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shelbygt
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Iowa
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Howdy, newbie here. I just bought a 1973 LWB R75/5. I'm toying with the idea of a 1,000 big bore kit. Wondering if anyone here has experience with it. Would you need to do something with the heads to take advantage of the extra 250cc's?

schrader7032
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VBMWMO #7032
San Antonio, TX
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Doesn't look like it. Here's

Doesn't look like it. Here's the text from Ted Porter's website where he's selling the 1000cc kit for pre 1976 bikes with the 97mm case spigot opening:

Quote:

Up to 10 HP power increase for 1970-09/75 with 97mm block bore.
Just exchange Cylinders+pistons - nothing else has to be modified.

FITS: From 1970-75 up to 09/75 production date (97mm block bore)
Kit includes:

New Nikasil cylinders.
Stainless steel pushrod tubes.
Lightweight 9.5:1 1000cc pistons modified for your existing combustion chamber.

Kit is complete except for top end gaskets.

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Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2

shelbygt
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Iowa
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Yeah it looks like a plug and

Yeah it looks like a plug and play deal. I'm just wondering if the 1,000 cc bikes have bigger valves, better flowing intake and exhaust runners, etc. Any idea if the later heads fit the earlier /5's?

mark_weiss
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Here is a chart:

Here is a chart: http://w6rec.com/duane/bmw/engine.htm

Increasing valve size a small amount (r75 to r90 exhaust) is quite simple. Any machine shop equipped to rebuild cylinder heads will be able to insert larger valve seats and blend them to the heads. The 750 and 900 heads are the same casting, the rest of the port is unchanged. Early R100 heads are different and fit larger spigots to the intake side and the 40mm exhaust heads have a slightly larger bore as well (some R100s take larger header pipes, most don't). Late (monoshock) R100s use the same carb sizes and exhaust pipes as an R75/90, so no fitting problems there. Then there are the camshafts...

Staying with smaller valves will produce a larger increase in torque than in horsepower. Bigger valves will boost top end horsepower at the expense of low to mid range torque. If you go for horsepower and will be using higher rpm, consider that the larger bore will produce a lot more crankcase windage (air in and out through the breather system) and just may be a bit much for the passages cast into earlier crankcases. More stress on gaskets and seals will be the result, especially the rear crankshaft seal. The rear seal should be updated to the latest version to better withstand the pressure.

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khittner
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Or, Shelby, you could leave

Or, Shelby, you could leave it alone, sell it to someone who prefers the inherent smoothness of the 750, buy an R100 with more horses and gears, and probably have money left over.

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Konrad

Dan McCarthy
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See how you like 750cc first!

Apologies for adding nothing to the original question, but my experience has been that airheads get sweeter as they reduce in cc (apart from the very different R45/65).

Believing size was everything I only came down from 1000cc to embrace what ended up being a pair of beautiful and iconic R90s bikes. Performance-wise they seemed as quick as the 70hp big valve 1000cc bikes (who rides these at 100% anyway?), but there was generally less vibration and an absence of "shuddering" that I seemed unable to tune out of their bigger sisters. Then I got my R75/5 and now I figure that for a small decrease in performance I get a much smoother engine that's generally nicer around town and still acceptable at highway speeds (drum brakes on the front notwithstanding).

My latest airhead is an R60/6 - still some months away from completion having been in pieces since 1995. I'm really looking forward to getting this bike on the road and finding out just how it compares to my 750... slower but even smoother perhaps?

I noticed something similar with round barrel Guzzis too - the 750s I had shook less than the 850 Le Mans, which was noticeably smoother than the 1000cc Spada.

Big jugs on aircooled twins aren't always a good thing!

Dan

shelbygt
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Iowa
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This is very helpful. Bike

This is very helpful. Bike has about 50k miles on it. Should probably at least change valves/seats for today's unleaded fuel. Just thought it might be easier to swap heads. Maybe just go to R100 heads? Thinking hard on Mikuni's. Does engine have to come out for cam swap?

shelbygt
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Iowa
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Dan, you're right I should

Dan, you're right I should probably get some ride time on it. Like you, I'm into Guzzi's too. Have a 73 Eldorado and a new California touring.

