AL­LEN­MILL­YARD

Allen' s jour­nal on mak­ing the amaz­ing RC3 4 con­tin­ues with top tales about the tank and seat con­struc­tion.

Classic Motorcycle Mechanics - - COTENTS - Allen Mill­yard

Our Allen sorts the tank and clocks on the RC374.

Ihave been fol­low­ing my model mak­ers’ pic­ture book through­out the build of my RC374 and I have been won­der­ing for some time what I was go­ing to do about the tank. The pho­tos seemed to show a ba­sic tank shape which be­lieve was a long range tank spe­ciae cally made for the 1967 sle of Man TT. The tank was uite at-sided in shape with­out many com­pound curves. had never made an alu­minium tank be­fore but thought why not have a go mak­ing one from scratch? There are a few videos on­line, show­ing peo­ple mak­ing alu­minium tanks which was a great help, and re­mem­bered do­ing a few months in the tin-bash­ing shop dur­ing my ap­pren­tice­ship many years ago so un­der­stood the ba­sic prin­ci­ples in­volved. bought a sheet of 2mm thick 1050 grade alu­minium, choos­ing 2mm over 1.6mm for added strength and ease of weld­ing, es­pe­cially the edge-to-edge joints. The rst thing did was to make the un­der­side of the tank to t the frame: this was the eas­i­est part of the tank to make it be­ing ba­si­cally at shapes welded to­gether. made s ome card­board tem­plates then cut out the shapes us­ing my heavy duty tin snips and band-saw. The edges were then Aeled s mooth and Aet­ted t ogether with min­i­mal gaps and the parts welded to­gether us­ing my TIG welder, check­ing for a good AET on the frame as went along. Af­ter a few parts had been welded to­gether it was pos­si­ble to re­shape and straighten any dis­tor­tion and trim to size, (it' s al­ways best to cut out pieces over­size then trim af­ter). The un­der­side of the tank was pro­gress­ing nicely and was get­ting a feel for how the alu­minium sheet works and ows. cut out and welded on the front

hoops that would lo­cate onto two spig­ots on the front of the frame. The bot­tom of the tank had to run par­al­lel with the frame about 10mm above the top tube on both sides to al­low room for six rub­ber pads. With the main un­der­side panel made cut out a sq uare hole at the back ready to weld in the sump. The sump was folded up from one piece of alu­minium sheet in the shape of an open top box and welded in place. The sump en­sures a con­stant ow of petrol to the bank of six carbs. A 25mm hole was drilled in the cen­tre of the sump ready for an alu­minium threaded boss to be welded in place for the petrol tap. then made two in­ter­nal bafø e plates that will pre­vent petrol slosh­ing around and stiffen up the struc­ture es­pe­cially by sup­port­ing the top panel, which spanned around 600mm in length. The bafø es were shaped to t over the in­ter­nal cen­tral chan­nel, and were pre-drilled with sev­eral holes to al­low petrol ow be­tween sec­tions. The bafø es were then welded in place and the top folded edge ad­justed to the cor­rect height. The front bafø e was welded in place di­rectly un­der where the chin pad would be lo­cated to sup­port any loads and pre­vent deø ec­tion or dent­ing. The evolv­ing tank struc­ture was trimmed and ad­justed to t the frame sq uare and in line ready for the top sec­tion to be made to t. The top sec­tion was made from one piece of alu­minium sheet with a rolled edge on both sides and com­pound curves at the front and mid­dle. cut out the top panel over size by sev­eral inches to al­low for bends, shap­ing and trim­ming to size. The nished size shape was then marked out and the sides formed over the edge of my bench us­ing a wooden mal­let.

The com­pound curves were formed us­ing a boss­ing ham­mer on a cloth bag lled with sand. This took sev­eral at­tempts to get the shape right but soon with per­se­ver­ance the shape started to look right. grad­u­ally trimmed back the edges with tin snips to t closely to the bot­tom sec­tion at the front and rear. At this point a 50mm di­am­e­ter hole was marked out, chain drilled and led to size in the top panel ready for the petrol cap body to be welded in place from un­derneath. A de­pres­sion was also formed for the rub­ber chin pad to sit in: this was achieved with a punch and die for­mer, made from ply­wood. The top panel was marked out and placed be­tween the punch and die. The for­mer was then pressed to­gether in my hy­draulic press, prac­tic­ing rst on pieces of scrap sheet to per­fect the shape and pres­sure req uired to form a per­fect de­pres­sion. The top panel was laid on its side on a piece of ply­wood and the edges tapped at and level with a mal­let, then card­board tem­plates were made ready to cut out the two side pan­els. The side pan­els were shaped and trimmed then welded in place. The top sec­tion was then of­fered up to the bot­tom sec­tion and grad­u­ally trimmed and shaped to t to­gether en­sur­ing a close t with min­i­mal gaps. When was happy with the t the two parts were welded to­gether. The welds were led and sanded to make a smooth joint and any de­fects re-welded. blocked the out­let port and lled the tank with wa­ter to check for leaks and was pleased to see only a cou­ple of mi­nor leaks. These were welded up and the tank dried in the sun. The tank was placed onto the frame and the rear se­cur­ing hook and strap was made copy­ing the orig­i­nal de­sign as shown in my model mak­ers' book and welded in place. With the tank com­plete the seat was by com­par­i­son an easy task, the base was made from a sin­gle sheet of 2mm thick alu­minium folded and shaped at the edges with holes drilled to x to the frame, the hump was formed by cut­ting, weld­ing and form­ing at sheet with a boss­ing ham­mer to pro­duce the de­sired shape, then ling and sand­ing to re­move any sur­face marks. My RC3 4 was re­ally tak­ing shape now and was feel­ing conae dent about mak­ing the fair­ing, but that' s an­other story!

The lower tank parts.

el­d­ing th e lower tank tog eth er.

Lower tank sh ow­ing th e baf­fle plates.

rial fit of top prior to trim ing to size .

il­ing ole in sum p for th e petrol tap boss.

rial fit of tank onto fra e prior to fi­nal weld­ing .

Close- up of top panel trim ed to fit bot­tom of tank and ready or weld­ing .

Pun prac and die for form ing th e in- pad de­pres­sion tic e de­pres­sion sh own in be­tween!)

e fin­ish ed seat!

Rear tank strap ount­ing .

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