E: Engine Partial Single Seizure? Safe to ride?

[davidd]

Bernd,

As long as you mentioned it, it was your experience that I spoke of earlier. I had mentioned all these seizures to Pat Manning, and when he spoke to Terry last night he mentioned them to him, Terry was surprised and concerned. I think that using aluminum liners does help with my engines, but even with the ductile iron liners I have never had a seizure with a Comet. I have never used the oil hole either.

Vincent had some problems in the US when they delivered B's with only .004" clearance. I don't know what the bumped it up to, but the problem went away.

David

 

[Vincent Brake]

Btw.
Was your muff good round?
E.g. did it get good contact with the liner...
I ve seen stange things when i took some apart...
Since than always skim the liners and have me made up some nodulair cast os liners.
With 0.15-0.18mm interf fit .
Honed in pressure plates.
And the machine head and muff so there is an 0.025-0.035mm to press up before the fanges touch.
I dont like the grinding in so much.

 

[royrobertson]

One thing worth checking is that the bore is absolutely square to the base flange. In my case, many years ago after a major blow-up, the new liners were bored "drunk". This made a race winning motor misfire and a very frustrated mid-field runner. As they had been done by a reputable re-bore company in Brighton it took me FOUR years to find the problem. It was only by reading Tuning for Speed again that put me on the right track. I "borrowed" a large steel bar from work and turned it between centres on my lathe until the barrels just slid on. One bore was 8 thou out of true and other 5 thou. The poor pistons were trying to go up the bore slightly sideways, the ring edges rounded and not sealing properly,
(Note 0.006-0.008 inch bore to piston clearance for racing on iron liners or for my Aptec plated 90 bore aluminium liners 0.003 inch)
To get racing again I just cleaned up the base flanges and the high sides of the liners and left the tops until later. With new rings fitted the performance was back.
I have checked and machined quite a few barrels over the years as this fault has been found even with original bores. Hopefully when I get the hang of it, I will be able to post a photo of my latest light weight mandrel soon.
Cheers Roy

 

[oexing]

I have been aware of "drunk" bores relative to the base cylinder face elsewhere so when I came to finish the two new sets of B-Rap cylinders that I got 30 years ago from the Spares Co I checked them and found same faults too. Cure: I set them up in the lathe gripped with the three jaw from the head side and checked for runout as deep inside as the clock was readable plus same check at the bottom end. My three jaw chucks resp. the mounting plates for the lathe are all relieved by one millimeter play to be able to get any workpiece into good runout position via tapping the chuck with plastic hammer or alu block while the three bolts that hold the chuck onto the mouting plate are only lightly nipped on. No mandrel used here but checking with indicator at various stages.
When all was satisfactory I skimmed the base face true. For finishing the bore for honing I utilized an old car brake disc - you find them in all dimensions in scrap containers at garages for free.
The outer face of the disc was skimmed for good finish so I bolted it onto the standard face plate that comes with new lathes . Finally I could machine the bore of the disc for clearance fit of the liner and skim the end face of the disc for zero runout and mount the cylinder onto the brake disc. So in the end that is perfect for boring out the undersize liner plus finishing the spigot of the cylinder liner to get the required preload of 0.02-0.03mm for sealing onto the head - no grinding paste for me.
Another topic: I cannot see why the Vincent engines should really need that much piston clearance of more than 5 thou, that does not correspond to other engines I know. After all, you have an alu cylinder with an iron liner - like modern 2 valve BMWs or Ducati , Guzzi etc. that run happily with ca. 3 thou. So my gut feeling is that possibly the honing operation with the long liner reaching deep into the case is not done with due care. It is extremely important to have all conditions spot on , the correct hone oil, VERY soft grinding stones that don´get blunt and remain free cutting. Otherwise you get all sorts of geometrical defects - and seizures in consequence. I do not use pressure plates and the like, don´t see the use BUT take care to have extra soft stones and lot of oil and keep them free cutting - plus my Zündapp 6 stone production hone.
Just thinking: Could there be some distortion of the liner by torquing down cylinder and head onto the crank case due to funny minmal clearance liner-to-case slide fit?? Positioning the lot via these head studs looks a bit questionable, all drillings not an absolute precision thing it seems.

