Max ring end-gaps for a CD175 A4/K4; 15 or 31 thou?

[mike73]

Hi everybody,
I have recently acquired a 1972 honda CD175. In fair-to-middling condition, 22000 miles on the clock but with poor compression. When I got it- checked there was oil and it would turn over- attached my comp gauge and got only 30psi! Something amiss I feel. I took the head and cylinder off- the head is clearly gummed up and will prob need new valves and guides. Happy to go ahead with that as its clearly needed... my problem is the cylinder. The bore looked fine- using my bore gauge it comes out at 51.08mm with no sig out of round or taper- so far so good. Haynes states wear limit of 52.1 so I'm inside that and my 1971 Honda shop manual agrees with this figure. The problem comes when I check out the ring end gaps- Strangely Haynes gives the ring gap as 15 thou (compression rings) but later says to replace them if they are over 14 (Duuhhhh?). I checked the Honda shop manual and it gives ring gaps of 16 thou and replace if over 31.5!!! Well mine are 18 so they are either well within limits or totally shot!! Who do I believe? 31 thou does seem huge for an end gap.
I will add that although the motor did have loads of oil in it, when I drained it prior to dismantling the thing the oil was really thin and watery- it just p**d out as soon as I removed the drain plug. Its not water so I suspect someone has been trying to use flusher but never drained it- thin oil would obviously not give a good ring seal and as I didn't do a wet compression test (being so shocked by 30 psi and expecting the bores to be scored anyway) I don't really know whether to change them or not. For this old bike new standard rings are hard to come by now and pretty expensive so I don't want to change if they are still OK even though that would be more satisfying.

Incidentally I cant detect any movement in the big ends and the gudgeon pins seem fine too so I am hoping that the motor is basically sound despite this odd oil phenomenon.
I will make sure to clean out the clutch basket and oil pump (if I can get at it) as I suspect there is a lot of silt in the crankcases.
Anyway guidance appreciated guys!
Cheers
Mike
____________________
If you have eliminated the impossible- hit everything that's left with a hammer

[Fizzer Thou]

These old bikes are getting a bit old now.I remember when they were being sold new back in 1975 when I first started riding bikes.

Pistons and ring sets are still available

https://stores.ebay.co.uk/D-G-COMP-SHOP/HONDA-PISTONS-/_i.html?_fsub=146873016&_sid=147480266&_trksid=p4634.c0.m322

As for the end gap,my Haynes manual (#067) for your bike states the compression ring end gap should be 6'-15'

There is another thread in the 'Show and Tell' section of someone else going through a CD175 restoration

https://www.bikechatforums.com/viewtopic.php?t=233806
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[mike73]

Thanks- oversize pistons and rings are pretty much available as you have also found- I guess for the older bikes not many people are after standard sized so its these that are hard to get. Have found one source at 70 quid a pop but would like cheaper.

Haynes ring gap in your manual is also different from the figure in the honda book so I remain unsure of which is correct. It may be simpler to bore to first oversize.

Incidentally if you get an oversize piston is the gudgeon (wrist) pin oversize as well? I see these bikes have no little end bearing which seems a little odd?
Cheers
Mike
____________________
If you have eliminated the impossible- hit everything that's left with a hammer

[Fizzer Thou]

The little end of the con-rod does not need to be caged like on a 2 stroke as it relies on splash thrown up by the crankshaft splashing oil about as it rotates.The gudgeon or piston pin does not need to be oversize and there is normally some sideways movement of the piston once located on the con-rod.Ensure that the small holes in the little end are clear as this is what helps to lubricate the plain bush.

But it does look cheaper to buy a 1st oversize piston kit and to have the bores machined and honed to size instead of attempting to buy standard size rings,only to find that the bore is worn.A recommended rebore shop would be able to measure the bore accurately to ensure that the 1st oversize pistons will be sufficient.If the bore is too worn,you can then go to 2nd oversize and bore accordingly.
____________________
Just talk bikes.What else is there?

Always have a 'Plan B'

[YBR Ric]

If I'm unable to ascertain the correct ring gap I use the rule of 0.003" per inch diameter of bore for new and three times that for worn.

With a 2" bore that would work out at 0.006" min, 0.018" max. So with the sets of figures you've been given I'd personally go with the Haynes figure of 0.006-0.0015"

The reason for the low compression reading is more than likely as a result of the rings being gummed in their grooves. Once cleaned up, reassembled and free to expand into their bores you will get a much healthier reading. With just 0.018" end gap I'd just refit them.
I've worked on engines with far greater ring wear (and no tell tale signs of it) in the past which were running along quite happily prior to being stripped down to sort out other problems.

[Fizzer Thou]

Just found this picture in my photobucket account.The price was for c.1976

https://i666.photobucket.com/albums/vv23/WiNot_Rhencullen/Snaps/HondaLeaflets028.jpg
____________________
Just talk bikes.What else is there?

Always have a 'Plan B'