Bukh DV10

[EASLOOP]

Just finished re-seating the two valves in my engine. Spec calls for the valve surface to be 1mm below the surrounding casting; my exhaust valve is 2mm below. Checked with Bukh (they are exceptional people) who tell me I will have some loss of compression as a result and that this may cause some starting probs. Has anybody else done this? and what was the effect?
Look forward to replies.
John

 

[jerryat]

Hi Easloop,

I ground in the valves on the Bukh 10 I had in my last boat and had about 1.5 mm depth from the surface of the casing. I can see where Bukh (they ARE terrific aren't they?!) are coming from since the lower depth of the valves increases the volume of the cylinder lowers it's effectiveness at the compression stroke.

However, I could notice no decrease in power or difficulty in starting after re-assembling my motor, indeed the mere act of improving the seal of the valves and therefore the compression, gave a slight overall improvement. /forums/images/graemlins/smile.gif /forums/images/graemlins/smile.gif

I shouldn't worry, particularly as the only alternatives are to have (or DIY) new valve seats fitted then grind the valves to these, or replace the head!!

Hope it gives you years more service.

Cheers Jerry

 

[supermalc]

Maybe I'm being a bit cynical, but I wouldn't have thought lowering a valve 1mm would make a significant difference to the compression ratio.

If you have the specifications of the engine this should be relatively easy to work out. If for example the compression ratio is 20 to 1, then the combustion area is 1/20th of the swept volume of the piston (area of piston x stroke) add the area of the valve x 1mm to the combustion area. Divide into the swept volume (volume of piston) and you will know if the compression ratio has dropped a significant amount.

 

[jerryat]

Hi Supermalc,

Quite agree, that was exactly my point. Bukh are right to point out the potential difficulties, but as I said, I noticed a slight improvement in mine despite a lower than recommended valve bed and therefore advised Easloop not to worry!!!

Cheers Jerry

 

[Seal_surfer]

The best answer is to take the head to a machine shop/engine reconditioner and ask them to skim the head by the corresponding amount, providing that hasn't been to excess previously. Cost was in the order of £25. This ensures you've got a perfect face for the head gasket also.

 

[jerryat]

Hi Seal surfer,

I'd be very cautious about doing this considering the amount that the head would need to skimmed. I did a quick check in the workshop manual, but the max. skim amount is left blank(!) though from memory there is a fairly tight limit to how much you can take off the head before you risk the piston hitting the open valves. Easloop would have to check again with Bukh for their advice.

Frankly I do not believe this will be necessary if the motor was previously been operating ok. These motors are remarkably resilient (my current one, the 20hp, lasted from the Cape Verdes, through the Caribbean and back to the UK on only three rings per cylinder, for example) and unless the engine has badly over-heated and therefore warped the head, I wouldn't do anything.

Cheers Jerry

 

[supermalc]

To put it more simply, if the general condition of the engine is ok, and it is only the valve (s) that need grinding in, dropping the valve a little should make no difference to the starting or performance of the engine, as Jerry says.

If the engine is difficult to start due to poor compression, caused by cylinder and piston wear, then grinding the valves will doubtfully be the remedy required.