Lowering compression for a milled cylinder head

Hello all. My first topic post, and I need some help. I have an RH10 Cylinder head that has been so milled to up compression, that it's actually flat. Otherwise, it's a good head, no broken fins or any holes drilled. Be nice to use it, but not at the high compression it now would have; plus the valves would probably foul into the pistons if not modified. Any ideas, advice, or recommendations on how to use this head, but have the standard 8.5 : 1 compression ???

You could fit a 28thou base plate - part 06 6591 - which are in the current price list so should be available from your Andover Norton stockist. With a base gasket, I think this should give you enough clearance. When I stripped down my '76 mk3 interpol , there was one of these fitted even though the head was standard RH4.

Regental,

I sure appreciate the fast reply. I might just have to do the baseplate like you say. I put a baseplate on a 750 Commnando (Combat) I rebuilt years ago ... and it worked out really well. I'm hoping some other members have some more ideas too , any one out there ? Cheers !

Double copper head gasket? Never done it, but I'd guess you would need to seal the two together with some Permatex Copper-Coat spray and use the same stuff on the outsides to seal things up. Or maybe have one custom-made in double thickness.

Jim Schmidt does different thickness head gaskets. Maybe a combination of both base and head gaskets could do the job. He is jseng on this forum.

hi there , here you could find the direct link about Jim's gasket ..............the way to go to lower the comp! Pierre

http://users.gotsky.com/jimschmidt/jsmo ... ession.asp

hi again, just made the calculation, by adding his thickest head gasket and base gasket you can add 2.1mm in total , just measure the actual squish band depth and compare with std (of courser you can't now , but if you need , I had at home an untouched RH10 head where I can measure : 3.18mm approx with my caliper.........), hope this help, Pierre

Measure the squish band step. Stock is approx 3mm (.120") deep for 8.5:1 CR (commonly refered to as 9 to 1).

I can supply any head gasket thickness you want by special order. Or you can order a .062" head gasket and a .021" base gasket online as Pierre suggested in the post just above this one - see his measurements.

See the link below

Jim

Once the spacers selected remember to check the valve/push rod length/ rocker centering geometry. Also there is some excess meat I think in the factory pistons for more fudging CR. An expert like Jim Schimdt likely knows off top of his head how much is safe to mill off and how much is too much to race on long.

To all, I sure appreciate the posts ! When I use the head, I think I'll go with Jim's custom made base and head gaskets. Once the CR is down, I should be OK with pushrod length and rocker geometry, although I will still check that. Thanks for the reminder Hobot. And thanks to everyone for their input, cheers !!!

I'm trying to keep it all straight too, so repeating it helps me stay current. Nothing better for head removal/attaching in place than trimming some rocker end space to easy pass the push rods. I zip tie head to spine while diddling the gasket/goop/thread. Then stretch rubber bands over head onto rocker ends to hold push rods set in place once head lowered. Push rods get zip tied together till time to clip and drop down. Crooked wrench/special spanner is a treat to nip upside down nuts.

Unless you are burning some 3rd world econo fuel that's part diesel, a good tune and heat range plug can tolerate at least 9:1 w/o much worry on standard cam. Base gaskets are just another seam to seal and squirm and heat barrier to cross to me.
Tighter squish is known=famous for its anti-detonation function. Smaller chamber is also slightly more anti-detonation too. Ponder possibilities.

What solution you decide on will depend on if you should hone your cylinders. With a compression plate at the base, the ridge left from the rings must be removed or trouble could insue. Even if you cannot feel it, it is there.

-avoid thicker headgaskets . by using one there the copper will be exactly in your combustionchamber = very hot
you will blow them (i know from experience)

-you can or should use a alloy (or steel ) base gasket (spacer) to jack up the barrel and increase the squish doing so This also eliminates the need for honing the barrel as the pistons do not rise so high up in them If your barrel has got no real edge on top then i would not hone them (we ran an engine with shortened barrels (at the bottom.nsu engine) and ran it for a season without bad results to the rings or pistons
honing ruins more barels then anything else (bore or leave alone)

-imho your head does not look like it is really shaved to the limit . bolt it on your barrel with some plasticine on pistons and measure (bolt it on without headgasket and if you can turn the engine by hand then there is enough clearance when you assemble WITH headgasket) do similar with valves .
but I am quite sure you will be ok and running such a head (cr) with a std cam is FUN without being hard on the engine (check your pushrodlenghts !!!!!)

Thank you for sharing your deeper engine recovery insights lynxnsu.
Ring bore dodging and combustion exposed head gasket. Get copper bright red hot and it gets almost wax like. So I can see it losing work hardening after head clamping. First reasonable review in favor of base plate to me and something I may face juggling down CR in a special.

[quote="lynxnsu-you can or should use a alloy (or steel ) base gasket (spacer) to jack up the barrel and increase the squish doing so This also eliminates the need for honing the barrel as the pistons do not rise so high up in them If your barrel has got no real edge on top then i would not hone them (we ran an engine with shortened barrels (at the bottom.nsu engine) and ran it for a season without bad results to the rings or pistons
honing ruins more barels then anything else (bore or leave alone)

[/quote]

nortriubuell, I believe there are 2 ends to a stroke. Raisng the barrel to avoid the upper ridge only serves to expose the lower. I am all for avoiding honing when not needed but make darn sure there is no lower ridge. Telescoping gauges are ok but an inside micrometer may be needed to eliminate the doubt and the need for the infamous honing procedure.

Lynxnsu, are you saying that you would change rings without deglasing?