R45 (1978) cafe racer/scrambler project

[Bikeshed]

Hi everyone - new to this site and looking for some advice.
I am currently building a cafe racer from a basket case R45. Nothing shocking there. However, I hoped to beef up the low power output with some R65 barrels and pistons. All was going well until I fitted the upgraded barrels and pistons. It all went together well but on turning the engine over something was fouling close to tdc. When I loosened the head bolts on the left cylinder the fouling ceased.
I could not identify what is fouling, so I removed the gearbox and clutch - still fouling. I removed the alternator and cam chain cover - still fouling.
The right hand cylinder does not seem to be part of the problem.
Am I being stupid? - probably.
Is this conversion possible? Only the bores are different between the two engines although there is some difference in valve diameter and piston skirt length.
Any sensible thoughts on this issue will be most welcome although I have already considered the accidental spilling of unleaded allover the thing.

 

[Nin]

Have got the camshaft timed correctly?

 

[Bikeshed]

Hi Nin.
I have not disturbed the crank or cam shafts. However, thinking about it, it is probably a good idea to check the valve timing. A task for tomorrow evening!
Thanks

 

[tuftywhite]

Someone will be along to correct me, I'm sure, but I think there is a difference between the early 45/65 engines and the later ones. If you're mixing them up, then you may have issues.

 

[mikeyboy]

I would suspect the combustion chamber is smaller on the 45,
The oversized piston is probably making contact at tdc.
It can be sorted by machining the squish clearance,but tbh you won’t be getting much of a power gain by fitting the 65 barrels with the stock 45 heads( small valves/ports/carbs.
You’d be better off sourcing a pair of post 81 65 heads and some 32mm bings

 

[Bikeshed]

Thanks so much for these replies. I am at the head scratching stage. I think a bit more scratching before I dive in for R65 heads and larger carbs.
This project is a bit of a learning process and explains the millions spent on R&D.
Cheers.

 

[Steptoe]

Just a thought..Are the barrels and pistons you're fitting to the early engine the later nikasil lined versions ?

If so the bottom of the pistons hit i think a part of the crank or the crank hits the bottom of the barrels.. i can't remember exactly which it was as it was 20 years ago when i had the same problem on a customers bike .

Took me a while to diagnose the problem.

 

[theoneandonly]

and the cheap fix ( may work ) add a spacer to the base gasket.

 

[Bikeshed]

Well, barrels, heads and pistons removed (again). Planning to refit the 45 items and see if the problem resolves. I am unsure of what year the 65 items actually fit.
All contributions greatly received.
It maybe that the bike remains a 450 until the project matures and then the engine may get some more attention.
Will report back on developments.
Thanks.

 

[dogshome]

Hiya, The 450 heads have a much smaller diameter combustion chamber and smaller valves. They will definitely not fit. The reason I know is that I sold some barrels and pistons. I looked into it before selling so whoever bought them was aware. I will guess that the entire dome of the piston is touching the smaller diameter of the 450 head.

If you get a pic of a 450 head and compare it next to a 650 head it is quite obvious. I'm glad you found it before destruction! You might be able to machine the pistons down if there is enough ring land, but I'm guessing with that and probably not an option. Cutting back the 450 heads is another way, but you run into the valves in the wide flat band so can't make it a squish.

I think the only simple answer (apart from unobtainium R65 heads) is a thick copper head gasket. I'm guessing 3 or 4mm or so, which could be cut by hand.

Or get some made e.g. https://www.gteek.com/custom-made-copper-gaskets https://www.stephensgaskets.co.uk/gaskets/cylinder-head-gaskets

The other possible complication is valve cutouts. The 650 are in different positions, but I'm not sure how different. They look close in photos I can find. The 650 valves are bigger so you might be lucky there.


I think it's all doable, but get some plastigauge or soft solder in to check clearances

 

[dogshome]

You can see the difference in the R65 and R45 head photos below.

The large pic shows how the 45 uses the 65 gasket (in a pretty poor way) and also the size difference.

 

[dogshome]

Hey I work in Retford!

 

[Leyther]

Why don't you contact Steve Scriminger , I don't think he is far away from you.

 

[Kenny]

Just another option......

https://www.flatracer.com/bmw-airhead-boxer-860-performance-kit-r45-r65

 

[Bikeshed]

Wow, thanks for all this technical advise. I have sourced some 45 barrels and pistons which may be a cop out but will do until I can find some reasonable 65 heads - and then maybe the bigger carbs.
It's all work in hand as clever folk say.
Thanks to all. The project continues.

 

[mikeyboy]

I’ve got an r65 engine
I believe it’s the post 81,but I’d need to check further.
Get in touch if it’s of interest