Cummins 6BTA Blow By

[GeneM]

Sorry for jumping in another post, so here's the issue.

I was planning to do a full rebuild because of low compression in 2 of 6 cylinders and lots of blowby from the oil fill cap.

I did the other engine and thought this (port) was OK until I saw how it compared to the rebuild.

The thread I jumped into talked about honing to get back compression. I learned something new there and was hoping for a cheaper fix.

The engine is a 6BTA-M2 370 HP, installed in 1998, having 2000 hrs. The previous owner ran it with risers that let water back in. The stbd engine was worse, so the done rebuild. I thought I saved the port one, doing a first class riser job on both engines. Alas, not so.

It burns a qt every 4 hrs at 1200 rpms (cruising is 2200 rpms). After 40 hrs, the oil is so thick, it won't lube the turbo and won't come up to cruise rpm.

I think I have to do the rebuild - pull it and new sleves (not wet). Thanks for the hope of a brighter day.

[Sailmonkey]

As was mentioned in the other thread, honing in place is do-able, but the real problem is cleaning up afterwards.

Besides if you’ve been running with oil so thick that your turbo won’t spin up, you need the full rebuild anyway.

[jimbunyard]

6BTA is a parent bore engine, the only sleeves available for it are so-called 'salvage liners', used if the block is so far gone that it can't be fixed by boring out to 1 mm over size. .5 mm oversize is also available if the wear can be cleaned up more easily.

Depending on your budget it might be possible to get by with just honing the cylinders; it depends on the actual nature of the wear and how far out of spec the cylinders actually are. The only way to tell for sure is to take the engine apart and measure it.

As for doing it in situ, that depends on your situ. It can be done without removing the crank, and I've done it on rare occasion, but never on an engine as big as a 6BT. It is better to be able to manipulate the block so the honing fluid runs back out the head end of the cylinder, and to move the block that much you'd likely have to remove it from the boat, in which case you'd probably be better to have the whole thing rebuilt more traditionally.

With 2000 hours on the engine, and from your description it sounds like the complete rebuild is going to be the better, most economical-in-the-long-run choice, but you never know until you see inside the engine. If it were me I'd pull the head to get a better idea of where I stood and plan from there my next steps...

[GeneM]

Thanks guys. I'm back to being prepared to rebuild.

I talked to an engine guy I liked and he said he would take a stepped approach, maybe I'll luck out.

The rebuild on the other block was done under advice of not trusting the original 1998 cylinder metal, so they bored it and put in liners.

Seems to be working well.

[Lepke]

I hone in place with the crank in. On some engines the crank has to be rotated between cylinders for hone clearance. I make a pan to fit over the crank and add absorbing material to catch the honing oil and any metal. Sometimes I make the pan out of tinfoil to be able to fit the engine. After honing, a good cleanup and you won't have debris problems or an damaging effect on engine life. Oversize rings are available form many companies as a custom ring. Most racing ring manufacturers will make them. When I have rings made, I try to maintain stock clearances after honing. If the cylinder is .003" bigger, I have the rings made that much wider. End gap needs to be checked carefully. There will be taper in the bore, but unless it's excessive, it won't effect performance that much. Most of the taper will be at the end of the stroke when pressures have decreased.

[Compass790]

Originally Posted by GeneM

Thanks guys. I'm back to being prepared to rebuild.

I talked to an engine guy I liked and he said he would take a stepped approach, maybe I'll luck out.

The rebuild on the other block was done under advice of not trusting the original 1998 cylinder metal, so they bored it and put in liners.

Seems to be working well.

I agree with the good advice that both Jim Bunyard & Lepke offered. Ignore what I said in other post about air cleaner. Thats clearly not your problem if you've had water ingress & have two cylinders low.