Vic

 

[davidd]

Another topic: I cannot see why the Vincent engines should really need that much piston clearance of more than 5 thou, that does not correspond to other engines I know. After all, you have an alu cylinder with an iron liner - like modern 2 valve BMWs or Ducati , Guzzi etc. that run happily with ca. 3 thou.

Vic,

I think the difference is that those are very different engines. Singles all run hotter than twins. If you compare twins to twins, increased the Vincent finning, made most of the stroke above the crankcase, ran large air filters and used a better liner, you could tighten up the clearance just like the modern twins.

It might be better to do a cost/benefit analysis and list all the benefits of running very tight piston clearances.

David

 

[royrobertson]

Please be careful if you are holding a barrel just on the inside jaws of a three-jaw chuck as they can and in Ian Hamilton's case years ago will come adrift (it hit him on the arm bruising his tendons badly. I think he was actually trying to shorten the liner. Some sort of stepped plug with a centre in it supported by the tailstock will keep it in place.

 

[oexing]

David, you are quite right, better accept more clearance than have seizures. But I just want to know WHY there seems to be that problem with Vincents. I am running 5 thou clearance in the 600 cc four valve Horex single in an all cast iron cylinder and 6 thou is in the 3.8 l E-Type all iron block . Commonly you will find quite a bit less necessary in alu cylinders with shrunk in iron liner, like Yamaha XT and SR, same range of bore, 80 to 90 mm. I do not think the long liner extending into the crank case matters much, all hot parts of effective stroke remain in free air and the bottom part of the liner is oil cooled in the tight fitting case. And finning is not too bad on the alu barrel, quite a lot less to be found on other types. I do not believe in Vincent mystery and uniqueness that causes that trouble of clearances, instead some defect in machining like miserable honing quality with that fitted liner. Unfortunately these defects are not easy to measure.
Roy, I only skimmed the alu cylinder base with barrel clamped in three jaw chuck. That liner had some undersize so I could grip it tightly for that job. Otherwise I prefer the base plate to mount items like head or cylinder.

Vic

Horex 600 cc four valve single, 1935:

 

[greg brillus]

As discussed before on this forum, later engines like the Yamaha use a much higher flow of lubricating and cooling oil to the cylinder head. The large diameter (relatively) cylinder hold down studs as well as supporting the entire forward mass of the engine, also act as heat sinks and certainly can be a common place on the inner walls of the liner to cause four points at which the piston will readily "Grab" and seize quite easily. The Vincent cylinder head is not really a large casting by mass and has many large holes cast or machined into it, thus the heat verses material is probably quite a poor ratio, much of this heat is absorbed by the cylinder muff, but also by the piston. There are many that have suffered piston issues, not least because of ignition or bad carburation, but lack of piston clearance is common, the specs by the manufacturers is not in the "Real world" of what actually happens in the engine. I have seen brand new barrel/piston kits for a Harley Sportster engine that had one thou piston clearance...........Would not have made it to the end of the street.

 

[royrobertson]

On my little 5 inch swing lathe the chuck jaws are short so when used gripping inside diameters I would not take a chance hence the mandrels. It does mean one for each size although I have found shim sheet works if they are oversize bores. I made a mistake at work doing a "homer" years ago when I was lightening a Panther hub to fit my Norvin racer. I did not clamp it well enough on the mill and nearly twisted my the index finger off my Left hand.
I now have a "claw" hand and have been a lot more careful since.

 

[vibrac]

You only have to see the marks on a liner outside when it "falls" out of the muff to know why 6 thou is a useable clearance on a vincent.
They did not know about nickasil back when the bikes were built but they weren't afraid to push the boundaries.. no cast iron barrels for the Phil's!