range response at the expense of a slight loss of top speed (which cannot legally be used anyway).

 

For this reason I don't see why an 1100R motor wouldn't work perfectly well in an RT. After all, in the UK they fitted the 850 motor into RTs intended for fleet use, so an 1100R motor should be perfectly adequate.

[4wheeldog]

If I were considering this swap, I would confirm that the RT exhaust header fit the R heads.......If it does, I would figure I was good to go. The R motor will give a little less top end, but will make it up in midrange and the ability to use lower octane fuel.

If the exhaust will not swap, then you are into a cylinder head swap. Checking parts #s will tell the tale whether the pistons are different or it is all in the heads.

[NonComp]

According to the Haynes manual, compression ratio differences between the R and RT in the 1100 series engines was 10.3/1 vs. 10.7/1. Not sure about the 1150's (not covered in my manual) but are likely similar.

 

Looks like the 1150's have different part numbers for pistons and heads. Check the online parts fiche at Max BMW Motorcycles for head differences.

 

Max BMW Motorcycles parts fiche

[oldyam]

According to the Haynes manual, compression ratio differences between the R and RT in the 1100 series engines was 10.3/1 vs. 10.7/1. Not sure about the 1150's (not covered in my manual) but are likely similar.

 

Looks like the 1150's have different part numbers for pistons and heads. Check the online parts fiche at Max BMW Motorcycles for head differences.

 

Max BMW Motorcycles parts fiche

 

As well as those quoted above,my Haynes quotes 10.3:1 for 1150 R & GS and 11.3:1 for RS, RT & 1100S so a slightly larger difference in CR

[Dave Faria]

Thanks Everybody!!!

[kmac]

If your motor in your RT is tired, and you have a complete long block from an R/R, I would just out the long block and not worry about the compression. I was curious why you would want tear it apart and mix and match the parts if you dont have to?

 

Seems to me that the fastest and simplest way to get your bike freshened up would be to just bolt in the new motor complete and deal with any minimal loss of HP from the slightly lower compression ratio. I doubt it would be much if any real different feel.

[notacop]

Isn't the motor the main bit of frame on these bikes? So swapping engines is like using a sky hook to hold everything up while you r/r the engine?

Not that a swappo isn't a good idea, but I'd be sure the old engine was good and dead first. May as well put in a new clutch while the parts are apart.

[NonComp]

A friend was touring on an older 1100RT when he discovered a crack in one of the heads (patched over with silicon by a previous owner). Since he was far from home and the shop was unable to obtain a new head on short notice, they ended up pulling the jugs from a scrapped GS and mounting them instead. He said the bike runs fine. I don't know if they changed the pistons or not.

[kmac]

NC,

I think it is a good bit of work to swap out the engine long block since it is a structural member of the chassis but i dont think it is alot more work than swapping out heads and barrels. And with the long block you get a fresh crank and bottom end bearings. I am sure some of the more experienced gentlemen here may be able to help more than me on that topic tho. It looks to me that a floor jack and some propperly placed jack stands or blocks and you should be able to get the block out.

 

With my current clutch issues i would swap out the clutch while i was in there if your clutch has alot of miles on it since you are already there. I would rather ride than wrench a second time.

[NonComp]

From the pics I've seen (Haynes) I think if you are doing an engine swap, you are removing the front and back subframes from the engine, ending up with an engine sitting on a block. You can suspend the front and back sub-frames from the ceiling (along with the wiring harness).

 

It would probably be easier to swap pistons and heads if that's all that it take and the rest of the engine is still good.

[kmac]

If Daves bike has alot of miles on it to the point where he bought a complete engine to refresh the motor, personally i would want the new bottom end as well. And by swapping out the long block it may be a bit bigger job, but no gasket set needed and i have always found any motor that has not been opened just seems to be more reliable. Once you start doing head gaskets and things it seems to shorten the life to the next head gasket. A factory sealed long block is factory sealed and tight.

 

I do not hesitate to pull a head if that is what is required, but if i had a basically new long block sitting there and all i had to do was pull the chassis apart to swap it, i would be in for that...JMHO