valve pocket size / position in pistons for K1100 head
Turbo Mini Forums

HI,
So many experienced engine builders here.... fantastic!
Many builds with K1100 heads too. Currently I'm preparing my own engine with this type of head.

Recently I bought the SC building manual. Regarding the pockets in the piston for the valves, it is only mentioned the deepest part of the pocket needs to be 4,5 mm

Anybody who can advise me which diameter mill to use for these pockets and on which location?
Regardless the piston size (std or 73,5mm dia) it is known the head will not bring the valves exactly in the middle of the mini engine, nevertheless you see only pistons from companies like MED with symetric positioned pockets. It makes me think the diameter of the mill is quite large...

Thanks!
Bart

I would just send the pistons to Carl at Force considering the price on their website.

Thanks!
Knowing he's doing a splendid job, that would be worth considering... however I'm a quite stubborn idiot who likes to do as much as possible himself...

Do a dry build and use clay on the Pistons to get a reference, or pull the valves out of the head, and make a pointer stick from a piece of rod the same dia as valve stems. With the head (and gasket) in place, One tap on your pointer will reveal the centre point. Also measure a depth reference of how far in it touches the piston. Now you just need to set the Pistons over on the mill, and with the spindle axis aligned to the mark, install an appropriate cutter (2-4mm larger on dia - your choice - & with suitable corner rad relief) will give you your cut-out. For depth, do a little math and measuring based on your pointer stick in conjunction with the max lift you will see with the piston at TDC and five degrees before/after... Or just cut it to a depth that feels right lol...
Simple enough?

Edited by TurboDave16V on 31st Jan, 2015.

Yep. Punch down the guide every time. You can normally make them symmetrical and over sized anyway unless your going for big c/r.

Thanks guys! I had already ideas in my mind like these.. happy to have them confirmed!
I try to get a dynamic c/r of 9:1, engine will stay normal aspirated

Hi, why such a low compression being a N.A. engine.
I'm doing the same conversion but on a k1200 head and aiming for 10.5 to 11:1CR NA also.

Edited by R.Rodrigues on 18th Feb, 2015.

The dynamic ratio isnt the same as the static. It takes into account the valve timing.

My bad, din't notice the dynamic...
I have the 284 deg cams also, from what I understand is the inlet cam we should advance a couple of degrees. I think the 11:1 static its fine to me, thinking in use the BK450 trying to keep the build as low cost as possible.

11:1 static is like original BMW. With a clever conservative ignition timimng it shoudl be possible to drive on 95 ron fuel.

And indeed, dry decked you can stay with the good friend BK450 as head gasket. Isn't such a hassle to get extra holes in that.

Hi I also wanted to do as much as possible myself, I wanted to do the complete conversion by making and modifying everything myself, I just cut the pockets in the original 1275 Pistons and I coppied the bmw pistons. It was an engineering challenge more that a quest for power. At the risk of ridicule I'll post a few pics of how I did it in the 'garden shed', please be kind with your comments.

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Once I go the jig and tool set up I simply 'production lined' the set.

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ok i get how you held the piston. but you didn't use the router for cutting the pockets did you???

terry

erm,

so you copyed the position from the BMW piston to the A series piston?

if so you havent allowed for a series pistons not being in the same positon as the Bimmer ones

I think Nick did a nice creative job. I understand Joe's concern, as the position of the piston to head is not accurate in centre when installing a K head to A series engine. All depends what radius is used for the pockets. I had a look at the pics on the site of MED, giving a good idea about universal usable pockets.

Terry yep I did use the router :) , tungsten tip cutter, same princible as a tipped milling cutter, worked a treat , the Pistons are only aluminium type alloy, soft for any cutter, you not like the idea.
Joe I made them symmetrical around the centre line, for ease of "production lining". Yeh I figured out the head/combustion chamber wasn't exactly on the centre of the bores, I alowed for this and cut a little bigger than the sample but then the Pistons on the bmw at tdc where flush with the block, the a series Pistons at tdc are about 1 mm low and it's only clearance, again negating the need for being too accurate. As I said the conversion was an exersize in garden shed engineering not a quest for power. Although it has now become the latter.

nick. think the idea is great..just could not beleive me eyes. i'm doing the same conversion but on the cheap. so far i've i have spent about £200 hoping to keep it below £500..

Actually we would need Siryun in this discussion: talking flow. What about the flow characteristics around the valve in the pocket...How many space would be optimum? Or is it just microeffect? So, how many space aside and below... all counts for the CR too.....

I was on the defensive lol, just sharing my project, garden shed engineering isn't to everyone's taste, but it's all about the challenge.
I managed to build mine to a road usable standard for about £600. With that I bought a complete 1275 engine with all the bits, ( ran it up on the garage floor when I got it home to check it worked) , a complete k1100 engine with loom and ecu ( I thought I would try to get it going on the BMW ecu but that soon became apparent it was never going to work) Honda CBR 600 carbs. And a couple of second hand bike exhaust manifolds to use to make one for the new setup.

It first ran on the standard dizzy and the CBR carbs, and was like that for the first year, then I got bored lol

I don't know, just give it more boost, that fixes everything :)

I coppied the bmw piston cut outs exactly...... No problems here

yeah but you moved the head...