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clearance of cylinder liners in crankcase mouth

charles d cannon jr

Active Forum Website User
VOC Member
I'm rebuilding a 1950 Black Shadow and recently bought some oversize cylinder liners from Coventry and took them to my machinist to do the key cut and drill the oil hole, etc, and took him an old liner to show how it is done. He brought to my attention that the undercut at the lower aspect of the liner was smaller than my old one so I brought the new ones home and slid them in the crankcase holes to check them . They were easily rocked back and forth in the mouths and using feeler gauges, I was able to measure 10/1000 inch gap. I'm a pack rat, so I found some old liners from my first project, Rapide 697, and was able to appreciate a gap of 5/1000 inch. I spoke with Bill Parr, technical director and he felt that .003-.005 was the upper acceptable limit for clearance. I bought some new liners from the Club and they seem even looser. I've found a firm in the US (where I live) that will do reverse engineering from a sleeve to create a new sleeve, and I'm considering sending them one of my new sleeves and just asking them to reproduce the sleeve with less undercut at the lower aspect but this seems like a big deal. Are the vendors making them smaller so everyone can "get theirs in" thereby creating difficulty for those who have slightly bigger crankcase holes? Any comments regarding similar experiences or use of other vendors or sources for the sleeves will be greatly appreciated. Thanks, Charles Cannon

chankly bore

Well Known and Active Forum Website User
Non-VOC Member
Phil. Irving always maintained that the liners should be snug in the crankcase mouth and the Victorian Section has the correspondence to prove it! Series "D" liners were slack- presumably for cheapness,ease of assembly/disassembly or because it was felt that the oil intended for the back of the piston was better off in the bottom of the crankcase.Also it pays to check your crankcase for bellmouthing.


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VOC Member
To offer another perspective, I prefer at least 0.005" (the more the better) clearance between the liner and crankcase as normal elastic distortion occurs here when you tighten the head bolts which can cause distortion of the liner and influence piston to bore clearance. I even go so far as to blank off the oil feed to the cylinders to direct more at the cams. I know others will have a different perspective but then each to your own. Early Vee-twins utilised front cylinder direct oiling based on the idea the rear cylinder stole most of the crankcase oil mist and this is evidenced in greater front cylinder wear typically occurring. I cannot see why extra oiling is needed in the rear cylinder. Please don't interpret my comments as wishing to go against someone as esteemed as PEI, rather just take it as another opinion which may well save you needlessly wasting both time and money.


Well Known and Active Forum Website User
Non-VOC Member
Albervin said:
redbloke1956 said:
In an old MPH (forget which one) is a nice little quote from Bob Dunn; "liners should be 0.005" less in OD than the crankcase mouth into which they fit".
On my "A" I have to relieve the crankcase bolts to remove the cylinder but not on any of the post war Vincents I have. None has a leakage problem.
The above from Albervin.
Personally my old liner had a snug fit in the mouth...but my brand new liner has a .005" clearance ?, I read in one of the Vin publications that "Early liners were a snug fit but later C's and D's were a loose fit" or words to that effect .
Just another observation.


Well Known and Active Forum Website User
VOC Member
Terry Prince supplies oversized liners to fit the various alloy muffs going around. Machine to fit muff and give sufficient clearance of crankcase mouth. Note that when fitting the liner in the muff there is a distinct chance of distortion so further measurements need to be made. This has all been discussed before and can be found using the search tool at top of page.


Well Known and Active Forum Website User
VOC Member
My engine’s a 1950 Rap. The barrels have always been a “snug fit” and need the crankcase studs slackening before I can remove the barrels. I don’t suppose 5-10 thou will make too much difference, but to me, the crankcase bores should be the datum points for the barrels and top end, they’re the only parts machined relative to main bearings etc.
I haven’t made a conscious effort to make them this way, it’s just the way they’ve been for 38 years, and I can’t attribute any problems to the fit.



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VOC Member
My latest project is to reduce the 600 Racing comet back to 500 (you get more points for a second on a 500 than a win on a 600) this will involve an alloy plug to fill up the 7mm annular space twixt crankcase and 500 barrel (a bit like a top hat without the top) this would give me an opportunity to add an O ring or two somewhere. still thinking but I have ordered an alloy blank and sorted some time on a bigger lathe
NB It also occurs to me that in the standard set up a large clearance between the crankcase and the liner will reduce the amount of oil passing across into the barrel via the holes provided if .005 is large in this context I am not sure.

ray vinmad

VOC Drawings Manager
VOC Member
If you are looking for a `snug' fit make sure that the crankcase mouth is dead square to the mounting face. Otherwise you will effectively be trying to put a kink in the liner when you tighten the barrel down.

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