13 replies to this topic

[Mr Dave]

Hey all,

 

Looking for people's experiences with bore wear, in particular the lip that develops above the top ring reversal.

 

Stripping my 998 A+ - Having cleaned the bores out, I did a quick fingernail test and it seems like the only perceivable lip is on the side of Cyl 1 facing the rad and Cyl 4 facing the flywheel. Is this a typical wear pattern on these engines (i.e. bedding in) or should I be concerned? I was not planning a rebore (tight budget) if I could get away with it.

 

I can kind of understand that the thermal gradients and therefore expansion would be different here so may be the cause of the uneven wear (there is no lip in the Siamese area). Haven't gauged it yet so will try and borrow one from work to investigate further.

 

Thanks

[jaydee]

Not very typical..can you measure how big is the 'taper' with a bore gauge? Will be helpful to determine whether a rebore is necessary or not

[Cooperman]

The 'step' indicates excessive bore wear which can probably only be sorted out by having a re-bore & new pistons.

Otherwise it'll be a 'bodge' job and further work will soon be needed.

[Mr Dave]

@jaydee - Not sure how the bore gauges we have would work, will have to chat to the techs about it. I guess it would need to be a series of two point measurements to profile it. We do have a CMM but I'm not trained on it. Maybe I can slip the guy a tenner to run some scans!

 

@Cooperman - I don't think the wear is that great as I'm pretty sure the honing marks are still there (will have to check and photo when I get home), but if it comes to it I guess I could at least take the opportunity to go straight to +60 or +80

[Ethel]

Bores can wear into any shape, measuring the diameter between where your bore gauge touches is a fairly rough guide - think how you can bend & twist a length of plastic pipe without noticibly altering the bore diameter.

 

Maybe the wear is down to how the rings are sat, or the offset con rod axis A's have, or maybe it's just because there isn't another hot cylinder next door, to heat and expand that bit.

[Mr Dave]

@Ethel - Yes that was my thinking. I guess the rings will bed into this shape eventually anyway

 

No progress on gauges, CMMs etc yet, but I have done a very crude "run a DTI across the step and see what the change is" test and the maximum I saw was 0.1mm / 0.004". Would this normally be enough to warrant a rebore? Not that it necessarily makes a difference but I was going to renew the rings anyway...

 

On the honing marks front, the marks are still evident except towards the top of the bore (i would approximate this to the area between top and oil rings) where it's not really clear.

[Ethel]

Bore wear tends to be worst about half way down, where it's pushing/pulling the big ends furthest away from the centre line. The lip isn't an issue, it formed because the piston rings don't reach it, but that could change if you put a new ring in there.

 

It's whether you're willing to fork out for new pistons really

[jaydee]

4 thou is on its limit

Assuming its 4 thou the max step you have

Usually the taper is a lot less, barely measurable, on healty engine

Might get away with new rings and a LIGHT honing, and be ok for long time or might burn oil after a few thousand miles

Have it inspected at the local machine shop (if familiar with the good old A series engine) to get accurate measurement and decide if its worth it to bore one (or two) size up

Edited by jaydee, 02 March 2015 - 02:05 PM.

[ACDodd]

This needs a rebore, period.

Ac

[Cooperman]

This needs a rebore, period.

Ac

what I sed  .

[ACDodd]

The biggest mistake people make is starting an engine project knowing they only have a limited budget. You strip and inspect the engine. At this point this directs you to the work needed and the parts required. Only then can you set your budget. If you don't have the funds, you will not have a successful build, and your rebuild stops here.

AC

[Spider]

The biggest mistake people make is starting an engine project knowing they only have a limited budget. You strip and inspect the engine. At this point this directs you to the work needed and the parts required. Only then can you set your budget. If you don't have the funds, you will not have a successful build, and your rebuild stops here.

AC

 

Very good sound advice. It will also work out the cheapest.

[Mr Dave]

Thanks all. Will get the machine shop to check it anyway, but will start shopping around for pistons.

 

@ AC - I agree that is very sound advice. I always knew that a rebore was potentially going to be required so it's not a massive blow but it is just extra cost. Please understand when I say "budget" I mean in the sense that I am restricted to how much money is left at the end of the month, not by the amount I am willing to spend, so it is really more a time penalty than financial.

[Cooperman]

There always seems to be too much month left at the end of the money  .