888 CH/base gaskets - how thick?
 

Hello,

I have 2 sets of base gaskets, one set 0.3mm (78610391A) and the other about 0.9mm (78610571A). The new cylinder head gaskets I have just bought are part no. 78610542A.

Which base gaskets are correct for my '92 888? The 0.3mm ones were bought from a shop that closed a few months ago, (the head gaskets were out of stock) the 0.9mm one came as spares with the bike a good while back.

The bike shop that supplied the new head gaskets said the part no. of the 0.3mm gaskets applied to a monster, but these are very similar to my old scrap gaskets. They also said the 0.9mm gaskets were the right ones to use.

I'm a bit confused as my part numbers on my parts list have been superceded. I cant compare the thickness of the old/new head gaskets as the bike shop dismantled the heads/cylinders to overhaul them and I never got them back.

Thanks in anticipation!

Angus

Angus,

If you buy new head gaskets you can get 916/ST4 gaskets which are plain steel and thinner than the original black fibre 888 head gaskets. The originals are 1.3mm but I don't have the thickness of the steel head gaskets. I would guess they are 0.7mm which would give the same head to piston clearance using 0.9mm base gaskets instead of 0.3mm.

Standard clearance from head to piston is around 1.4mm but you can reduce this to 1.0 mm safely. Using 0.3mm base gaskets with 0.7mm head gaskets would reduce the clearance to only 0.8mm which is too close, but check the plain steel head gasket thickness. 1.3mm head gaskets and no base gasket works out fine.

To be sure check the squish with solder between the head and piston.

as mike says, there is really no correct thickness base gasket as they are installed in the required thickness for piston to head clearance. 1.00mm is supposed to be the ideal squish setting with steel rods (titanium rods req a different setting) so use the ones that give the correct clearance.

Thanks for the replies, I've opened up the packet of the old, thick base gaskets and they are 1.0mm thick. The new head gaskets are black and slightly compressible, look like fibre and are 0.7mm thick.

If I am to check the squish, do I need to do this with the gaskets in place and the barrel torqued down fully? Would this deform the head gasket (assuming it is single-use)? Or can I just fit the naked barrel "good and tight" and add on the gasket thicknesses?

Angus

if you use the old head gasket, assuming it's the same type as the new one you are going to use, that should give an accurate measurement. use some solder or plasticine to measure it with. use base gaskets to alter the squish, often the tolerance is too large so machining the base of the barrel with an appropriate base gasket is the usual way to get to 1.0mm