13 replies to this topic

[Sterling M]

I have a slightly unique question regarding the end gap on new pistons and rings

Motor is a 998 bog standard with the first over size rebore many years ago the only modification.

Unfortunately when the boring machine was cutting the bore the power went off and the machine ground to a halt, leaving a round score in the bore.

I have lived with this for about 40 years of running but have decided this rebuild to hone the imperfection out.

I have purchased new pistons complete with rings at the correct over-size.

Quite a lot of honing later, the bore looks spot on, but has grown about 4 thou to achieve it.

The piston ring end gap has gone from 15 thou in the other three bores to 32 thou.

Is this really an issue given the state of tune for the motor?

Am I chasing shadows and worrying about nothing?

You thoughts/ideas please.

[DeadSquare]

.032" is far too large a ring gap.

 

.004" oversize bore is too slack for pistons.

[Steve220]

Yeh.. that piston skirt to bore size is way too big. They'll slap and wear prematurely

[weef]

As a general rule of thumb the ring gap is 0.004"/ 1" of bore diameter.

From your figures, presuming you have measured correctly. the ring gap well out of specification.

[Sterling M]

Hmmm, as I thought, What are my options?

The pistons are a good fit, is it worth trying to get some rings made for the extra wear in one bore, or go the full hog and rebore the block?

[Earwax]

For me , I think it is time for a full rethink.

 

Get a second opinion on whether the pistons are indeed a good fit . 

 

It may be cheaper to buy an existing running engine - if a rebore again, new pistons etc are the best way forward on that block/

 

As above I also think even15 thous ring gap is probably not ideal for a brand new set up ? 

[Sterling M]

OK, thank you everyone, appreciate the thoughts.

The only reason for getting rid of the groove was to stop the rings traveling in the piston grooves every stroke and introducing more wear in that part.

Unfortunately it looks like I have created another monster.  

Spent the day re-checking measurements and gaps etc.

 

Bore #1 is the most at the top at 2.571" and 2.568" at the bottom. (Ring gap at the top is 0.033"); (2nd Comp ring end gap is 0.029")

(Oil Scraper ring end gap is 0.037")

 

Bore #2 is 2.566" at the top (Top Ring gap at the top is 0.021"); (2nd Comp ring end gap is 0.018")

(Oil Scraper ring end gap is 0.029")

 

Bore #3 is 2.5655" at the top (Top Ring gap at the top is 0.019"); (2nd Comp ring end gap is 0.016")

(Oil Scraper ring end gap is 0.024")

 

Bore #4 is 2.564 at the top (Top Ring gap at the top is 0.018"); (2nd Comp ring end gap is 0.020")

(Oil Scraper ring end gap is 0.023")

 

Pistons measured 2.560" at the widest point 90 degrees from gudgeon pin below oil ring, these are new +20 thou pistons, which gives a 11 thou clearance on the worst bore at the top and 8 thou at the bottom.

I can't find in any of my reference books where the maximum clearance for piston to bore should be.

I'm asking this as I have read elsewhere on here that the 12 thou end gap is a minimum, can anyone advise what is regarded as a maximum please?

I guess I'm looking for an iron clad reason to not buy more pistons/rings/reboring the block as I would like to keep the original motor rather than sourcing a replacement.

Many thanks in advance.

Edited by Sterling M, 30 July 2023 - 03:27 AM.

[sonscar]

I would build it up and run it.you now have a well worn engine which may give years or service.Sure it might not be as good as it could be but you have the parts.If it is really bad you will have to rebuild it any way,cost some time and gaskets and other sundries.This is my cheapskate unskilled view.Steve..

[Steve220]

Those measurements are way out of tolerance. Piston skirt to bore should be 4 thou MAX. It'll burn oil, be low in compression and potentially crack a gudgeon boss due to excess vibration of the piston. Personally, I'd rebore it for +40 and do it properly.

[DeadSquare]

 

error

Edited by DeadSquare, 30 July 2023 - 09:37 AM.

[DeadSquare]

I would build it up and run it.you now have a well worn engine which may give years or service.Sure it might not be as good as it could be but you have the parts.If it is really bad you will have to rebuild it any way,cost some time and gaskets and other sundries.This is my cheapskate unskilled view.Steve..

You think that you might be cheapskate,  if I knew what was wrong, and it was working, I wouldn't have taken it apart.

[Spider]

While it may not seem right, a ring gap of 0.032" isn't that much of a worry, but what will be an issue with 0.004" + piston to bore clearance is keeping the piston square enough in the bore and keeping it quite.

[Sterling M]

Thanks everyone, after reading here and other comments on this forum I have decided to bite the bullet and rebore to the next size.

The rest of the engine is getting the "works bugger" so thought it was foolish to not tidy this up while on the job.

The ring gap (and for that mater the ring gap location), is a hotly debated subject and I found an interesting read from a link shared here which puts a whole new aspect on the argument.

Worth a read, if not for future reference.

https://diagnosticengineers.org/ring-gaps-vs-knowledge-gaps/

 

Thanks for the help.

[Shooter63]

Thanks everyone, after reading here and other comments on this forum I have decided to bite the bullet and rebore to the next size.
The rest of the engine is getting the "works bugger" so thought it was foolish to not tidy this up while on the job.
The ring gap (and for that mater the ring gap location), is a hotly debated subject and I found an interesting read from a link shared here which puts a whole new aspect on the argument.
Worth a read, if not for future reference.
https://diagnosticengineers.org/ring-gaps-vs-knowledge-gaps/
 
Thanks for the help.

That's one of my favourites to bring up when somebody starts banging on about how ring gaps must be blah blah blah, nice post

Shooter