Small Bore Over bored Caution

[970s]

Had just scrapped an early 998 cooper block due to off set bored a little to increase the clearance between 1 & 2, 3 & 4 cylinders, for the max +100 thou flat top pistons!
Anyone had the same experience?

Al

No.4 Hole resized2 .jpg

cracked hole.jpg

[bwaminispeed]

I think that's the oil passage to feed the cylinder head.....I believe Vizard mentions it in his books.....

[jayare]

Could that not be sleeved and re-cut to save the block? It looks further down than the rings would go?

JR

[whistler]

My 998/1098 12A479 block was overbored to +0.100" without any problems. Offsetting not necessary unless you're going bigger.

[diz]

Graham Russell mentions the possibility of breaking into the oilway when using 68mm pistons and suggests it can be fixed by sleeving.

https://russellengineering.com.au/project-68mm-part-1/

[spoon.450]

I think that's the oil passage to feed the cylinder head.....I believe Vizard mentions it in his books.....

That rings a bell with me……..Aralditing a piece of brake pipe in or something….???

[5portsrock]

Seems fairly normal on big overbites to small bore blocks. You can save it by sleeving. Its generally below the ring lands so not to much of an issue

[970s]

Thanks for all your advise, had found another 998 block to over bore again without off set bore , just a lesson learnt to avoid off set at all for small bore block!


Al

[Alex]

When offsetting a 998, you don't go as far on the oilway end of the block - it's in the Bibles.
From memory it's 10 thou less than the distance of the other movements (.025"/.025"/.025"/.015")

[970s]

I should have studied the Bible more frequently before I machine the block, never though of 998 block will cause an issue.
Finding another 998 block may be more cost effective than have the block sleeved and re bore.

Al

[bwaminispeed]

Looking at that pic, I'm pretty sure even at .010 less, you still would have been into that gallery....