Dan McCarthy
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Shelby,Bigger brains than

Shelby,

Bigger brains than mine will advise you about the valve seat recession issue, and Snowbum gives a pretty encyclopedic account on his website (if you can find it...). Speaking purely from my own experience, I have yet to discover valve seat recession on any airhead I've owned over the past two decades. Two things might be factors here - I use a fuel additive on every tankful (Castrol Valvemaster), and for prolonged high speed use I use a BMW R850r oilhead.

At 50k your bike should be just about entering middle age and if the valve gaps don't close up between 1000 mile inspections then you have little or nothing to worry about. Konrad's post earlier - which went on while I was writing mine - is tongue in cheek, but carries much truth! These 750cc BMWs aren't the quickest, but their performance is perfectly appropriate to the levels of handling and braking provided by the archaic chassis and running gear!

Quick point about the carbs... I couldn't get mine to work well and again Snowbum's site gives a comprehensive account of the foibles of the early Bings fitted to these bikes. The answer for me was to source and fit a pair of very late R80 Mono carbs - and all of the poor running issues disappeared.

As a Guzzi guy you will surely come to love the similarities, and the differences between the German and Italian aircooled twins.

Dan

shelbygt
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Iowa
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Found the Snowbum website.

Found the Snowbum website. It's gonna take some digging but I'll bet there's a lot of information!

I dig all bikes. Worked in a motorcycle shop from 7th grade through college and worked on many brands. I like American, German, Jap, British, you name it.

mark_weiss
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Arizona
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A /5 should not have any

A /5 should not have any valve recession issues, that came with later engines.
Changing to 1000 heads will drop compression and lower performance.
Mikunis are a nice upgrade. Rigging a choke cable system can be tricky. Or, just live with the levers.

Heavy flywheels mean smoother running. Pre-nikasil engines have this. That is why early 1000s run more smoothly than later editions. Fitting a 1000cc kit to a 75/90 base sacrifices very little in overall smoothness. The Siebenrock kit is a very nice setup. The pistons are designed to work with the 75/90 combustion chambers.

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schrader7032
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I think people confuse valve

I think people confuse valve recession, which was the issue with pre 1981 engines, with the situation with the 1981-1984 engines which was valve face plastic deformation. With the absence of lead, the pre 1981 engines will eventually suffer with the valve seats slowly wearing away...the recession part. In 1981, BMW came out with unleaded seats that didn't transfer heat into the head very well...plus I think they got the valve grind angles wrong and the contact surface was razor thin. Thus, the head was retained more by the valve itself and the face of the valve took the beating and was prone to deforming or tuliping. In 1985, BMW finally fixed the problem with the proper metallurgy for the seats and valves.

The 1981-1984 bikes didn't all implode...it was mostly the 1000cc bikes and those with fairings...the additional heat from the liter bikes and the fairings tended to trap the heat. It also had something to do with how the bike was ridden. By Dad had an '81 R100RT that he put 65K miles on. BMW did all the service. He was not hard at all on the bike. He never experienced a problem with the valves...at least BMW didn't say anything to him.

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Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2

shelbygt
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Iowa
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Thanks guys! I'm going to

Thanks guys! I'm going to chew on this some. If I'm going to do the Mikuni's I should decide If I'm going to do the siebenrock kit as I should be looking for bigger carbs.

Decisions decisions.....

RickR90s
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Farmington Hills, MI
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Shelby, There's an old

Shelby,

There's an old saying, "If you want a faster motorcycle, buy one".

Having been a R75/5 owner for a number of years, I always felt it was more than fast enough to put a smile on my face. I was always surprised and pleased with the low end torque, the unbelievably smooth motor and whiny, clunky four speed box. That being said, hot rodding the bike, any bike, usually results in compromises somewhere.

What you gain in horsepower, might be lost in low end torque and ride-ability or reliability. Although many systems are advertised as bolt-on, in most cases, other parts of the bike are affected and might need attention to balance the equation, so to speak.

Lastly, a stock, vintage BMW will always be worth more money than a modified one.

Just my $.02.

Rick

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mark_weiss
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Having installed the

Having installed the Siebenrock kit, it is indeed a bolt on affair. Will it last 100k miles? I have no idea but did not see any reason why not. Will it put more stress on the bottom end? Yes, more power will bring more pressure. That the bottom end was largely the same from the R75 through the R100 would lead me to believe that while the conversion may not last as long as an R75, it should last as long as an R100.

Should extra work be done? Sure, refreshing the cylinder heads would be an excellent step. Have the whole top end in fully refreshed condition will yield the best running.

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