Lapping Compound Grit Size?

[rhitee93]

For those of you who lap the bores of your cast iron cylinders, what grit sizes do you use?

The boring has left me with a good surface finish, but I have about 1 thou of taper over the 2" length. I was hoping I might be able to use a lap to open up the tight end a bit, but have never done it before.

Thanks!

 

[rhitee93]

Never mind. I forgot to search here for this first :

 

[tomol409]

Beware of compounds supplied for valve grinding. The fine of some is as coarse as the coarse in other packages. I use 400 grit for model diesel steel cylinders and this will give almost a mirror finish. Depending on the finish you have achieved, you may need several grades of compound. 100 would only be used on a poor finish initially. If your finish is good, I would reckon nothing less than 200 grit would be needed. The compound more or less grinds itself to a finer finish as you go along. To remove the taper you will need to make an expanding lap. I have the appropriate taper pin reamers and drill and ream the lap then fit a taper pin, the end of which is threaded and fitted with a nut to expand the lap, which is split. Check each end of the lap carefully to ensure it is parallel. Use a twisting and reciprocating motion of the lap. Do not overdo the amount of compound. You can buy diamond lapping compound now fairly cheaply, but I have not used this as yet.