250 N/C cyl head refurb

[Mike]

This is really a continuation of this thread: viewtopic.php?f=3&t=3549&p=30971#p30971 - but because it has moved on beyond just the carb, thought better to start a new topic.

So - the story so far - confirmed oil in the combustion chamber, and I've now stripped down the head.




There was evidence of a leak on the valve seals and some obvious signs of the seals being damaged, so they definitely need replacing. I'm taking the opportunity to give the head a good clean whilst it's apart and have been checking the tolerances against the manual specs.



My valve stems are both about 7.94, which, if I'm reading this table correctly, makes them within tolerance, but there's something funky going on with my intake guide, which appears required some persuasion to extract the valve from it. So - I'm thinking that it might be worth my while replacing the valve guides.

@themoudie - Bill - I see you posting a while back about K-liners (here: viewtopic.php?f=3&t=3717&p=29676 ).
Knowing that you're just up the road from me, I wonder if you might share who your engineering company was for this? I'm guessing that they would bore the existing guides to fit the liners? - should I consider fresh valves also?

I'm also a bit surprised at the specs given for the piston rings. They don't seem to tally at all with what I have installed. I suspect my piston is probably not OEM. I think the table is suggesting that the 250 Monza doesn't have an oil control ring, but I do. Furthermore, the ring gaps when installed in the bore is 2mm, much more than the tolerance suggested in the table.

Interested in views out there by those who know more that I do..!





Thanks,
Mike

[George]

Worn inlet guide would explain oil on valve. I always fit new valves if replacing guides as new guides would have to fit over unworn part of the stem and be loose on worn part. 2mm ring gap far too much. Also check clearance between piston and bore. Might be time for rebore. Sorry to be depressing but you can't do half of the job. We have an expensive hobby.

[Mike]

I always fit new valves if replacing guides as new guides would have to fit over unworn part of the stem and be loose on worn part.

Yes, that makes perfect sense, thanks George - I'll check the piston/bore clearance tomorrow evening.

It has been a (long) while since I've had to buy parts for this bike. It's good to see that Nigel continues to have a well stocked store.

[blethermaskite]

Mike, your piston certainly does not look like a std. Ducati monza piston, and your measured ring gap is very excessive, agree with all 'other' George has suggested' looks like you have found your reasons for the 'oilyness' .
Cheers,
George

[themoudie]

Aye Mike,

So the diesel 250 was the answer!

K-liner's for the guides do not involve removal of the guides. As I found to my cost, blow holes in cylinder head castings and bodge jobs can be disguised! So, rather than removing the guides, I had the liners installed, with no problems to date. I also have had them installed in my Yamaha SRX600 heads. The engineers that I use are in Dundee, Engine Resources, whilst this may be a wee bit far for you, I'm sure something could be worked out. On the other hand J.E.M. Engineering, Polmont Road, Laurieston Road, Falkirk, Tel: 01324 633266, and Pitcairn Engineering, Orchard Farm Works, Banknock, Bonnybridge, Tel: 01324 841483, are both in your neck of the woods.

Rather than measuring valve stem diameters and trying to measure the valve guide bores, I put the valve in the head and lift it clear of the seat, to its working height, then check for "wobble" at 90° intervals. If there is more than 0.08mm (0.003") play then either the valve stem and or the valve guide are worn. Personally, for the cost of a new stainless steel valve, I would renew the valve and have the guide K-Liner'd. With engineering labour costs of at least £60 + VAT per hour now, it is the labour for setting up the job, expertise to know what to do and the actual doing of the job that will empty your wallet. They are also a rapidly dimishing "species", as it involves physical labour, dexterity, nouse and experience, all being in short supply now due to the lack of "practical engineering training, 'elf and Safety legislation and general "Jobs worths"!

As for the piston/rings: I am not familiar with the detail of 250 pistons and with the number of "standard" or "racing" items that have been available down the years, few do! However, the ring end gap that you quote (2mm) is excessive. This may vary depending upon the ovality of your cylinder bore and where the piston ring gaps are around the circumference of the bore. Ideally they should be equidistant around the circumference of the piston.

Also are your piston ring lands (piston ring grooves) worn? This would exacerbate the oiling problem.

NIgel Lacey recommends, for road use: "A piston to bore clearance of 0.003" to 0.0035" (0.075 ~ 0.090mm).

Ring end gaps (maybe slightly larger, but NOT smaller):-
Top ring: 0.012" to 0.014" (0.03 ~ 0.35mm)
2nd ring: 0.014" to 0.016" (0.35 ~ 0.4mm)
Oil control ring: 0.016" to 0.018" (0.4 ~ 0.45mm)"

I hope this helps, good health, Bill

[Eldert]

Hi Mike

your piston is a aftermarked piston made by MC supply

Eldert

[Duccout]

Bill,

This is slightly off topic (apologies) but my partner's niece's boyfriend is just finishing four years at university studying mechanical engineering, and knows nothing - he has done a 'bit of lathe work' and amazingly (to me) has never heard of Brunel!!!!! How on earth can someone go through school and university and study engineering without having heard of Brunel? I wonder what they do teach them nowadays?

Back to topic.


Colin

[themoudie]

Aye Colin,

I rest my case! Sad, nae, very disturbing!

The same can be said for my silvicultural profession. Trees? OH! you can design it all on a GIS, why would you want to actually grow productive trees?

Back to the important topic of getting Mike's 250 back up and running, so that he can wear it out again!

Good health, Bill

[Mike]

Wow - a huge amount of useful information - thanks folks!

Also check clearance between piston and bore

I don't have a bore gauge, so I've made approximate measurements with a set of callipers at the top of the barrel - I realise that this will only be measuring the top of the bore, and therefore will be quite inaccurate as it'll not capture any wear further down. I measured the piston skirt, just below the pin hole, both across the pin hole and at 90 degrees to it to try and a get a feel for the ovality of it.

If I've been measuring correctly, and based on that recommendation from Nigel..

NIgel Lacey recommends, for road use: "A piston to bore clearance of 0.003" to 0.0035" (0.075 ~ 0.090mm).

...then this piston is precisely the wrong size for the bore. Particularly if the bore is worn, then these will be the minimum numbers, and it could be higher than this.



Checking the seats of the rings, I get this:



So what's next? - I'm expecting that you'll confirm that this piston is useless. I'm also expecting that I should probably get myself a bore gauge to figure out whether the liner is also junk. What do you think?

By the way, for interest - I didn't have the tool for locking the camshaft (I wanted to remove the bearing and check it), so yesterday I designed a tool to do the job, and 3d-printed it. It's definitely a one-time use tool, as I broke it whilst doing the job yesterday evening.. but it did the job. The idea is that it bolts into where the cap is, then an aluminium drift can be inserted to lock the cam in place.



Regards,
Mike

[George]

No need to replace liner buy oversize piston from Nigel Lacey. This will include rings and pin. Give piston and barrel to your local engineering shop and have barrel rebored.

As a guide if you measure piston and slide feeler gauge between piston and bore add together this will give size of bore and you will know which oversize piston to buy.
Good luck.