Fitting a stronger big end to a 350

[Wali.Badger.Taylor]

Is it possible to fit a stronger (Jawa?) big end to a 350 crank? I broke one in half yesterday sprinting at Eelmoor.

[brenton.roy]

Hi Wali, ouch, that must have sounded expensive...
I looked into this a little, a while ago. It was too expensive for me in the way the engineer explained it (and not practical given my riding), but it could be done.
The idea (apparently) is to get flywheels cut from billet and a whole new assembly made up. You can then use your big end of choice.
You can use the Ariel flywheels and have them bored, but they may be your next weak link.
The Jawa big end looks good. Others have used Norton, I think Chris Reynolds uses a Sunbeam assembly in his Hartley VB.
The biggest issue will be finding a rod that is more or less the right length (you could shim the barrel a bit). You can get a Carillo made, but you might have to mortgage your grandmother.
http://www.bsagoldstar.co.uk/price_list ... ist_12.pdf
regards, Brenton

[adrie.degraaff]

There is a man in the Netherlands who has this combination; Ariel crankcases, Hedlund flywheels, Jawa bigend, roundfoot COMP barrel and 1955 alu head.
I don't think it has cost him much.

[Knud.Degnbol]

See here how it is done. The photo was taken during the German Arielrally this August. I don't know who made it.
Knud

[brenton.roy]

There you go Wali - choices!
I forgot to say that you could probably go the budget version and have the flywheels bored for a VH big end.
Brenton

[david.anderson]

Wali
With the VH engines it is generally regarded that the maximum safe compression ratio is 10.5 to 1. Australia’s most successful Ariel rider initially used 12 to 1 compression but after problems dropped back to 10.5 to 1 and had no further problems. He did say that the flywheels may still flex at 10.5 to 1, but he never broke another crankpin. With a VH crankpin in a 350 you should be able to go higher than 10.5 to 1.

I do know that several Australian tuners made their own bigends. As you have access to a lathe it is not a difficult or costly job to turn a crankpin from suitable steel slightly oversize and have it hardened and ground to size. It was apparently normal to use 3/16” rollers in these bigends. (Vincents run 3mm rollers so you could also consider 1/8 x 1/4 rollers) The result is a 1/8” oversize crankpin which is a lot stronger than standard (or 1/4" larger if 1/8 rollers are used). And while you are on it have the flywheels machined to parallel fit (possibly also larger )as mentioned by Brenton. It is most important when machining a crankpin to ensure a large radius at the cheeks or it will break there.

A friend recently had a new crankpin made for his bike by a local machine shop. They charged him $250 for the crankpin and he reused the outer race as it had minimal wear and he fitted new rollers. A new big end for an Ariel will cost us about $400 Australian dollars with current exchange rates so to have one made from scratch is viable.

I do know that one Australian tuner fitted a speedway JAP bigend to his Ariel but cannot elaborate any further.

That said, a few years ago while holidaying in Tasmania I got talking to a Tasmanian Australian Ariel Register member. He had a very hot 37VH that was running 12 to 1 with a standard bottom end. That bike had recently been clocked on the track at 126mph.
David

[ian.scott]

Hi Wal
When I was racing a Cooper-JAP I had a straight 1" press fit pin fitted together with an INA needle roller big end . That was a common mod at the time . You try the 500 owners club for some contact who still does them . Mine was done by Bill Needham

Ian

[Wali.Badger.Taylor]

Thank you all for the advice, I am looking at the options.

I believe one of the problems with putting too many horses through an Ariel engine is the single bearing on the timing side. Drive side has two and holds the shaft well but on the other side it can move a fair bit. There is plenty of room to fit a needle roller behind the half time gear it justs needs machining out. This will be done and extra web will be welded in to suport it.
Wali

[nevhunter]

Wal, it's only the bearings close to the flywheels that take most of the load. The one you propose to add may help alignment being maintained, but it might be better to just let it distort as you might cause the tapers to be loaded too much in bend. The roller type main bearings recommended for the VH are high load bearing . I think the case would fail before the bearings would. ESSO or Jawa bottom ends have been fitted to many motors for racing and are good, but I though they were getting expensive and in shorter supply.
IF you are really serious about the bottom end a set of milled 4130 steel wheels with a pressed in pin and the mainshafts pressed in also and welded at the inside face only could be the go, plus a nice titanium rod should see you safe to revs where your valve gear will become a problem. Art Senior in AUS made a cammer with the chain running through the pushrod tubes

[Simon.Gardiner]

This will be done and extra web will be welded in to suport it.

I had some extra welding done in the timing chest, the welders had a lot of difficulty as some of the castings were seemingly quite porous and had absorbed an amount of oil. Two of my three timing-side crankcase halves ended up as scrap. (That was a few years ago, maybe over a couple of decades techniques have improved!)

I'd think the main bearings themselves were up to the job (maybe replace the timing side with a lipped roller as per VH), but the bigger problem is that there's no way of locking the shaft in the bearings, which is why the shafts wear even in normal use.