big valves

[boubou50]

I'M search for goods heads at different site on internet and i see several heads whit small valves (38mm-36mm i think ) and on every photos i see the same part number than mine ( I have the latest one 81 on whit big valves )
Is it possible ?
tanks

[Barry]

Is it possible ?  

I wouldn't have thought so as the 78-80 heads with 34/38 valves have a different part number to the 81 - 85 heads with 36/40 valves.

[nhmaf]

One does have to be very astute / careful with internet sales as there are many who are selling things which they truly know little-to-nothing about, but they will proclaim themselves to be experts.  This happens alot on ebay, especially.

You can also try ibmwr.org/market where the signal to noise ratio is much better.

Where are you located?  Depending on where on the planet you are, we may be able to help direct you to some local sources.

[boubou50]

I'M leave in Montreal Canada
I see the part number on the photos on ebay  but the vendor did not tell the heads year they just mention the size of the valves 38- 34

And on my heads the part nunber are the same

[nhmaf]

I'll take a look at a spare head that I have in the garage later this weekend.

[Barry]

A head with 38mm intake and 34mm exhaust valves is the early pre-81 head so it seems you and the ebay vendor may both have the earlier heads. As long as both heads are the same it would probably work just fine with a small loss of power at  the top end compared to the larger valve head.  If you don't use 7000 rpm very often you might not notice much difference. If you don't have the big valve head in the first place there won't be any difference.

Anybody ride both 45HP and 50HP R65's back to back and can describe the difference in power characteristics  ?

[tvrla]

What size are your valves?

It's possible your heads were swapped some time in the past. And if they've got the larger valves, there's no reason the heads couldn't have been modified with larger seats and valves.

Are you sure the newer heads have a different number?

[boubou50]

Mine are big valves 40mm-36mm But i don't know the entire history of my bike and don't know the millage ( may be  100,000 kilo ) and the valves are recessed very badly

I don't know if the newer heads are the same  part number but for buy it on internet i want ( if it's possible ) the same heads .
but if you said the difference are very little may be i take the heads wirh  the smaller valves ,

And if i replace it by R 80 heads ?  ( 42mm -38mm )

Do you have see this before ?

[Barry]

Even if they were to fit (different exhaust size for a start) who knows what R80 heads would do to your compression ratio and larger valves would lower gas velocities probably resulting in lower torque and power at low to mid range revs.

No one came back yet with a performance view of small valves vs big valves but what sort of engine characteristics do you want ? If you like low to mid range grunt over top end power generally small valves are better and if you are not planning to fit proper unleaded valve seats the original valve seats fitted to the small valve heads are less prone to recession than the ones first fitted to large valve heads. On the other hand if it's all out maximum power you want stick out for large valve heads.

[boubou50]

I just want to rebuilt my motor with the best heads that can find and to be honest cheaper is better because i don't have big money for this job.

and in the other side in the past i have big cycle with big HP and now for me 45 or 50 are enough and i don't want more and more HP just a good and reliable motor .

Barry and you said the small valves and the big have not a ''big difference '' and i think i'm search for small valves heads .

[nhmaf]

I'd make sure that both sides matched, regardless of whether you go small valve or big valve.   Easier to make the engine run smoothly if both sides are running the same way.   Personally, I'd go with the bigger valve heads as any other parts are also 'newer' in the production scheme of things - easier to find and less confusion in the long run, plus they really aren't that hard to find, in my experience.

I've still gotta get out to the garage and take pics of my heads - will do so tomorrow.

[steve hawkins]

I have later post 81 heads fitted to my 1979 R65.

This and the later airbox is apparently worth 5 bhp.

My old heads had 90k + on them, plus a wrecked exhaust thread and some missing fins, so when I rang Motorworks for a quote for refurbishment - they mentioned that they had a refurbished post 81 pair on the shelf with valves....All I had to do was swap the valve gear over....Less than £300 at the time, which was less that the refurbishment of the originals.

Rev. Light

[Barry]

Steve

How did that extra 5HP feel on the road compared to the stock 79.

[steve hawkins]

It was so long since I rode it in the small valve config - a full Cafe Racer conversion over winter - I barely noticed.......I was also still running the clamshell airbox with a K&N.   And I did not run the bike for long before that.

However I was able to break the 'ton' regularly on the local bypass in the new config - which I never would have attempted with the pre Cafe Racer setup.

I would say that there was an improvement, to be sure, but it was one of many....Less weight, crouched riding position and experience - holding onto the revs longer in all gears and not short shifting.



Rev Light.

[nhmaf]

OK - managed to take some pictures of the numbers on the very dirty pair of spare R65 heads which I have awaiting cleanup/refurb for installation on my bike.

Right side head number casting is "1335234"


LEft side head casting is "1335235"


Now, perhaps the simplest way to tell (maybe?) the large valve heads (1981->)from the small valve heads (1978-1980) is to look at the underside of the exhaust port - as far as I know, none of the bikes had pulse air injection of any manner prior to late 1980 and the introduction of the flat black airbox.   I didn't think that the earlier heads would have had any sort of port machined into them beneath the exhaust port for fitment of the pulse air plumbing, which didn't exist then, correct?   So, if the head has a hole/port in this location (or is plugged by a bolt as shown here as when someone disconnects the pulse air system) it should be a large valve head, no?


Looking at the A&S online fiche, they all seem to have the same part numbers, and the drawings all seem to show the head with the pulse air connector&port, so I am thinking that BMW may have just obsoleted the small valve head altogether and only stocks/sells the large valve heads now - it would make sense, I guess.   What seems really odd is why the list price for the left head shows as $485 while the price for the right head is $809?  Hmmm.