Volvo Md7a Rebuild or Repower

[barnakiel]

Jeff PLS if you find the rings / pistons or other related parts could you post the source here.

Same engine here. We will overhaul but I want to find rings / pistons up first.

Too bad I do not know if and how many times our engine was overhauled by now.

There are 3 sets of piston rings. I believe they go with 3 sets of pistons(?). So you can play the trick 2 times only.

b.

[jeffjd]

Thanks for all the posts. Grant - good to know about the piston material.

I will post the final results and parts purchased. If we really want to keep these engines going it would be in our best interest to group our buying power and purchase custom pistons, rings and bearing. I am pretty sure if we had four people we could have a rebuilt kit made for 30% of the penta price. It would not be cheap but a lot cheaper than penta.

Last time I put the head back on I dropped a valve steam cap in the oil slump. Bought a new one to get it going forward - $50 plus shipping which I overnighted so let's call it $75 for a part that I could not cost more than $0.50 to make. Time is money so I paid up.

Currently the exhaust manifold is being pressure tested.

I agree I need to pressure test it, but I need the adaptor. I plan on trying to borrow one from a local mechanic ... If I can find one. I have had 5 mechanics turn me down on looking at the engine.

BTW does anyone have a piston out of the engine and does it have a part number? I might have a lead on pistons but they asked it there was a serial number.


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[gjordan]

Jeff, You may not need new pistons at all. Wait until a machinist checks them out, before you put out the bucks for new. I went back through the thread and almost gagged at $900 for a set of bearings. I am sure any good bearing house could find you replacement bearings for less than that. If you have good oil pressure there is no need to change the mains, and if clearances are within spec you may not even need new rod bearings. There was mention on another thread, of replacing rod bolts , but NDT testing will tell you if your bolts are good. Every airplane that people have ever flown in will have had parts that were NDTd and found safe to put back into service. Engine failure in an airplane has much more danger than in a sail boat, so I would trust NDT testing. Just another 2 cents worth. _____Grant.

[weephee]

Could you please explain what NDT testing is. I have my md7a apart right now waiting to have the head cleaned up and the valves done. I pulled the pistons and I'm my mechanic is looking for a set of rings. I'll look for a serial number on the piston and get back to let you know what I found. Once again, what is NDT testing and how is it done. Thx

[glenn.225]

Interesting thread.

NOT is non destructive testing. Usually done with penetrating dyes for engine parts. Other types are x-ray ultrasound magnetic etc.

jefjd If it isn't solved after all this testing it could turn out be a fuel delivery problem.
And i would get a new head basket and a torque wrench. Refusing headgaskets is generally not a good idea. but been there done that!

[barnakiel]

Yep.

Get the bearings measured and then gooogle them out. There are huge supplies of all sorts of bearings from various sources. Some are at least as good as original ones and a fraction of VP price.

Here:
Volvo Penta MD6, MD6A, MD6B, MD7, MD7A, MD7B - MarinePartsEurope.com

you may find pistons' VP catalog numbers. I am not sure they are the same as part numbers but the mechanic may know.

my 2 cents, I am an engine ignorant, and all ears

barnakiel

[gjordan]

NDT is "non destructive testing" which has several different methods. For steel parts it is usually in a chemical bath with electrical current run through it. Any flaw in the bolt, rod, crankshaft,etc, will show up visibly. I have sent many aircraft engine parts out for testing and certification, but I dont remember specific prices. At Volvo prices, I suspect that getting your rod bolts tested would be less costly than just buying new ones. Rod bolts came up on another Volvo thread, so I brought up NDT testing. Failed rod bolts are very rare, but have been known to happen, so they would be the only part of a marine diesel that I would bother to have tested. Fanatical racers often send brand new parts out for testing before installation, since manufacturing flaws are not always obvious to the naked eye. That is a bit too annal for small marine diesels. _______Grant.

[weephee]

Checked out the pistons from my md7a. On the inside skirt is the number R021 and 76L30 and MAHLE. FYI the numbers on the connecting rod are. #3875315 on one side and PFW 8.76 on the other side. Hope this helps

[gjordan]

Great work weephee, mahle is a huge builder of pistons, and probably one phone call to them will tell you what other engines take the same piston. Any other brand of engine will have lower prices than our GREEN DEMONs. An automotive machine shop might be able to identify the maker of the rod, and maybe give us a better idea of how to get less pricy bearings. I am going to be doing the ring and bearing rebuild on mine after the first of the year, so maybe all of these sources will be worked out by then. It will benefit all of us Volvo owners. ____We all thank you._____Grant.

[gjordan]

Just had another thought! Are there any identification marks on the bearing shells? And how do the bearings look to you? There are usually a manufacturers code or name stamped on the backside of a bearing shell, and if it is undersize (from previous crank grind) it will probably have a number stamped on it. Maybe .01 or .02 or something like that. Bloody detective work going on here!!! _____Grant.

[jeffjd]

Re: sourcing parts - I will call MAHLE to see if they still have pistons. It would be great if we could put together a part list and make rebuilding these things cost effective. I have seen several posts mentioning that Nissan rings work but no part number.

Anyone know if there are shims for md7a? I am a little concerned if I need to deck the block.


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[jeffjd]

This thread is getting long. Summary - blew the head gasket, replaced and now I have white smoke and power is way down. Only hits 1100 rpm in gear.

The exhaust manifold pressure check can back good.

I ordered an adaptor for the compression test, so I will do it on Saturday. I will post the part number if it works.

I thought my head gasket got a good seal but now that the exhaust manifold is good it must be the head gasket causing the white smoke? I will do the compression test but if it is low couldn't it be either the rings or head gasket? I have a new head gasket (2nd one (btw french creek $20 cheaper than marine part express)), but I do not want to waste the 2nd gasket. In other words, if the head gasket leaks can the compression test tell me anything or can I get a full compression with a leaking head gasket?

I realize this could be fuel, but that system was rebuilt last year. If the compression is good I guess back to fuel trouble shooting.

Anyone know how restricted a exhaust system needs to be to cause a decrease in power? I checked my lift muffler and soot/grease was taking up 20% of the entrance to the muffler. I will clean it out, but I doubt this caused enough back pressure to effect the performance.

Condition of the block & head surface. Had the head surfaced but checked it after torquing and the was a slight contour which was caused by a contour on the block. The block should be decked, but I did not check originally. I may try to deck the block with a board and sandpaper, the machine shop suggested.

Another thing we realized was our torque wrench was not reading correctly. I realized this when trying h the boat's method of torquing the head. Then I switched to a non click type torque wrench.


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[jeffjd]

Sorry - mahle was a dead end for me. Someone else may have luck finding someone to cross refer the part numbers.


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[gjordan]

Jeff, 20% blockage of the exhaust system (anywhere in that system) can cause significant reduction in power. Might run O.K. at low speed, but not at higher throttle levels. You probably should disconnect every joint in the system and clean as best you can. Even the thru hull at the outlet could have restrictions. The water in the head, and the white smoke may or may not have anything to do with the exhaust system, but the reduction in power could easily be from a semi plugged exhaust. You may have found , at least one of your problems. Good Luck. ______Grant.

[gjordan]

Small side note here, I just looked at my MD7, and the fuel filter on the back of the engine is a FRAM P4102A. I have no doubt that it costs less than a Volvo filter. I intend to find a bulkhead mounted remote oil filter setup, since I have never figured out what kind of engineer designed the side mount filter that cant be changed without spilling oil. _______Grant.