16 replies to this topic

[James_1]

Okay, I've got my 998 engine partially stripped to clean up and rebuilt after the gearbox went. The bearings are all 'as new' and the journals are unmarked so I'm not going to do anything to them. The oil pump is wrecked so that will be replaced with a new one. However, I want to really clean the block up, ideally get it chemically cleaned and I also want to balance the rods and pistons if possible - BUT - on a seriously tight student budget.

I looked at just taking the pistons out and leaving the rings in their grooves, balancing and then carefully refitting without honing (there are still hone marks in the bores) which initially made sense. A bit of searching online found loads of people saying no don't do it but weirdly no real reason why not other than it's bad practice.

Other theories include added stress to the rings from opening and then closing or that rings don't rotate so unless they go back into the bore in the exact orientation that they ran in in they won't seal correctly. If this is true is it also impossible to remove the conrods leaving the pistons in the bores? They are the push fit circlip type.

I want to do this build properly but the price of rings seems to have doubled since I last did a build. There is no wear at all to the bores. Is it totally wrong to reuse the rings and only going to end in an oil burning nightmare? Has anyone even tried it?

With a planned red-line of 6500rpm based on the MG Metro cam's power-band is balancing of the rods and pistons even really needed?

With regards to cleaning the block and also drilling/tapping the galleries is this really sensible with the pistons still in the bores? I really don't want swarf floating about!

Sorry for the long post, I know it's been done to death but I can't find anything definitive or even backed up, other than the generic "I reuse rings all the time and get better performance because they get better as the get older" type nonsense.

James

Edited by James_1, 28 June 2012 - 11:35 PM.

[Ethel]

I can't see any problem with refitting the pistons. The rings are free to move in the grooves, so it's unlikely the gaps would stay in exactly the same place - if they did, the top & bottom ring gaps would eventually wear little ridges at the ends of their stroke. Assuming the rings & bore have worn together, why would an existing ring at a bit different orientation take any longer to bed in than a brand new one? If anything, you'd expect it to fit better.

[James_1]

This was always my thinking. I think the idea that the rings having to go in in the same orientation is a bit far fetched. Could removing/refitting damage them in any way?

I've bitten the bullet and ordered a set of rings though with the new oil pump and timing chain so I have the option to fit them if it's deemed necessary.

If anyone has personal experience or scientific explanations I'd be really grateful! I'm determined to put this thing to bed with proper answers.

[Ethel]

My preference would be to reuse the rings if the compression & gaps are still ok. Cylinders are bored round, but they wear oval because the crank is trying to throw the pistons through the back of the block with centrifugal force. An oval ring will fit better than a round one as long as it's somewhere near the same orientation. It's the same reasoning behind putting the gaps to face the front (dizzy side) of the block.

[James_1]

That is a very interesting point Ethel. In those grounds I would assume it is worth noting the basic orientation of the rings as the come out and ensuring that on refittment they go in the same way, as two ovals 90 degrees 'out' will not work well at all.

[Ethel]

Yes, most engine builders have a pet theory about where best to put the ring gaps, so you may notice a pattern across all 4 pots.

[bmcecosse]

It will be fine - re-use the existing rings - just put them back in the same bores. At 6500 you hardly need to balance - but no harm doing so. I used to run my 998 engine to an estimated 9000 - but it did have a centre main strap. The pistons on a good fine scale balance - and the rods end to end and then overall -rechecking the end to end . You must fit new bearing shells - never refit old shells. And a lightened flywheel makes a big difference to a 998.

Edited by bmcecosse, 29 June 2012 - 09:39 AM.

[James_1]

Thanks, the engine is having a CMS fitted. I've got some digital scales that are good to .01 of a gram so getting them accurate should be fine. I also have a balanced and lightened flywheel.

Much in the same vain as the questions RE rings, why must bearings never be reused? They will go back into the same rods/caps the same way around onto the same journals and have no wear at all to them. No grazes etc. I haven't heard of not reusing shells before and have seen them reused when in this state by a top racing firm in a racing MGB engine, not as a bodge but because they didn't feel it was necessary.

EDIT: Full build here: http://www.theminifo...ld/page__st__15

EDIT2: I'll get the engine stripped down completely today and cleaned, then I'll plastigauge the bearings to check that they are as good as they look.

Edited by James_1, 29 June 2012 - 12:51 PM.

[Ethel]

I don't see any issue in reusing shell bearings either, but it could well be false economy not to replace them while you have the opportunity. I'd but a higher priority on new conrod nuts 'n bolts.

[bmcecosse]

If the oilpump was wrecked - the oil pressure was surely down..... But that's my advice - re-use the shells at your own peril. Bolts and nuts should be fine !

[Cooperman]

Fit new cam followers. Don't chemically clean the block until and unless it is completely stripped. Balancing is not necessary with a 998 unless it is going to really revved, like 7000 rpm plus.
If it was not burning oil or smoking before removal, rings should be fine. However, always fit new big-end, -main and -thrust bearings and set the crankshaft end-float to manufacturer's recommendations by using oversize thrust bearings as necessary.
Always fit a new timing chain and a top-quality oil pump.

[James_1]

New 'high performance' followers are going in with a new oil pump. A new chain and duplex setup is going in.

The block has been completely stripped down to nothing today and been cleaned and painted. The engine wasn't using any oil but I need to plastigauge the crank and check the end-float tomorrow.

The oil pressure before strip down was 70 on startup dropping down to 60 at any revs and 40 at idle. When HOT the pressure would drop as low as 20 at idle but anything over 1000rpm would see it return to 35-40 and then 60 at revs.

What is the reasoning behind not reusing bearings? As I say, I've been there when a supposed expert deemed it to be perfectly acceptable practice on the grounds that there was no wear and the oil pressure was good.

None of the bearings are marked in any way so it would appear to have been just the pump which is really wrecked.

[Ethel]

That pressure sounds fine, might have been the pump, or just the oil pressure relief valve causing it to drop.

[James_1]

The pressure relief valve was slightly scratched so is being replaced, along with a new spring. I've got a ball bearing to replace it, is that acceptable or would it be advisable to run the stardard type. I was only concerned about the drop when hot so may fit an oil cooler if the problem pursists after the engine is reinstalled.

[Cooperman]

A lot of people have had problems when using a ball bearing in place of the plunger.